C220 94, idle surge when warm.

I have 1994 C220, with 137K miles. Car runs and idle's good when cold, but as the temperature reaches 80 deg. c on the temp gage or at the steady temp, the engine idle speed surges up and down and one can smell the unburned fuel all over. The process of rough idling continues till the engine dies.

I have changed the plugs, air filter and cleaned plug wires. Also I did change the Over voltage protection relay last year, which solved my earlier prob. of not starting randomly.
Please help me on this.
Thank you and eagerly awaiting reply.

[blackmercedes]

Sounds like a "mixture" or fuel trim problem. That's a typical symptom for a Mass Airflow Sensor (not sure if 94 HFM systems use MAS or Air Mass Meter, but they're not that different in their purpose) gone bad. It could be an O2 sensor as well. My understanding is that under cold start, the system is not "actively" working using the MAS and O2 sensors. They come into play when the engine is not under cold start conditions.

Are you getting a check engine light?

Thanks for a fast reply.

The check engine light is no lit.
Some one local told me to spray carburetor cleaner in the air in take after the sensors while the engine running to clean up any gunk in the idle actuator. What do you think about that? Also I noticed a black layer of oil and dust on the butterfly at the intake which I cleaned with a rag.

check the fuel pressure regulator, pull the vacuum hose while the engine is running and see if any fuel is leaknig from the regulators vacuum fitting, you may also try tapping on it lightly and look for fuel leakage, it may also leak with the engin off.
Ray
mercedessd@cox.net