Ok, here is the deal, you're cruisin' down the road at 70 Mph, you got the T-Tops out and the stereo blaring. Life is grand. Then out of nowhere comes that annoying bright orange "Service Engine Soon" light. You are shocked. You pull over to the nearest gas station to do a quick inspection of under the hood and find nothing. You take off again and the light is off. What's going on? You go home and check for codes and what pops up but the notorious code 32. The purpose of this article is to teach you the theory, operation and diagnostics of EGR in hopes that you will not be needlessly swapping parts. I will try to not make this a boring science class or a useless lecture but an informative tool.

EGR Theory: EGR serves one purpose and one purpose only. That purpose is to reduce Oxides of Nitrogen (NOx). Under normal combustion, Nitrogen(N2) Oxygen (O2) in the air and Hydrocarbons (HC) in the fuel combine into water (H2O) Carbon dioxide (CO2) and the Nitrogen remains unchanged. Under very hot combustion temperatures, the Nitrogen reacts with the other two byproducts and forms Nitrogen oxide (NO). After being released into the atmosphere, it picks up another Oxygen and becomes Nitrogen dioxide (NO2). In the presence of sunlight, it combines with other compounds like Hydrocarbons and forms Smog. Since exhaust gas is inert (very stable) it doesn't burn again. So by being introduced into the combustion chamber, it will lower combustion chamber temps enough so that the Nitrogen doesn't react with the other compounds and is passed unchanged out the tailpipe thus not contributing to smog. Now, since exhaust gas doesn't burn, it doesn't exactly help with combustion. At higher RPMs, this really isn't noticeable, but at idle, the reintroduction of exhaust gas will cause a very rough idle and can cause stalling if to much is introduced into the combustion chamber.

EGR Operation: Like mentioned before, EGR flow is good at higher RPMs, but not good at idle, so some sort of control needs to be place on the EGR system. Earlier systems used ported vacuum straight to the EGR valve. At idle, the throttle blades are closed, so there isn't any ported vacuum. When the throttle is opened, ported vacuum starts to build. The more throttle, the more ported vacuum. This is how vacuum advance distributors work. As throttle is increased, the EGR valve opened further. Once the throttle is closed, ported vacuum is lost and the valve closed. Most Third Gen. F-bodies use the basic diaphragm EGR valve, but instead of relying on ported vacuum, it relies on vacuum that is allowed to pass through a solenoid. The solenoid is controlled by the ECM. When conditions are right (engine temp, TPS position, RPM, etc...) the ECM will ground out the solenoid. The solenoid is a Pulse Width Modulated (PWM) meaning that the amount of vacuum is controlled by the computer rapidly switching on and off to ground. The more vacuum the ECM wants to go to the EGR valve, the more time it lets the solenoid remains grounded, measured in Duty Cycle. With a scan tool, you can command the EGR solenoid to say 50% Duty Cycle and actually feel the pulses if you put your finder over the vacuum port of the solenoid.

EGR Diagnostics: Now that you understand what EGR does and how it does it, lets see why sometimes it doesn't do it. Since exhaust gas if being introduced into the intake manifold, things like manifold vacuum and airflow reduce slightly. Speed density computers can recognize EGR flow by looking for a drop in vacuum via the MAP sensor. MAF systems can see a reduction in airflow since exhaust gas is displacing fresh air. Some systems have a temp sensor. Since exhaust gas is much hotter than fresh air, it can pick up EGR flow by sudden increases in temp when the valve is open. Highway speeds are when the most EGR flow is commanded. When cruising on the highway, the computer will pulse the EGR solenoid so that vacuum will pass and go to the EGR valve. After commanding the solenoid on, it will look for signs that the EGR valve opened by one of the means mentioned above. If it does not see the change in MAP, MAF, or increase in temp, it knows that exhaust gas is not flowing even though the ECM is commanding it. This is when the light comes on. What are some possible causes of no exhaust flow? Well, first, make sure that the solenoid is getting vacuum. A plugged or broken vacuum line will make everything else inoperative. If vacuum isn't getting to the solenoid, it surely will not get to the EGR valve. Once this is established, make sure the ECM has control over the solenoid. Visually make sure that the connector is firmly seated into the solenoid and that the terminals inside the connector are not corroded or damaged. Usually you can rev the engine while it is in closed loop and it will be enough for the ECM to command EGR so you will be able to feel vacuum at the solenoid. If you feel vacuum, than the ECM has control over the solenoid and adequate vacuum is reaching it. If you do not feel vacuum, you may need to drive the vehicle with a vacuum gauge hooked up to the solenoid. If you are driving in closed loop, you should see some vacuum. If you don't the solenoid is probably bad, or the ECM is not controlling it (bad ECM grounds or ECM) providing you had vacuum going to the solenoid. If all is OK so far, inspect the vacuum line going to the EGR valve for plugging or being broken. It is not uncommon for previous owners to plug these vacuum lines with sticks, BBs, screws, or anything else to try and cure a "rough idle". The lines can also become plugged with carbon deposits over time. If it is clear and free from defects, check out the valve itself. Make sure it isn't seized by manually lifting up on the diaphragm. If it moves freely, put a vacuum on it. It should move and stay there (hold a vacuum). If it doesn't move, try lifting a little on the diaphragm (it may be a positive backpressure valve, lifting on it some will act as the backpressure). If it still doesn't move, or wont hold vacuum, the valve is bad. If everything still checks out OK, then the only other thing that will limit exhaust gas flow is plugged up passages in the intake manifold and cylinder head. This is a common problem with the V6's and will leave many technicians scratching their head because the passages can't be seen. If all else checks out, get a rifle cleaning brush and a shop vac and start cleaning. You will be surprised the amount of carbon chunks that will come out of there.

Other notes: Most people like to disable the EGR because they claim that hurts performance. In actuality, disabling the EGR can hurt performance. Here is why. As we already know, at certain throttle positions and RPMs, the ECM will command EGR operation. This is to cool combustion chamber temps under load. Well, with cooler combustion chamber temps, we can further fuel economy by advancing the timing. We know that to much timing will cause "pinging". But when we keep the combustion temps down, the timing can be advanced without the "pinging" effect. At highway speeds, the ECM commands EGR operation and will advance timing accordingly. With a blocked of EGR, the computer thinks it is flowing when it is not and will advance timing. Now that the combustion chamber temps are much hotter, the advanced timing is no longer a good idea and detonation occurs. Since detonation can severely damage an engine, knock sensors are used. When the knock sensor detects detonation, it will retard timing. It takes more to stop detonation that it does to cause it and this is where it hurts performance. For example, at highway speeds, your total advance may be, lets say 30° BTDC. If the computer advances it one more degree to 31° and it detects detonation, it can't just go back to 30° to stop it, it must retard timing to like 25° to try and stop it, and if it still occurs it will further retard timing. If the EGR was working properly, the temps would have stayed cool enough to operate at 31° with no problems.

It takes lots of practice and experience to become a quick and proficient troubleshooter when it comes to complex engine diagnostics, so I tried to break this subject down so that anybody would be able to get that darned light to stay off. Good luck!

This article was written by Bernard Tripp (aka GMTech on the ThirdGen.Org Message Board). I am an ASE Certified Master Technician with Advanced Level Engine Performance (L1). I work for a Buick-GMC dealer and a long time F-body Fan.