Diesel EGR Service

For independent automotive repair shops, diesel exhaust gas recirculation can be a boost to the bottom line.

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Diesel EGR Service

Exhaust gas recirculation (EGR) is an emission control system that we love to hate. On the one hand, EGR tends to reduce both power and fuel economy. EGR systems have also become more complex over time, making them more challenging to diagnose and service. However, EGR has made it possible for diesel engines to meet strict government NOx emissions regulations, leading to an improvement in air quality in our major centers.

For independent automotive repair shops, diesel EGR has been a boost to the bottom line. Enough goes wrong with these systems that EGR has turned into a good revenue source for shops that offer diesel repair services. Getting some training in diesel EGR service basics can pay off quite nicely for both the technician and the shop owner.

A Little History

Exhaust gas recirculation systems have been in use in gasoline engines since the mid-1970s. Automobiles were identified as a significant source of oxides of nitrogen (NOx) emissions, which were bad enough by themselves but were also an ingredient in the formation of ground-level ozone and smog. EGR was an effective method of preventing NOx from forming in the engine cylinder, but tended to decrease engine efficiency. It was a common belief in pre-OBD days that maximizing power and fuel economy were far more important than what came out of a vehicle's tailpipe. Thus, it was common practice to disable a vehicle's EGR system despite Environmental Protection Agency (EPA) warnings against tampering.

Most diesel engines in the U.S. didn't use EGR in these early years, despite the fact that they produced a great deal of NOx. The diesel emissions regulations of the time were such that the required NOx reductions could be achieved using methods such as charge air cooling (CAC) and injection timing adjustments. The bar was gradually being raised, however, and in the 1990s all eyes were on the EPA 2004 regulations that required a major decrease in diesel NOx output. It was quite likely that EGR was going to have to be utilized to meet EPA 2004, but engine manufacturers were planning on crossing that bridge when they came to it.

The situation changed in 1997 when the EPA discovered that most of the major diesel engine manufacturers were "dual mapping" their on-road engines. This meant that the engines were calibrated to meet EPA emissions standards when run on the 20-minute certification test, but would revert to a different calibration when operated under highway driving conditions. According to the EPA, some manufacturers had been engaging in this practice since 1990, resulting in millions of tons of illegal pollution being produced. The charges led to a settlement that cost the engine manufacturers hundreds of millions of dollars and, more significantly, moved the timeline to meet the EPA 2004 regulations to October 2002. The diesel engine manufacturers were now under the gun, and many adopted cooled EGR as the technology to use for meeting the accelerated standards.

The EPA regulations have only gotten tighter since that time, and since 2007 all new on-road diesels use cooled EGR as part of their emissions control strategy.

How EGR Works

A diesel engine typically uses a stratified mixture for its combustion process. Fuel is injected into superheated air during the compression stroke and creates a broad range of air-fuel mixtures in the combustion chamber. Areas at the interior of the fuel spray will have rich mixtures, which take more time to mix with air and ignite. These rich areas also tend to burn cooler and produce more particulate matter (PM) emissions.

The outside edges of the fuel spray are a different matter. These areas tend to be lean (up to 100:1) and the fuel vaporizes and ignites very quickly, creating a hot flame front. The extreme heat generated in these areas is what causes normally-inert nitrogen to react with oxygen and create oxides of nitrogen (NOx) emissions.

The basic idea behind EGR is to meter exhaust gases into the engine intake air stream. The recirculated exhaust gases have a high carbon dioxide and water content, which tends to limit NOx formation in the cylinder by:

1. Absorbing heat from the combustion process, which lowers peak flame temperatures.

2. Displacing oxygen from the intake air charge.

Diesel engines equipped with an EGR system also utilize an EGR cooler. The EGR cooler is a liquid-cooled heat exchanger that removes heat from the EGR gases and transfers it to the engine cooling system. When EGR gases are cooled, their density and heat absorption capacity is increased. Cooled EGR gases are thus able to absorb even more heat, further limiting NOx formation.

Sidebar 1 Bypassing the EGR Cooler

There are some modes of diesel engine operation where it is undesirable to send exhaust gases through the EGR cooler. One example is cold starts, where hot EGR gases can be used to help warm up the combustion chamber as well as the diesel oxidation catalyst (DOC) and diesel particulate filter (DPF).

Another time that the EGR cooler might be bypassed is during DPF regeneration, when high exhaust gas temperatures are required. If post-injection is used to initiate an active regeneration of the DPF, it also may be necessary (as in the Dodge/Cummins 6.7 liter) to bypass the EGR cooler to prevent fuel from contaminating the cooler element.

Most EGR coolers have an integrated bypass valve. This valve is used to allow EGR gases to bypass the cooler element and proceed directly to the engine intake air stream. The bypass is generally a two-position valve (either on or off) that is operated using either an electromechanical or vacuum actuator.

Diesel EGR System Design

Gasoline engines use anywhere from 5% to 15% EGR gases in the intake air charge. Because the EGR volumes are relatively low, newer gasoline engines are able to use an EGR system which does away with the valve and associated piping. Instead, internal EGR uses variable valve timing to retain exhaust gases in the combustion chamber during valve overlap.

Diesel engines use much higher volumes of EGR, and can use it across the operating range. In some diesels, up to 70% EGR can be used, and the highest flows are often seen at idle and part load. Since diesel EGR volumes are much higher, a conventional external system must be utilized.

The most common EGR configuration in diesel engines is known as high-pressure loop (HPL). HPL EGR taps exhaust gases from the exhaust manifold (before they enter the turbocharger) and then sends them through the cooler and EGR valve on their way to the intake manifold. All current domestic heavy-duty pickup diesels use cooled HPL EGR systems.

The Achilles heel of HPL EGR is intake manifold boost pressure, which is often high enough to limit EGR flow. Two strategies are employed to increase EGR flow during high-boost conditions. First, a variable geometry turbocharger (VGT) is used to increase exhaust backpressure, and second, an air throttle valve is used to lower intake manifold pressure. Thus, HPL EGR flow can be regulated through the operation of the EGR valve, the nozzle in the VGT, and the air intake throttle.

Despite these measures, some light-duty diesels need more EGR than what can be provided with HPL systems. For increased EGR flow at high engine loads, some diesels (such as the VW 2.0 liter common-rail TDI) also use a low-pressure EGR system. Low-pressure loop (LPL) EGR takes exhaust gases from the outlet of the diesel particulate filter and sends them into the turbocharger air inlet. LPL EGR may also incorporate a cooler to further decrease NOx formation.

LPL EGR has numerous advantages, including the relatively cool temperatures of the exhaust gases after they leave the DPF, and low pressures at the inlet of the turbocharger. Volkswagen goes one step further and uses a throttle valve downstream from the DPF to increase exhaust backpressure, which serves to increase flow in the LPL EGR system. While attempts have been made to use LPL EGR alone, it appears to be most effective to use a combination of HPL and LPL systems to achieve necessary EGR flow across the engine operating range.

Diesel EGR Service

Obviously, your customer is not likely to tell you that their diesel engine's EGR system isn't working. Instead, common diesel drivability complaints include white smoke, black smoke, exhaust odor, surges or lacks power. Verify the concern, and then do a visual inspection of the vehicle to look for obvious problems. Technical Service Bulletins (TSBs) can play a critical role in diesel drivability diagnosis, so retrieve any DTCs early in the game and do a thorough TSB search. Narrow your search to powertrain management/emission control TSBs, and then look over the results for symptoms and/or DTCs that match your problem vehicle.

When diagnosing a diesel drivability concern, you should consider what role the EGR system might be playing in it. Use a scan tool with sufficient capability to perform bi-directional testing of the EGR and air management systems. A good example is diagnosis of Ford Powerstroke diesels, where the technician's best choice is the Integrated Diagnostic System (IDS) scan tool. The IDS can perform an EGR Systems Test and an Air Management Test that will quickly confirm proper operation of the various EGR components as well as the turbocharger, etc.

Keep in mind that EGR malfunctions are often just a symptom of another problem. For instance, a common failure is for the EGR valve to hang up due to coking. While the valve will likely need to be replaced instead of cleaned, you still need to ask what made it coke up in the first place. This could be caused by a number of issues, including:

1. Poor quality fuel

2. Excessive fuel

3. Unburned fuel

4. Excessive oil consumption

If poor quality fuel is suspected, try an OEM-approved fuel additive to see if it helps. If the additive goes into the fuel tank, it may take 20 miles or more of driving to see a difference. Some technicians recommend pouring the additive directly into the secondary fuel filter for more immediate results.

If the engine is consuming coolant, you should suspect a leaking EGR cooler. There are often factory service tools available that are made to pressure-test EGR cooler elements to confirm your diagnosis. Factory service tools also come in very handy when removing EGR valves. While you can occasionally get away with using pry bars and screwdrivers, there are some EGR valve designs that will sustain damage if the recommended tool is not used. Be sure to use a stiff brush and a vacuum to clean out the EGR passages before reinstalling the new valve.

Eventually, a TSB may lead you to the point where you need to reflash a module (or two). A tool like the IDS works perfectly for this, but you may also use a J2534 pass-through device to get the job done. Make absolutely sure you have a ripple-free battery charger connected to the vehicle, because if the battery goes dead during the reflash process you will likely ruin the module. Also, keep in mind that changing the calibration on one module may require a reflash on another as well.

A New Frontier

EGR systems are being used in virtually all new diesel engines. While the systems are becoming more sophisticated, an organized approach can help you diagnose and repair any EGR problem that comes your way. If you get the appropriate training and the tools, diesel EGR service could be a great source of revenue for your shop.