1991 Mercedes Benz 300sl



April, 22, 2009 AT 2:34 PM

Engine Performance problem
1991 Mercedes Benz 300sl 6 cyl Two Wheel Drive Automatic 106000 miles


since I saw a number of related questions in the same regard: my car (inline 6, perfect mechanical condition engine) hesitates to accelerate after the first warm up phase. Take off on intersections is difficult. The operation is too lean. Check: when enriching the A/F by pressing the throttle, the power comes back (revs up to 7200 rpm). Driveability is horroble. When cold (first 90 sec.) Power is good, enough fuel coming.
We (mechanic and I) found out that O2 sensor plug is hanging off without finding the counterpart connection to the ECU. Where is the cable to be found? (All wiring for grear and ECU go to passenger side into sealed engine compartment. No routing along frame. Can cable have been cut by preowner when installing aftermarket radio?
All other regular stuff (fuel filter etc. Changed). One or both fuel pumps are singing (pressure not measured).
Problem worsened over time, after winter storage radically bad.

Read also about over drive: does not work. Shifts too quickly from 2nd into 4th. Downshift is like waiting for tax refund from government. Any suggestions please? Thanks

2 Answers



April, 23, 2009 AT 7:00 PM

1991 Mercedes Benz 300sl Lean engine operation after warm u

A lean fuel condition can be caused by: * Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (Use a fuel pressure gauge to check fuel pressure at idle)

* Dirty fuel injectors. (Try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR valve)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor



February, 18, 2011 AT 7:21 PM

Aloha, sounds like you have covered everything. Let me ask you; when you replaced the fuel filter, is the replacement filter for a tandem fuel pump configuration? Regards.

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