Originally posted by Bob Blaylock:
So, what wisdom can any of you offer me as far as the EGR system istelf? I'm rather hazy on just what it does and how it does it and why. I think I understand that, in certain circumstances, it causes some exhaust gas to be fed back into the intake, and that this somehow causes the engine to run cooler, and to thus produce lower amounts of nitrogen oxides. I understand that cooler means less nitrogen oxides, but how does feeding exhaust gas, which must be very hot, back into the engine, make it run cooler? And under what circumstances is this being done?
What can you tell me about the actual repair work I have ahead of me? How difficult is it to get to, and remove the EGR vave? Should I just plan on buying a new one before I start, or should I expect there to be a very good chance of my being able to repair the existing one? What's a new EGR valve likely to cost?
You are correct in the purpose of the EGR valve. The way it reduces combustion temperatures is by mixing in exhaust in the intake stream, the exhaust is mostly inert and contains little oxygen. Thus, it has a tendency to richen the combustion mixture, thereby reducing combustion temperatures, and thus reduce nitrogen oxide emissions. The computer's timing map is programmed to consider the EGR effects on combustion temps. When an EGR valve sticks closed or the ports are blocked, the combustion chamber temps increase, resulting in detonation or pinging. My V-6 had clogged EGR ports resulting in no EGR flow at all, and it would ping even on 93 octane. I took the intake apart and cleared the ports, and it solved the problem.
I'm not familiar with the Zetec, but the EGR valve itself was about $40, and should be roughly the same for your motor. At your mileaege, I would just install a new one anyway - they don't last forever and tend to get gunked up with carbon, and the vaccuum diaphrams tend to crack and leak due to the heat. It's a really simple device and is easy to replace. It basically mounts to the intake manifold with two bolts, and has about a 3/4" exhaust tube connected to it, and a small vacuum line up top. That's it. Assuming its easy to get at on the Zetec, it would probably take you 15 minutes to replace.
Again, I'm not familiar with the Zetec, but you may want to check the EGR ports behind the throttle body. If its anything like the Duratec's arrangement, expect to find ports clogged with carbon. These must be clear or the EGR won't work right.
