Mercedes C180 2L : revving issue on startup

dubliner24

Hi Guys

I'm trying to troubleshoot an issue on my fathers car, MB C180, 2L, 2001.

On start up it revs high (~2000 or more rpm) for a second or two then drops to near cutting out (~200rpm) for a second or two then high again and low again etc....

This keeps happening until the car is driven or else until the revs are held high with the accelerator for ~10seconds while stationary. On releasing the accelerator revs hold at idle as normal thereafter.

Anyone have any ideas?

Thanks for your help

dubliner24

Hi Guys

I'm trying to troubleshoot an issue on my fathers car, MB C180, 2L, 2001.

I've already put this in the DIY maintenance section but didn't get any replies so I'm hoping someone here might have some suggestions.

On start up it revs high (~2000 or more rpm) for a second or two then drops to near cutting out (~200rpm) for a second or two then high again and low again etc....

This keeps happening until the car is driven or else until the revs are held high with the accelerator for ~10seconds while stationary. On releasing the accelerator revs hold at idle as normal thereafter.

Anyone have any ideas?

Thanks for your help

VeVeX

We've seen these symptoms (and many others) caused by oil in the wiring harness on that model.

dubliner24

Oil in the wiring harness... that's an interesting one.

In that case did the problem persist as long as the engine was running?

In my case, the revving only happens during startup. The engine doesnt start up and go to idle like it should (and used to!) but once the car is driven at all its fine.

Thanks for your help VeVex, willing to try anything at this stage!

Bigserious1

Any engine light on? Had erratic rpm on an E200 of mine that was caused by a split pipe that caused the problem.

dubliner24

No engine light...

I was thinking it could be a leak in a vacuum hose? Finding a leak there would be a real headache?

dubliner24

Hi guys,

Just an update on work done to tackle this issue over the weekend.

I replaced the air filter, cleaned the MAF sensor and cleaned the throttle body. Reassembled everything and so far so good! that horrible idle surge hasn't happened since! Really delighted with the result. That said, it was always an intermittent problem, but it's looking good.

During the work I had the engine covers off and the air intake housing off. Discovered a few leaks.

As can be seen in the attached picture, I found a coolant leak at the coolant temperature sensor. I removed it, cleaned it up and reinstalled. This explains why the coolant level in the reservoir had dropped, so glad to have that addressed.

The other sensor on the front of the engine however, can anyone identify it for me? I think it must be a cam position sensor? There appears to be an oil leak from the bottom of this housing. I initially thought the oil leak, as can be seen in the picture, was coming from the valve cover itself and was preparing to redo the gasket, but on closer inspection it appears to be the sensor. Is this something that should be removed and the gasket redone?

Finally, I recorded a quick 30 second video of the engine running. https://www.youtube.com/watch?v=advQJBURBkg

For the first 10 seconds the engine is idling. A noise can be heard - almost sounds like the cams hitting off something inside the valve cover.
At around the 10 second mark however I got my helper to increase the revs slightly above idle and that noise completely disappears.
On settling back down to idle again the noise comes back. Can anyone shed any light on what that might be?

Thanks again for all your help to date.