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EGR system FAQ for VW and Audi TDI
VW-Audi TDI EGR system FAQ: how it works, tied into the DPF, and how to disable it (some notes applicable to all light diesels)
Where is the EGR and what does it do?(top)
EGR stands for Exhaust Gas Recirculation. The system is located on the firewall side (rear) of the engine on almost all TDI engines. The main purpose of the EGR is to reduce NOx emissions produced during diesel or gasoline engine combustion.
By mixing exhaust gasses back into the engine, it lowers peak combustion temperatures and reduces NOx emissions anywhere from 50%-75% vs. a non-EGR system. The newest 2009+ models work with the exhaust or Adblue system to reduce NOx even further. The reason why putting exhaust gas (hotter than ambient air) into the intake stream reduces NOx is because the free oxygen would otherwise turn into NOx during combustion. Because a diesel runs very lean (much more air than fuel for efficiency), there's more oxygen present vs. a gasoline engine which means more NOx. The MAF air intake sensor also sees a reduction in air intake and adjusts fueling. Everything else being equal and within limits, in a diesel, more fuel = more power = higher temperatures (more potential for NOx formation). In a gasoline car, more fuel at the right ratios = lower temperatures.
The 1997-2003 1.9L TDI systems take in exhaust gases at the exhaust manifold, send them through the EGR cooler (except North American market 1996 passat), and then into the intake manifold through the EGR valve at the intake manifold. The 2004-2006 EGR system takes in gas at the exhaust manifold, sends it through a diverter valve, then the EGR cooler, then through the EGR/diverter valve into the intake manifold. These later cars used an electronic EGR and intake flap "throttles" that regulate intake air for EGR metering purposes. Unlike a gasoline car, the throttle has no direct role in controlling engine RPM - it's only for the EGR because all diesel engines are throttled by fueling, not air. Earlier cars used a vacuum and solenoid operated EGR. These are standard routings in most cars, both diesel and gasoline, turbo and non turbo. The newest engines have a high and low pressure filtered EGR system that works with the diesel particulate filter DPF system. See 1000q: DPF FAQ for more details on the newest 2.0L engines. For the 3.0L TDI engine, see 1000q: Adblue with DPF FAQ.
The system is regulated by the car's computer to open or close to varying degrees depending on load and other factors. At light load or idle, up to 30% of the intake air flow can be recirculated exhaust. At heavier loads, the EGR valve begins to close and at a certain point, the car's computer closes the EGR valve completely. In the newest common rail TDI engines (2009+), the EGR closes all the way during an active diesel particulate filter regeneration cycle. Since the EGR is opened and closed at various engine loads, the presence or absence of the EGR does not have a significant effect on engine longevity. In theory, it lowers temperatures at lighter loads where there is less fueling and lower cylinder temperatures. In most cars, at heavy loads with more fueling and higher temperatures, the EGR system is closed so it does not reduce peak power at all. Again, because diesels run so lean, there's plenty of oxygen to burn even at high EGR flow.
Before going into the engine, exhaust gasses normally pass through a cooler. A side benefit of an EGR cooler is faster engine warm up because the hot exhaust gasses give their heat to the coolant. This is great for a diesel because cold engines have greater engine wear and worse fuel economy and emissions. Diesels have a lot of piston blowby and once the engine is warmed up, the engine seals tighter and gets better fuel economy.
In the mk3 Jetta and Passat TDI, the EGR is part of the intake manifold. EGR metering is a calculated value by the car's computer. In the mk4 and pumpe duse mk5 cars, the EGR is a separate unit bolted to the intake manifold and can be replaced as a separate unit if necessary. These cars also note how much fresh air is coming in from the MAF to determine how the EGR is working. This is why blocking off the EGR on the TDI can cause a check engine light/malfunction indicator light.
The introduction of the pumpe duse engine in 2004 also saw the addition of an O2 sensor in the exhaust. Unlike O2 sensors in gasoline cars, the TDI sensor is not used to determine the amount of fuel injected. Again, it does not directly control fuel metering. The TDI O2 sensor is used to calculate the potential for NOx formation and regulate the EGR valve with more precision, resulting in cleaner emissions.
Here is a picture of a pumpe duse (2004+) diverter valve and EGR cooler. More pictures can be found in the FAQ articles that show EGR cleaning or removal 1000q: a4 egr cleaning and removal. If you have a mk3, see 1000q: mk3 intake and egr removal.
The 2009+ Jetta, Golf diesel, and Audi A3 TDI have a more complicated EGR system. The low pressure EGR gasses are filtered before being sent through the cooler and back into the intake. The high pressure gasses go straight from the exhaust manifold to the EGR valve and into the intake. The low pressure is used more at higher engine rpm and load. It's too new to know long term reliability but it should be relatively maintenance free because of cleaner fuel and less carbon buildup in the intake.
Some believe that the EGR and DPF tuning is the cause of occasional engine stuttering in the CRD engines. This usually occurs at certain partial load situations like hesitation when cresting over a hill.
Below is the visible part of the 2009+ CRD engine EGR system. The green highlighted box is the throttle valve. To the right is a cutaway picture on a demo picture. Its primary purpose is for EGR metering and not engine rpm metering. Some older engines had problems with the gears stripping (gears and throttle visible on the cutaway picture, below right) which caused air delivery problems. The blue box is the EGR valve which regulates recycled exhaust gasses from the white-wrapped tube (high pressure EGR) and restricts air during a DPF regen cycle. At high engine rpm or load, the visible high pressure EGR valve slowly closes and the system switches to the low pressure EGR.
The high pressure EGR gasses are taken before the turbo. The low pressure EGR gasses are tapped after the DPF, through a filter (shown below), a cooler (the silver horizontal cylinder), and then recycled back into the intake path right before the turbo.
How to check proper operation of the EGR valve(top)
The system is normally controlled by the car computer and a solenoid. The solenoid controls vacuum to the EGR valve. The mk3 and mk4 early ALH engine cars use a spring loaded valve that moves up and down. The mk4 later pumpe and mk5 cars use an electronic rotating diverter valve type EGR. This is less suspect to clogging than a spring type and fairly reliable since the electric motor moves the valve.
To check for the valve proper operation, with the car off, remove the intake piping before the intake manifold to see the EGR valve. Take a look inside. If you see a lot of carbon buildup, the intake manifold will need cleaning. A little buildup of a 2-3 millimeters is normal and does not require cleaning. I wouldn't clean it unless it's already off or there's more than 4 mm of buildup or enough to cause restriction.
For the cars that use a spring loaded valve, remove the vacuum line on the EGR valve and apply suction with a vacuum pump. Don't try sucking with your mouth since there isn't enough force and the hose is dirty. The valve should move towards the vacuum diaphragm when you apply suction. Since it's spring loaded, it should return to closed when you release suction. If it fails to fully close, it means there's a problem.
EGR valves that are electrically operated have sensors to show their position and should trip an error code in the car's computer. It's possible for the internal gears to strip.
If the EGR valve works but the system still doesn't work, check the solenoid, electrical connectors, and wiring. A vacuum leak can cause malfunction of the EGR valve. A clogged line can also cause malfunction. If none of these solve your problem, then the computer (ECU or ECM) or other sensor may be faulty.
Removing or cleaning the EGR system and intake manifold(top)
If there is excessive carbon buildup on the intake manifold and EGR, it must be removed and cleaned. There is no set time, mileage, or factory recommendation on when to clean it since accumulation can vary according to type of engine oil used, driving style, quality of fuel used (ultra low sulfur diesel, biodiesel, etc.), and even by car. The reason why VW diesels seem to be more prone to buildup than other cars is probably due to engine oil mist in the intake system meeting the recirculated exhaust gases. The oil mist comes from the CCV (Crank Case Ventilation, similar to a PCV system) system.
The main component of the CCV system is on top of the valve cover and looks like a black plastic hockey puck with a hose. Diesel cars naturally have more piston blowby than gasoline engines which pressurizes the engine crankcase. Since diesels don't have a strong vacuum in the intake system like gasoline engines, they use a vacuum pump to run the brake booster and other items. Gas engines are throttled by air - they use a throttle plate which creates a vacuum and pumping losses as a byproduct. The vacuum pump creates vacuum on one side and pressure on the other - this air pressure goes into the crankcase. Air pressure from these two sources must be vented or else it could blow out an oil seal. The CCV hose is attached to the intake piping before the turbocharger so that crankcase vapors and air pressure are drawn out of the engine crankcase. There may be a heating element on the hose to prevent it from icing up. (The oil in the crankcase vapors is why it's normal for a little bit of oil in the intercooler to be present. However, excess oil could indicate a problem.) The soot from the exhaust and oil mist combine to form the carbon buildup. Again, quality of fuel (biodiesel or ultra low sulfur diesel), engine oil, EGR system operation, and driving style all contribute to carbon buildup which must be cleaned. Since the introduction of ultra low sulfur diesel in the US, the likelihood of severe intake manifold buildup has reduced.
To clean the system, it's best to remove the entire intake manifold and EGR system but you can get away with just cleaning the intake manifold, EGR cooler, and EGR valve. You MUST remove the components from the engine when cleaning them. Do not clean them on the car or use de-carbonizing chemicals to clean the intake manifold. If a large piece of carbon breaks off and gets stuck in a valve or turbo, it will result in severe engine damage! The liquid or solids can also go into a cylinder and cause hydrolock. Dealers used to do "on car cleaning" until they realized that some engines were damaged from this. If you didn't get the message - you must remove the parts from the car to clean them! Remember that the pistons are almost touching the cylinders in a diesel engine due to the high compression, so even a small amount of standing liquid may cause engine damage.
You can also just replace them with new parts instead of cleaning them. It is recommended for the mk3 cars to replace the intake manifold because the EGR valve can malfunction and leak oil and is not removable from the intake manifold. On the mk4 and mk5 cars, you can reuse the manifold since it's just a cast piece of metal and the EGR can be replaced as a separate unit as needed. Cleaning them is a very dirty process so wear clothes that you don't mind getting dirty. Refer to the FAQ for DIY articles for each model.
Another problem for the mk3 and mk4 TDI is leaking oil out of the EGR weep hole. The EGR has a diaphragm inside that moves on a shaft. The other side is vented to the atmosphere. As the shaft wears, it can leak oil that comes out the weep hole. You don't want to plug the hole or else the EGR will not function correctly. The mk4 EGR is removable from the intake manifold. If it weeps oil, replacement is suggested. The mk3 EGR is not removable from the intake manifold. If it weeps oil, you could replace the entire intake manifold or reroute the weep hole by running a small hose from the hole to into the intake tubing upstream of the turbo or onto the ground or a catchcan.
The EGR on the newest 2009+ TDI is tied into the emission system to a greater level than earlier cars because of the DPF system. Give a hoot, don't pollute.
How do people modify the EGR and why?(top)
People remove the EGR system for two main reasons. Removing intake restrictions will reduce pumping losses and create more available power. Removing the EGR will also greatly reduce the amount of buildup that you will see on the intake manifold. It's important to not seal off the crankcase ventilation hoses because the crankcase builds pressure that must be vented! Some people vent the hoses into a catchcan. Do not vent the hose onto the ground because you'll have an oil slick under the car and on the road. This is like dumping used oil onto the road - this contaminates the ground/groundwater.
There are a few ways to modify the operation of the EGR system. The first is to use the adaptation function of a Ross tech VCDS cable to adjust the degree of EGR system operation. The computer may react to the change in EGR flow and adjust timing. People report a mileage loss with this method of adaptation. It is possible to shut it off completely with a VCDS but this will set a check engine light. It only works on earlier mk3 and mk4 cars before 2004.
The other way is to physically remove the EGR valve and/or entire system from the car and replace one end with a block off plate and the other end with a straight pipe (race pipe). The problem is that it will set a check engine light and it removes the anti shudder valve. The anti shudder valve is the first thing that might stop a diesel engine runaway (a type of malfunction) and could save your engine from being damaged. See 1000q: diesel engine runaway FAQ for more details on an engine runaway.
Pumpe duse engines (2004-2008 TDI in North America) use an electronic throttle plate called an intake manifold flap. This can cause running problems and possibly cut off air flow to the engine if it's not running correctly. To remove it you would have to use an aftermarket chip for the engine to run properly and disable the check engine light, contact chip tuners for more details.
Why I do not recommend removing the EGR system(top)
First, it is an emission device. It cost the manufacturer money to design and put on your car for the purposes of meeting emissions standards. This article is just the facts with the exception of this single section of opinion: I believe in personal rights more than the strict letter of the law and think a knowledgeable and consenting adult should be allowed to do anything they want to their property. However, emissions laws are in place because modifying things like car emissions infringes upon the rights of other people. Your rights end where your neighbor's rights begin and we all have to breathe the same air. In countries that have poor emissions controls and poor air quality, emissions are a major factor in disease and shortening the lives of people who are regularly exposed to vehicle exhaust, like police and road crews. Since most TDI are daily drivers, making changes to emissions devices has a much greater effect vs. special use off road or race cars. Here are some more facts to consider.
If you are thinking of removing the EGR cooler only so that the intake charge is hotter and in theory, less prone to carbon buildup, this defeats a large portion of EGR effectiveness since emissions and temperatures will be higher without the cooler. The EGR cooler also helps warm up the engine by giving heat from the exhaust to the engine. This results in a faster warm up, less engine wear, and better fuel economy due to faster engine warm up.
Removal of the EGR will turn on the check engine light which will results in automatic failure during emissions testing and can prevent you from registering the vehicle in many states. Disabling the light or taping over it won't work since they plug in an obd2 scanner which reads your car's computer. There are ways to trick the computer into not showing a check engine light like through a chip tune but tampering with emissions devices is illegal in most places. The newest DPF equipped cars require new computer programming for the ECU to bypass or delete the DPF filter.
The biggest gain is reduced cleaning of the intake manifold. However, ultra low sulfur fuel used in all US and Canadian diesel fuel has greatly reduced major clogging. Biodiesel users report no excess intake buildup although there'll always be a small film. There are only small mileage or power gains from disabling the EGR system since it is greatest at partial throttle and idle. If you are removing the EGR for reduced pumping losses and the small power or economy gains, bigger and cleaner gains can be made elsewhere. See 1000q: basic performance upgrades for a list of starting performance modifications. Also, the pumpe duse cars use an O2 sensor to regulate the EGR system much more efficiently than non pumpe duse cars. Even newer technologies on the common rail system cars allow more precise fuel delivery and burn, resulting in less intake clogging.
I have seen comments that suggest the ingestion of the sooty EGR exhaust gasses combined with diesel piston blowby can cause oil contamination. This is false since the soot gets burned in the combustion chamber. Diesel engine oil is also formulated to hold soot and combustion byproducts and the engine will have sooty oil whether it has an EGR system or not. Engine oil quality and engine maintenance is a much greater factor in proper lubrication than the presence of an EGR system.
One final thing to consider when modifying the EGR system: if you modify or delete the EGR system, you will almost certainly have problems getting warranty coverage on the engine if the dealer notices it. Even though it should not damage the engine, VW will probably claim that you caused the damage. If you wanted to pursue it, you would have to hire attorneys to take any further action and it's not economical to do so. It would be hard to deny a claim on the suspension if you modified the engine, but be prepared to encounter resistance if you make any claim on a modified engine. If your car is under warranty, I do not suggest making an EGR change unless you are prepared to change it back when you go to the dealer. Modifications like an ecu chip or larger nozzles are not easily detected and most dealership mechanics would never find them but a missing or blocked EGR can easily seen.
The future of the EGR system(top)
The EGR system is an emissions device and because it makes such a major difference in emissions, it's unlikely that it'll go away in diesel cars. While some gasoline cars have eliminated the EGR, they can also run tighter air/fuel ratios, something that would never be possible with a diesel engine. This is partly because a diesel isn't throttled the same way a gasoline engine is and because they run very lean/efficient (much more air than fuel). The clogging problem that older cars have doesn't seem occur on newer cars as much for a number of reasons: ultra low sulfur diesel fuel, more precise fuel delivery and burn, better oil separators in the CCV, and new EGR systems all reduce the likelihood of clogging. Some diesel trucks use a mini catalyst before the EGR to catch and burn up any soot, preventing most of it from entering the intake. Future gasoline engines are also raising their compression and approaching diesel-like efficiency levels to be like half diesel half gas engines.
Some people criticized catalytic converters for killing the 1970's muscle car but the truth is that poor engineering and high fuel prices killed the muscle car. And today, cars are far, far more powerful, efficient, and cleaner then ever.