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I had similar problems in my 1991 300E and it turned out to be the ECU. When it goes bad, it will default to a fixed AFR, it will not take into account any of the various inputs/sensors in the CIS system. The computer really never has control once it goes bad. The car is in open loop when it's cold, and the car is in open loop when it's hot.
The weird thing about the ECU's on these cars is that they almost always allow the car to be driven, whereas when the engine ECU fails on any other car, it's deader than a doornail.
Also on my car, the EHA was bad. It didn't leak externally though! When I put a volt-meter on it, it was pegging at full voltage when you opened the throttle. Replaced it with another EHA and now when the throttle is opened it doesn't peg.
I went from failing emissions by nearly 5000% (biggest failure the testing guy had seen), to passing with flying colors by replacing the ECU/EHA.
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