C180 PMS Varying Idle Speed

[Bernard Rother]

Hi All, Can anyone shed some light on the following symptoms ? A '94 C180 PMS. No catalytic converter to warm up, no mass air meter, new engine harness. Bosch diagnostics gives all clear on harness. Start vehicle, blip throttle regularly for about 2 minutes and idle speed drops back to 700rpm each time, normal. From a couple of minutes onwards, after blipping the throttle the idle speed settles at 1400rpm for 14 seconds. That will happen at each stop light etc. Fast idle at 1400rpm for 14 sec. before dropping to 700. Switch car off & restart, idle is back to 700rpm for +/- 2 minutes again, thereafter idling at 1400rpm for 14 sec. then down to 700rpm. NOW .... if I switch on the aircon while its at 1400rpm, the idle drops back to 700rpm ... aircon off = idle at 1400rpm for remainder of the two minute cycle.
Bernard.

 

[television]

I would have said the the idle switch inside the throttle air flow plate is faulty and the engine is not switching over to the idle circuit.

It is a known fact that these contact do wear, the cure is to file the stop faces so that the switch will close.

 

[Bernard Rother]

Hi Malcolm, we thought along those line as well. The MB mech. installed a new throttle body, same symptoms, swopped it out for another new one, no change. One mech said that due to the fact that it's a PMS system, the flywheel ( he described it as being different from a regular flywheel, forgot what he said. ) It looks as though it keeps its momentum for longer to assist gear changes when pulling off etc. He said it was ONLY on these particular models. I bought it 2nd hand and have been slowly bringing it back up to spec ...... and enjoying it ... apart from MB prices for electronic components, therefore I don't know how it behaved previously. Cheers for now.