Originally posted by Meza
About the EGR, I don't know how it could effect the low vacuum condition I have or drivebility! All I know is that it lowers NOx emissions buy redirecting some of exhaust gases back into the cylinders. The idea here is: If you lower the cylinder gases temperature, you will lower your NOx. The exhaust redirected by the EGR has H2O as a product of the combustion process and that what lowers the temperature in the cylinders. Anyways, How can I test it?
I will do some tests by this weekend and post results.
The EGR valve should only open at part throttle cruise, which is the condition for maximum NOx production. It should not be open at idle or WOT. If it's stuck open it causes what is essentially a vacuum leak at idle, which leans the mixture and causes combustion instablility.
I don't have EGR on my '88 2.6, so I can't speak specifically to the MBZ EGR system, but the first thing to do is a manual test of valve operation. Apply vacuum to the valve diaphragm to verify its integrity and disconnect the exhaust feed line and test for flow with the engine off. EGR valves are normally closed with no applied vacuum to the diaphragm. If necessary, remove the valve for a visual inspection and bench test. EGR valves are fairly common failure items on older higher mileage cars - they live in a tough environment. Also, there will be a vacuum switching valve that applies vacuum to the EGR valve diaphragm. You should be able to find it by tracing the vacuum line. I suspect that the vacuum switching valve is commanded by the ECU, but not sure.
Another cause of low vacuum could be a clogged exhaust system due to internal catalytic converter damage or a collapsed muffler baffle, however, I would verify proper operation of the EGR system first.