Stumbling when revving

Hi there

I am expreriencing a problem on my ‘72 450 SL (US spec). It has begun to run slightly rough, primarily in idle both when cold and warm. Also when I give the throttle a quick “jab” the engine seems to stumple and almost stalls. When I depress the throttle more slowly, the engine runs fine and revs without any problem.

I have verified dwell (32 degree), timing (5 degree ATDC) and fuel pressure (2 bar). I have also verified that all injectors are firing.

I have turned the the ECU knob all the way counter-clock wise and that has made the rough idle go away, but the stumbling when quick revving the engine is still there, so turning the ECU knob may only be a quasi treatment.

Any ideas?

Best, Peter

Hi Peter,

either there is an issue with ignition timing or more likely you have an issue with false air. Rheostat helps to correct in idle only. That is what does not let the engine rev up if mixture is basically wrong. Other options are wiring harness and injectors.

Hi Peter,
maybe the TPS enrichment-signals are missing.

Regards
Norbert

Hi Norbert,

not likely as it would not create such a bad mixture that car would not want to rev up. Remeber that we often recommend to drive without TPS connected to verify that stumbling does not come from TPS.

I have re-checked the fuel pressure and it is right on 2 bar. All injectors are clicking without misses.

When idling there is no problem, but as soon as I put it in gear (reverse or drive) it begins to stumble and almost dies. When in gear it practically has no power and barely accelerates.

When disconnecting the air temperature sensor the idle is practically the same, but now the engine acts normal when I put it in gear and accelerates.

Does it sound like I need to chase a vacuum leak/false air? 

/Peter

....I may be on to something. I just measured resistance on the engine temperature sensor. The area around the sensor was 53 degrees celcius and it measured 480 ohms. Even with a +/- 15% tolerance that appears to be too small a resistance. I measured both with/without connector plugged in and that made no difference.

I will measure the resistance again tomorrow when the engine is cold and see what resistance value I get there.

/Peter