HELP! Engine revving up and down at tick-over

[alex yates]

Hi All,

My 2000 W208 clk 230k has acquired a fault. When fired up, instead of ticking over as normal, it's revving up to about 2000 rpm then dropping back down to tick-over, as soon as it drops down it revs up again to 2k. It's as though the accelerator is constantly being pressed.
The frequency of the revving is about ever 2 seconds. Since it's done this the CEL has now pinged on.
It's happened about a year ago and I just disconnected the battery for a few hours to reset the ecu and the problem went away. Tried that this time and no joy.
Anybody any ideas?

 

[PeterC280]

I had a very similar cold start fault on a Vauxhall Senator years ago that was diagnosed as a faulty idle air valve that was replaced by the dealer under warranty. There was still some "hunting" afterwards albeit much less and I just remember realising one day shortly afterwards that it was working properly. I thought most likely some muck in the idle system.

PeterC280

 

[alex yates]

Hi,
Just an update. Took it to my mechanic yesterday, plugged in the code reader (of which there were loads of fault codes). Cleared them all so we could start a fresh and see what fault code came up, and resetting the OBD has cured the fault. Not impressed as I still don't know what was wrong.

 

[markyuk]

Hi,
Just an update. Took it to my mechanic yesterday, plugged in the code reader (of which there were loads of fault codes). Cleared them all so we could start a fresh and see what fault code came up, and resetting the OBD has cured the fault. Not impressed as I still don't know what was wrong.

Possible throttle position sensor or something similar? I once had a similar problem on another make and it ended up being that.