header photo

Big Thompson Diesel and Automotive

Loveland and Northern Colorado's Diesel and Automotive Repair Specialists

Blog Search

Blog Archive


There are currently no blog comments.

Modern Diesel Engine Emission Systems

I get the following questions and comments almost daily from customers and potential customers:

Why do we need all this emissions stuff?  My old diesel got way better mileage my friend says delete it and I won’t have any more issues!

            My truck is in constant regen and always has low power!

I have had the truck at 3 different dealerships and had lots of parts thrown at it, but it still has an engine light! Now they want to replace the particulate filter. What do I do?

            I just deleted my truck and now have a pile of issues! What do I do next?

I’d like to answer these questions for you and help educate you about the modern diesel emissions systems, laws, facts, and myths as they stand today.  I’d like to help you to make sure your diesel is always running at its best while costing you as little as possible.  I know emissions can be a hot button issue and my intention is never to offend anyone.  My intension is only to educate you and save you time and money in the long run.  I’d like to point out that I talk about diesel trucks in this article, however, this applies to all modern diesel power vehicles including diesel powered SUVs and cars from all manufacturers.

Clearing Up Emissions Laws

One of the common things I see people try to do is to register a deleted truck outside of an emission-controlled area.  Technically there have been laws in Colorado since the 70’s stating that if a vehicle is operated within an emissions-controlled area more than a certain number of days a month, you still need an emissions test even if the vehicle is registered elsewhere, even out of state.  To police this, they would have to stop every vehicle that crosses the border of emissions counties.  These days, that is not possible, but I recall in the early 80’s my grandpa worked at the AirForce base in Cheyenne and routinely had to drive to Denver for training and his carpool group used to get in trouble for driving across the border with a Wyoming registration and no emissions test.  They got around it by switching vehicles up every few days.  If they wanted to enforce this law, they could.

Here is a link to the diesel emissions program laws.  

Code of Colorado Regulations - Diesel Emissions

I would like to specifically point out:

“Part A of this regulation is to reduce air pollution resulting from emissions from diesel powered motor vehicles in the AIR Program area through opacity inspections or exemplary maintenance, by all diesel fleets registered, required to be registered or routinely operated in the program area or principally operated form a terminal, maintenance facility, branch, or division located within the program area as defined in 42-4-401 (8) C.R.S. with nine (9) or more vehicles over 14,000 pounds Gross Vehicle Weight Rating. Regulation Number 12 is a State-Only program and is not part of any state implementation plan with the US EPA.

I.B.20. "Routinely Operated" means operated for 90 days or more in any 12 month period.”

This is the rule for over 14000 lb. vehicles.  The rules for vehicles under 14000 lbs. are similar but seem to change often and can be even more strict. These same rules are in effect for gasoline powered vehicles as well!

There are also many state and federal laws about emission component tampering.  This means deleting vehicles is not just illegal in Colorado but is illegal nationwide. As a matter of fact, there have been more shops busted in non-emission states for deleting vehicles than in emissions-controlled states.  The definition of tampering as defined by the Code of Colorado Regulations is a follows:

What is the definition of tampering?

“Tampering is defined as the disconnecting, deactivating, removing or rendering inoperable any emission control device or element of design installed or engineered by the manufacturer on your vehicle, and is prohibited pursuant to CRS 42-4-314.

All vehicles certified for sale in the United States by U.S. EPA or the California Air Resources Board (CARB) have emission control systems that monitor and regulate the engine operations and exhaust gases to maintain air pollutants at strict levels. Tampering with these devices is illegal: it causes excess pollution, and may cause damage to other emissions control devices on your vehicle.

If you remove, disconnect, detach, deactivate, alter, modify, reprogram, or reduce the effectiveness of any emission control device installed by the manufacturer, or use less-effective replacement parts, then you are committing the act of tampering. This includes reprogramming or “re-flashing” the vehicle’s computer, or installing performance chips to circumvent or “defeat” factory settings, or to produce excessive exhaust smoke (diesel trucks). This also includes modifications such as installing an aftermarket exhaust system (pre-catalyst), or installing larger capacity turbocharger and/or turbo waste-gate modifications.

Tampering with a vehicle will also void its warranty. Any person other than a manufacturer or dealer who violates this section, or any person who violates the Clean Air Act (CAA § 203(a)(3)(B)), shall be subject to a civil penalty of not more than $3,750 per violation. Any manufacturer or dealer who violates this section shall be subject to a civil penalty of not more than $37,500 per violation.”

These are not new laws most have been around since the 70’s and are subject to change whenever new laws are voted in.

Why do we need all this emissions stuff?

I’m not sure about you but I really like to breathe clean air.  Yeah, it sucks that the government is involved but one of the biggest reasons that the diesel industry is getting hit so hard by government regulations is because of this!


This is the harsh reality.  Don’t get me wrong, I love diesel trucks. I own diesel powered vehicles and have been involved in diesel engine repair since 1992.  I have towed trailers of all shapes and sizes for hundreds of thousands of miles.  I am in love with diesel power.  But guys and gals, this makes all of us look bad!!!!  People generally do not like to see this and have made their voices heard.  Maybe if some of us weren’t out of control with this, we might not be where we are now?

Fuel Mileage

Another complaint I hear daily is the new trucks really don’t get the same mileage they used to. There is a lot that goes into fuel mileage.  For example. Back in 1995 the average curb weight of a 3500 series truck was 5000 lb. for a truck with a diesel engine and dual rear tires.  Here are the specs for the 1996 Dodge Rams:

The curb weight of a 1996 Dodge Ram 3500 is 5,517 lbs. The curb weight of a 1996 Dodge Ram 3500 extended cab is 5,849 lbs. The curb weight of a 1996 Dodge Ram 3500 club cab is 5,493 lbs.


These trucks also made 165 hp and 380 ft-lb of torque. 3 main things have happened to trucks in that time. They make a lot more horsepower, they weigh a lot more, and they are less aerodynamic. Here are some specs for the 2023 Rams:

The 2023 RAM 3500 specs indicate that at its lightest, this truck may come in at 6,056 lbs. Heavier trims like the 2023 Ram 3500 Limited Longhorn can have a curb weight of up to 9,252 lbs. The 2023 RAM 3500 dually is typically even heavier than their 4-wheeled counterparts.

Horsepower - These engines make 3x more horsepower and almost 3x more torque. To make this much power, it takes fuel. The more fuel you burn the more horsepower you can make.

Aerodynamics - There is another byproduct of horsepower, heat.  This is where aerodynamics comes in. The front of these trucks are almost 2x larger than they used to be just to get enough airflow through the radiator and intercooler to keep these trucks from melting to the ground.  The size of the transmission and power steering oil coolers has over doubled in size. The intercoolers have gone from nonexistent to massive.  In some vehicles, there are 2 radiators and an intercooler.

Weight – The added weight comes from many things.  Here are a few examples.  The larger capacity cooling systems, larger transmissions, creature comforts such as heated seats, navigation systems, and electric powered everything.  The weight of the emissions systems alone is heavier than a traditional exhaust system. The brakes are larger, and many additional components contribute to weight gain.

Let’s say your old 1996 ram 12 valve engine used to get 20mpg. Now load that truck to an equal weight of a modern truck empty. To do that you would need to put 4500 lbs. of weight in the bed. Now let’s go drive both trucks at 55 mph. I will bet they are closer in mpg than you might think. Now let’s go drive these trucks at 80mph. Wait. Will the 12v go 80mph? Maybe on the flats but let’s say we are driving from Loveland towards Denver on I-25. That hill outside of Berthoud will not allow a stock 12v truck with 4500 lb in the bed to go anywhere near 80mph. That truck simply doesn’t have enough horsepower. That new 2023 truck won’t even slow down or even downshift. It might only make 6 psi of boost which indicates it is not even working yet. Now what has the fuel mileage done. One of them is not even working and the other one is maxed out. Let’s turn up the fuel on the old 12v to make it keep up. Now more fuel equals less mpg and in the old 12v is now producing a lot of black smoke to even come close to the performance of the stock 2023.

The point is, the engineers that have made these new trucks have done an amazing job at making the trucks very clean out the tail pipe and making a ton of power while doing it. Yes, the byproduct is maybe not as good of fuel mileage but if you really compare apples to apples and make it an equal fight with weight and payload the fuel mileage would be much closer than many would ever want to admit.


When you are upset about the fuel economy of the new trucks consider that you are pulling your 5th wheel that weighs 25000lb at 80mph up I-70 towards Eisenhower tunnel while running 40” tires.  You have the multi zone HVAC system keeping all passengers at their own perfect temperature, the kids watching tv in the back seat with the dog sitting comfortably between them.  You can do this while not needing to wear ear plugs or taking multiple vehicles to haul the huge load. Truly, to get that much weight up the mountain at that speed, you will be way overloaded for that old 12v.  Yes, it will move the load but not safely and not at 80mph.  So now to carry that same load using the old truck,  maybe you have 2 vehicles to get the same load and same amount of people and equipment to your destination.  How fuel efficient is that?  Please really think about this part of the article.  I’ll bet you if you slow down to the speed limit you will get better fuel mileage. Bottom line is it takes more fuel to make more power.  A lot of those creature comforts take a lot of extra power. It also takes more fuel to go faster.  The faster you can go while moving an equal weight the more fuel it will take. Would that old 12v truck move the 25000 lb. trailer?  Yes, but not legally?   How would its fuel consumption look if it could pull it 80 mph?  Could it pull it 80 mph up I-70? Not a chance!!  Could the wife and kids be as comfortable?  Maybe if they took another vehicle.  Wait that second vehicle just killed your fuel economy all by itself? The cost of driving a second vehicle just offset any economy you might have thought you had.


Emissions Systems

Let’s get into the emissions system and what you can do to keep it happy.  First, if you are the guy pulling that trailer at 80mph up the hill, congrats!  You are not at all hurting your emissions system.  You are doing it a favor.  These modern emissions systems are designed to operate correctly when really hot.  You will see why here soon enough.  For now, let’s separate it into pre combustion emissions and after-treatment systems.  Pre combustion emissions include the crankcase ventilation system, the exhaust gas recirculation system or (EGR), as well as sensors that control fuel delivery and the fuel delivery system itself.  The after-treatment system includes the catalytic converter, particulate filter, SCR system, SCR catalyst, and sensors to monitor the operation. The other parts of the emissions system that can’t be really in either category are the combustion process and the turbo.

So how does it all work? What does it do?  Two primary focuses in diesel emissions are the reduction of particulate matter and the reduction of NOx emissions.  Particulate matter is basically visible smoke, black smoke, white smoke, blue smoke, or grey smoke. Black smoke is partially burnt fuel meaning there was combustion but there was not enough air to completely burn all the fuel causing black coal ash.  This is basically all carbon ash.  White smoke is usually burning coolant.  It can drastically and quickly destroy an emissions system.  Blue smoke is oil burning.  Too much of this can ruin an emissions system. Grey smoke is unburnt fuel.  This means fuel was delivered into the combustion chamber but for whatever reason the fuel did not burn.  Lack of compression or way too much fuel like a broken injector tip are the common causes of this.  All of these particulates are unsightly and unhealthy and are meant to be cleaned out of the exhaust system.

     NOx emissions is a major focus of the emissions system from the air filter to the tailpipe.

“NOx emissions are released by automobiles, trucks, and other non-road vehicles, such as construction equipment and boats. NOx emissions have direct negative effects on human health and indirect effects on agricultural crops and ecosystems.

NOx is a major component in the formation of ground-level ozone, which can cause severe respiratory issues. NOx emissions also contribute to the formation of smog and acid rain, and affect tropospheric ozone.”

Here is a breakdown of components and what emissions they affect.

Pre-Combustion Emissions

Pre-combustion emissions are components that alter the air charge into the combustion chamber.  Why do we do this?  The #1 reason is to cool the combustion chamber and reduce NOx emissions.  NOx emissions if formed under extreme combustion chamber temps.  The harder the engine is worked, the higher the combustion temps and the more the NOx emissions increases.  The EGR valve allows some of the exhaust gases to be recirculated as an inert gas into the combustion chamber to cool the firing event. The EGR system is a source of great controversy.  The reality is the EGR system is actually not all that bad unless the vehicle is not driven as it is designed to be driven. Lots of idle time, slow speeds, stop and go driving, and short trips wreak havoc on the EGR system.  There are two reasons for this:  

1.  This type of driving promotes lots of particulates in the exhaust that is now being recirculated into the intake of the engine.  

2.  There is not enough air flow to push all this dirty air through the engine fast enough, which allows it to find a surface inside the engine and attach to it like a cancer.  

The more carbon builds up the faster it builds up.  On the other hand, a truck that is working hard at a steady state speed making a good amount of boost has a ton of airflow.  Air is screaming through the intake and if the EGR opens there is less chance due to air volume alone for the carbon to build up inside the engine and the fact that the exhaust is cleaner at this engine load means not only are there less particulates entering the intake system, but there is also much less chance that they have time to accumulate on any surfaces.  The EGR system is very durable when the engine is working and becomes very temperamental and causes intake restrictions when the vehicle is not operated correctly.

The other pre combustion emissions system I mentioned is the crankcase vent system. This is a system that people tend to think needs to be deleted as well. This is nothing more than a PCV system like gasoline engines have been using for 70 years or more. For years, diesel engines just exhausted the crankcase blow by onto the ground and into the atmosphere.  This is a combination of unburnt fuel, oil, and all sorts of combustion gases just being vented to atmosphere.  These are a combination of particulate matter and poisonous emissions.  In modern diesel systems, these gases are being recycled into the combustion chamber where they can be completely burnt off and dealt with in the aftertreatment systems.  The oil is actually a fuel so it goes into the combustion chamber as a vapor.  When the diesel fuel combusts the air charge in the combustion chamber that contains the oil vapors it will simply burn off the oil vapors and it will never hurt anything.

Why do people think this system fails? The big reason is they believe it creates carbon build up.  This is where I will start to sound like a broken record. If the engine is being treated like it is designed to be treated, you will never see a single negative effect from this system.  If you let your truck idle all day, take short trips, stop, and go driving and in general just not working your engine then this system can cause some additional carbon buildup along with the EGR system due to the lack of airflow through the engine. When being worked hard there is adequate airflow to pass all this oil vapor and particulate emissions through the engine and it will never have time to slow down and attach to internal engine components. Also extended idle time promotes more blowby which will cause more strain ultimately causing this system to create and allow more oil and particulates into the intake causing faster and faster buildup.  The particulates and oils from the crankcase vent system and the EGR system can also attack other components. The entire intake tract including the intake valves and top of pistons can become very carboned up. I have customers with trucks that have 500000 miles and have no carbon issues at all. I also have trucks in the shop daily that have 60k miles on them and basically need the engine torn apart and the carbon chiseled out of every orifice.

Here are some images of the cancer eating the inside of these engines.  All of these are the direct result of too much idle and short trip time on the engine.  

This is an image of a dirty EGR cooler. There is absolutely no EGR flow through the EGR cooler causing engine lights for EGR low flow codes.



To the right is an intake manifold that again has had too much idle time. This is all soot from the EGR valve that has choked up the intake and is actively restricting airflow into the engine causing a low power concern.


This is oil and carbon buildup inside an intake passageway that is again choking the engine. 



To the right is an intake manifold dripping oil out of it. The oil is from the crankcase vent system that has failed due to excessive idle time and has been filling the intake with oil.



This is an example is a completely plugged EGR cooler again caused from excessive idling. This is the same cooler assembly compared to a new Bulletproof Diesel EGR cooler assembly.


This is a dirty MAP sensor. This sensor is designed to read boost and tell the engine control module if it can deliver more fuel or not. If this sensor is plugged, the engine will never run right.

The bottom line is, you can prevent all of these failures by simply driving your truck the right way, by using it what it was designed for.  The good news is if you can’t drive it like this every day that is ok.  Just be mindful and do your best.  We have a very thorough de-carbon treatment we can perform for maintenance to help eliminate this carbon from your engine.  The more short trips you take the more often we should perform this service.  I recommend a de-carbon for every engine, even the ones getting a ton of miles on them. How you treat your truck will decide how often we should perform this treatment.  If you have a lot of idle hours and short trips, I would do this every 30k miles.  If you have a good mix of short and long trips, maybe stretch it to 40-60k miles.  If you tow a trailer every time your truck is running and the average length of the trips is over 1 hour at highway speeds I would say you could stretch this service to 100k miles.

Here are some before and after images I have personally captured with a camera inside the EGR system before and after our cleaning process. Yes, I like to see the results for myself before I use any products. I usually will look before and after with an inspection camera just so I know our process is still working and that each individual vehicle has become as clean as I expected. If they do not clean up enough, we can either immediately run another dose or use a different plan of attack on that vehicles carbon issue. Again, this is a maintenance service and is not designed for extreme carbon issues. If used as a maintenance service before carbon gets completely out of control it will almost completely remove all carbon from the inside of the engine.

Before De-Carbon 

After De-Carbon 

Before De-CarbonAfter De-Carbon



If the carbon goes too far, this service will almost not even touch it.  Again, I suggest doing this before you think you need it. This can save you lots of money on repairs and fuel economy in the long run, not to mention if you are getting clean unrestricted air into the engine then the air burning through the combustion chamber and out the exhaust will be more efficient, thus, causing fewer issues downstream as well.


Here is an example of an engine that had an engine light on.  It had spent time at many other shops. It had a new intercooler installed and a new turbo installed.  The next step they were suggesting was new fuel injectors to get the truck running right.  The code says nothing about a fuel delivery issue so I’m not sure why they would go there other than the truck had absolutely no power.


Digging around and verifying the low power concern, the vehicle had no power. Checking the basics led to an airflow issue. No boost leaks found. I removed the intake and found the heat grid very carboned up. I went further and slid a camera in the intake. In this case I saw at least 3 intake valves that I could not get the camera past. We tried performing our cleaning process but unfortunately it seemed to gain only about 100 horsepower and never came fully back to normal.  This truck was very sluggish.  I slid the camera back into the intake and found a couple holes had cleaned up but there was no way to get the camera through the intake valves at all on a few valves. This truck needed the head pulled off and the carbon ground out and chiseled out of the ports and off the valves.

The Turbo

The turbo is used by both the pre and post combustion systems so I would like to talk about it here before we get into the post combustion information.  All these modern emissions diesels use a version of a variable turbo.  Variable turbos come in 2 main designs.  Some engines use a variable vane, some have a variable nozzle. For this article, I will use VGT as a generic term covering a variable exhaust turbo. The VGT turbos are very durable but have a weakness. Carbon buildup is their worst enemy. The turbo is coming right out of the combustion chamber and is ingesting the dirtiest air the engine has to offer. There is nothing pre-turbo that eliminates carbon except you. These turbos love a clean running engine. A cold engine, idling engine, stop and go driving, and an engine that has frequent short trips is going to be a dirty and sluggish turbo that will die an early and painful death.  To keep your turbo happy, drive the truck under heavy and steady loads for extended lengths of time. Use the exhaust brake (if your vehicle is equipped from the factory). This will ensure a full sweep of the variable components in the exhaust side of the turbo which helps clean the carbon off every time the exhaust brake cycles. This will not clean a stuck turbo, but it will help keep turbos cleaner for longer. We do have a turbo cleaning process along with the EGR and intake cleaning process also helps clean the VGT turbo. Some turbos, if stuck long enough, can damage the actuator that moves the turbo. Most turbos will exhibit low power concerns before there are engine lights related to a turbo sticking.  Pay close attention to how your truck is running. If it feels like it has lost a step it is probably getting carboned up. Let’s clean it out before it causes more expensive issues to fix.


In this picture this turbo was so plugged up with carbon that it would only run the engine for a couple seconds before it would cause the engine to stall. Sadly, the dealer sold them a completely new exhaust system and still did not fix the truck. They took this vehicle to an independent repair shop that sold them a fuel system. It was still not fixed after spending $15000 on repairs. Now with the turbo this plugged.  I can imagine the exhaust system would have been in bad shape.  If the exhaust gets completely plugged, we can have it cleaned with no need to spend $5000-$8000 for a dealer replacement.




This image is the same turbo before we pulled the rest of the exhaust housing apart. We most likely would have saved them a complete exhaust system and could have just cleaned it and we definitely would not have thrown a fuel system at the vehicle. This should have been a $2500-$3000 repair and not a $15000 plus the $3000 for the actual fix.

Post Combustion Systems aka The After-Treatment System

After treatment systems on a diesel started in the mid 90’s on some engines. These were simple catalytic converters designed to clean up NOx emissions. They were fairly inefficient but really never hurt the vehicles’ performance, especially if driven hard.  However, they came out when the common thing to do was to keep the truck idling all day. This caused them to plug and cause poor performance.  (See a pattern here?)  Vehicles then occasionally had just a generic cat until 2006 when particulate filters showed up on Duramax engines, followed by Cummins and Ford in 2008. These emissions systems were very misunderstood and caused the need for driving the truck while working it hard and made it so the diesel engine was no longer the start it up and let it run all day engine.  The particulate filter systems on these early trucks plugged all the time.  90% of this was because all the old timers that let their trucks run all day were not ready for this new system. I blame manufacturers and dealers for lack of education on this system and how the system differed from all previous engines. Now they really needed to be working hard to keep the emissions system working right.

How does it work?  The particulate filter has sensors that read the restriction across the filter. If the engine control module reads a large enough restriction across the filter it performs a regen. The regen process is basically the same idea as a self-clean oven. It gets the exhaust so hot that it burns off the soot from the filter and cleans the filter until it senses the filter is no longer dirty.  How does it make enough heat to do this?  This is one of the most misunderstood parts of the after-treatment system. It needs heat to clean the system and depending on the manufacturer they have a slightly different exhaust system make up. But they all work very similarly. The DOC is an oxidizing catalyst that helps convert gases.  Hydrocarbons and carbon monoxide are converted into carbon dioxide and water.  In order to do this, it needs to be hot. Only once the catalyst reaches a certain temperature does this really start to work. Usually around 650 degrees F to 700 degrees F. Once the DOC lights off the temp exiting the filter increases over the temp entering the converter. Usually by 100 degrees F or more.

Rohini KhobragadeSunit Kumar SinghPravesh Chandra ShuklaTarun GuptaAhmed S. Al-FateshAvinash Kumar Agarwal & Nitin K. Labhasetwar (2019) Chemical composition of diesel particulate matter and its control, Catalysis Reviews, 61:4, 447-515, DOI: 10.1080/01614940.2019.1617607

This process basically blows the hot exhaust gases at the particulate filter creating the “self-clean oven effect”.  This heat at the particulate filter bakes off the carbon turning it into ash and reduces the restriction across the filter. Over time the ash can be built up and the filter may need to be removed and cleaned to get the ash out. If the system is working right and being treated right this will be hundreds of thousands of miles before it is needed.  Now we know this system needs heat. If you are pulling a trailer or operating the vehicle under enough load the exhaust is hot enough to light the converter the system will basically take care of itself.  This means it is always keeping the filter clean and you will likely never see the dreaded regen in process message.

If you are not driving the vehicle in an appropriate manner and the filter is getting plugged, the system has many different levels of trying to clean the filter. In some vehicles you may never know it is happening in the first few levels of regeneration. The Cummins engine can go into level 1 regen without you ever knowing anything about it. They are firing the injectors during the exhaust stroke of the combustion cycle pumping raw fuel out the exhaust. The extra hydrocarbons help to light the catalytic converter even if you are not driving it right. Ford uses this same technology but will give you a message stating it is happening. Duramax engines and a lot of European diesels have an aftertreatment injector that injects fuel straight into the exhaust stream, usually giving a message but not always.  Either way, the engine control module knows the filter is restricted and since you are not driving it to keep the temps hot enough it has decided it needs to get involved to ignite the cat to help clean the filter.

The next level in regen, if the previous step has been unsuccessful, is to put a message on the dash that states filter full continue driving.  This means do not stop driving because it desperately needs to clean the filter.  It is not lying.  If it is in this stage, you do need to drive it until it turns the message off. It can plug the filter more if it is in the middle of this process and you shut the engine off. If you continue to ignore this, the next phase is to turn on the engine light and limit power.  

Why do I keep saying “you”?  It’s because if it has gotten to this point you are the problem. Something has been ignored.  Sure, there are some mechanical issues that can cause this, but they will also usually have a symptom and possibly an engine light associated that has been ignored to get to this point. When the engine is trying to force a regen the engine will be dumping so much extra fuel to make temperature that it is now getting really bad fuel mileage. It is also trying to breathe through a restricted filter that is causing more throttle to get the same amount of power out of it, again causing more bad fuel mileage. How long do you have to drive it to get the exhaust up to the right temp? That depends greatly on the load the vehicle is under. It also depends on the size of the engine. The small little diesels tend to heat up quickly; within 10 miles at highway speeds.  The bigger diesels take longer. I have seen 6.7 fords take 45 miles at 65 mph to get the cat to light without it being in regen. The same truck took 19 miles pulling a trailer.  The smaller diesels are under a heavier relative load just to place the vehicle in motion than the bigger diesels take. They are also the vehicles typically used for shorter trips, so it makes sense they need to make a lot more heat a lot faster.  The bottom line in all vehicles using this system is that it works really well when hot. You need to drive the truck usually pulling at least a light trailer to get the system working but when the engine is working hard it should never truly need a regen as the system will always be hot enough to be cleaning itself without the engine module forcing it to.

There are definitely a few things that can kill the filter. Oil contamination from a blown turbo seal. Coolant contamination from a bad EGR cooler or head gasket. Thermal shock from excessive EGT’s or from excessive raw fuel entering the exhaust stream like a cracked injector tip to name a few. Here is an image of a filter that has 2 failures. Thermal shock from coolant contamination has not only clogged it but has cracked it in a few spots. Unfortunately, the only cure here is a new filter.

The Cat and DPF filters still work together and are very close to the same as they were back in 2006. They have been updated and more reliable. The one thing that has helped the diesel engine regain quite a bit of fuel economy is the addition of SCR systems.  Yes, the dreaded DEF.  Hog piss, poison, and fairy dust as I have heard many of you call it. This system is not as bad as everyone thinks. By adding this into the system, the manufacturers have been able to control NOx emissions better. This has allowed a couple things that all diesel lovers should be excited about. Diesel engines create more NOx at higher combustion temps but tend to produce less particulate matter. They have increased power output which has made less particulate matter. This has resulted in smaller and less restrictive particulate filters. Meaning the engines are allowed to breathe a little easier which means even more horsepower and better fuel economy. To control NOx emissions, the system sprays diesel exhaust fluid (DEF) into the exhaust before the SCR catalyst which uses the DEF to catalyze the rest of the exhaust emissions. By the time the emissions come out of the tail pipe the exhaust is cleaned up to a small amount of carbon dioxide, nitrogen, and water. There are no more poisonous gases from the exhaust. All the particulate matter is gone. The exhaust doesn’t even smell bad. It smells like steam which is really what is left.

Janjić, Radomir & Makljenović, Radoslav & Bogićević, Branimir & Ilić, Zoran & Brkljač, Nenko & Stojanovic, Blaza & Bukvic, Milan. (2016). DIAGNOSIS OF DIESEL VEHICLES USING MODERN DIAGNOSTIC SOFTWARE.

What really goes wrong with the SCR system?  The heaters in the tank early on were a big issue.  As with all new technologies that first come out, there are issues. The issues have now been mostly ironed out. The sending units go bad, and pumps can wear out. Most of the failures I see with these systems can be cured by keeping the DEF tank full. I have a lot of customers that run it all the way empty. The issue lies in that the DEF is very corrosive as it is ammonia and distilled water. The ammonia makes crystals when exposed to atmosphere. If the tank is full, all components are submerged. When empty, the level sensors are exposed to atmosphere and splashing DEF. This starts corroding and crystalizing the components inside the DEF tank. This is the one part of the emissions system I am not going to tell you that you need to drive hard to keep it working. You just need to keep it full and most of the components in the tank will stay good and have very few issues. On the other hand, the rest of the system needs…you guessed it…heat.  The good news is if you are driving the truck so the CAT is hot and burning off the converter you will also be doing a good job at getting heat back to the SCR then it will be working right and will have very few issues. Slow driving, stop and go driving, idle time, and all around slow low load operation will result in an SCR system that can get plugged with crystalized def.  This can make the system throw codes and not work without being removed and cleaned. One of the biggest issues I have recently seen is the DEF injector hole into the exhaust is getting plugged with DEF resulting in poor DEF atomization. It doesn’t need a new injector or a new SCR catalyst like a lot of places will sell, it needs the hole cleaned. Unfortunately, nobody looks here for this problem. The following pictures are a before and after look inside the port from the injector into the exhaust.

The issue was the DEF was just dripping into the exhaust and not being atomized properly. This happened because the truck was idled a lot and the air volume across the injector was so slow it allowed the DEF to crystalize and build up over and over, like stalactites in a cave. A little cleaning cured the codes and cured the engine light.

Since starting to write this article I have attended many training classes for different vehicle technologies. For those of you thinking I’ll just go buy a gas-powered vehicle, guess what!  First, most gas-powered vehicles will get awful fuel mileage to perform the same amount of work as a diesel engine will. This is simple physics. Diesel engines are usually 30% more efficient than gasoline engines. Yes, gas engines have come a long way, but you are still immediately giving up 30% efficiency per gallon used. If they are not loaded, they can get similar if not slightly better fuel mileage. When they are loaded the mileage goes downhill quickly. Pulling a 20000 lb. trailer will be difficult and you will not only be killing the engine, but you will also be getting awful fuel economy doing so. Another reason is not only engine design but the fuels themselves are very different. Diesel fuel carries more energy per gallon. So even if your gas engine was as efficient as a diesel engine, (which it is not) the fuels they are burning are 13-20% less energy dense than diesel fuel. Now that 30% less efficient engine design is trying to so the same amount of work with a fuel that is another 13-20% less energy dense.  You would burn more fuel to do the same amount of work.  Here is the real kicker in the gas engine world that is going to affect all gas-powered vehicles as of the 2025 model year. Particulate filters. Yes, you heard that right. In order to meet emissions standards most if not all gasoline engine manufacturers will begin installing particulate filters on the gas engines. Guess what that means? You will no longer be able to use the gas engine as a grocery getter either. They will suffer the same issues that diesel vehicles have with the particulate filters. The filters are also going to make them less efficient.  I think this is going to be very interesting to see how it all pans out. I can only imagine the customer learning curve that will be needed when these systems start to fail on gasoline powered vehicles.  I am not trying to scare you with this article. I am simply helping you make an educated decision when it comes to which vehicle you should use for what purpose. I suggest if you have a modern diesel-powered vehicle that you use it when you know you will be working it properly. Along with this it would be advised to have a daily driver vehicle that you can do those short little errands with to help keep the short and harmful trips away from your expensive diesel truck. I am in the same boat as you are. I love my truck. I want to drive it every day for all trips.  I know a lot of those trips are killing it, so I choose to drive a beater every day and use the truck when it is needed to be a truck.

In Conclusion

I hope you have learned a lot and can help yourself keep your truck on the road. These systems are not scary, they are just misunderstood. The #1 thing you need to know is these modern diesel cars, SUVs, and trucks need to be worked. They are not your dad’s or grandpa’s diesels. They will work a lot harder and make a ton more power. Truly, they are designed to work hard. If you have a Chevy CRUZ diesel it will be working hard by driving on road trips. It will not need to be pulling a trailer obviously, but it would like to do more than just get groceries.  If you have a small diesel in a Colorado or 1500 series truck it doesn’t have to work quite as hard as their big brothers, but they would love to do a lot more than just take the kids to soccer practice!! If you own a Power Stroke, Duramax, or Cummins, you should be doing your best to keep it towing a trailer every chance you get. Try to eliminate all excessive idling and limit all short trips that you can.  Again, all of these statements can be applied to all modern diesel engines. I pick on trucks in this article. Many SUVs made by VW, Mercades, BMW, Jeep, and many others not listed have the same problems. These engines really need to be worked!  

A Note for Shops and Technicians

Some of this info will be copied and used by other shops as a diagnostic guide as has happened in articles I have written in the past. I just hope if you use the information I have provided here that you actually learn from it. Why do I hope you learn from it? Well frankly there are a lot of “diesel mechanics” out there at “Diesel repair shops” that might be ok mechanics. But do you even know what your truck goes through when it is working hard? Do you even own a diesel truck? I can tell you there are a lot of shops out there that don’t have anyone working there that has ever owned a diesel truck. If they do, that diesel truck has likely never actually worked hard a day in its life. Many of these shops also never send their techs to training. I truly hope if there are techs reading this you think of a few things. If the shop you work at will not buy you proper training to learn how these modern trucks work and you don’t own and work hard a modern diesel truck on a regular basis, please do not misdiagnose customers vehicles! Please don’t sell them a part you are not 100% confident will fix the issue! Please, please, please never tell a customer to delete their vehicle just because you do not understand how the system works or what it takes to diagnose the system!!!  



Go Back