[PKRWUD]

The EGR system takes a small amount of inert gas from the exhaust and allows it to re-enter the intake manifold to be burned again. This is largely for emissions, but it also acts to cool the temp of the combustion chamber. It is not used at idle, or during WOT, or when the engine is cold. It is primarily used during cruising speeds, after the engine is at or above it's normal operating temp. The ECM calculates this being added to the mixture, and relies on it for maximum performance. I just sent an email to a close friend telling him about a recent example of what can happen with an EGR fault:

A couple weeks ago, I had a job on a '93 Lincoln Continental (3.8L FWD) that sounded like it was going to be a lot of work. It started as a reoccuring MIL, and occasional "white" smoke from the exhaust. I spent a half hour on it with my scan tool, and pulled a half dozen codes, including 2 O2 sensor Lean limit (rich) codes, an EGR flow code, a fuel pump circuit fault code, and a couple others that suggested the system was rich. Ford had quoted him over $1800 to fix it, and I suspected it was going to be a lot of work. I scheduled an entire day for him. Before the job, I did a little research. I knew that when the EGR passage gets blocked on those, and an EGR insufficient flow code comes up, the system naturally goes rich (this explained most of the codes), but I couldn't understand how the fuel pump was also cutting out. That seemed to me like it would create a lean condition. Well, I found that when the system get's really rich, the ECM automatically shuts off the fuel pump in pulses, trying to control the fuel flow. Even though the ECM does this, it still logs a trouble code for it, which makes it even more confusing. Well, to make a long story a little bit shorter, I got back to the vehicle, started it up, and attached my vacuum pump to the EGR, and applied 15" of vacuum. Suddenly, there was a loud POP, and the engine died. The EGR valve had gotten stuck shut. I removed it, advised the customer, and cleaned it and reinstalled it at the customers request. I reset the ECM, ran it through the relearn process, and ran codes on it again. It passed everything. No codes at all. A simple sticking EGR valve caused all those codes, and potential headaches. The "white" smoke was caused by a bad PCV valve, so my total time on the vehicle for repair work was less than an hour.

Take care,
-Chris