The exhaust gas recirculation (EGR) system is used to reduce the amount of NOx created by the engine. It does this by diluting the air/fuel mixture with a small amount of inert gas (usually less than 10% of the total mixture); exhaust gas is used since it contains only a small amount of oxygen (and is readily available). Adding it has the effect of lowering the combustion temperature below the point at which nitrogen combines with oxygen to form NOx. When working properly, the EGR does not open when the engine is cold, at idle or WOT. A defective EGR can have a severe impact on the quality of idle.
The service manual has a nice EGR troubleshooting flow chart. This is close to the most complicated EGR you'll run across; it has a computer controlled vacuum solenoid and a position sensor. (It's perhaps the most expensive too; $200 list, $130 at the AC/Delco distributor.)
In my case, the unit passed the service manual tests but I later guessed it was leaking some exhaust at idle so I decided to shotgun it. I haven't seen a good test published but when I removed mine to install the new one, I found that the MityVac could pull 5-10" if I pumped a bunch. It would drop to zero in less than a second. However, pumping against the old unit would never deflect the vacuum gauge. (This was about the only time that shotguning paid off for me...) You're supposed to be able to clean the carbon out but removing and installing the EGR is hard (it's very difficult to remove the manifold tube bolts and to torque them back in) so I'm glad I just bought a new one.
If you remove the EGR and replace it, use new gaskets. These can be difficult to find. If you buy a new EGR, check the box to make sure the gaskets are there. When at the distributor, I saw at least one box that was missing them.
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