1996 E280T (124) lumpy idle

[Lawrence]

I have a 1996 E280T (series 124) with the HFM electronic engine management. I have just had the cylinder head gasket changed after overheating following failure of the water pump. Ever since then the engine has a lumpy idle which sounds as if there is a valve problem. There is also a constant clattering from the evaporator cannister purge valve. As soon as the engine revs go above 800 rpm it disappears and the engine runs as smoothly as it ever it has done. In D the engine revs are constant at about 600-650 rpm but in P and N, it surges to 1000 rpm and then slowly drops back to around 700 when the iregular (lumpy) idle starts again. The petrol consumption has also increased. Any ideas anyone??

 

[mjtray]

Both the E280 AND E320 with the M104 engine can suffer from a surging throttle.

Had problems with my E280 which was cured by;

1) Cleaning the throttle body.....it gets a build up of carbon inside it which stops the butterfly valve from closing properly.
2) OVP relay behind the battery may be packing up. The originals had a design flaw and they break down in the heat. Cost about £45 and are eay to replace......this part will fail at some stage!!!!
3) Regen valve that sits on the right hand iner wing near the ABS gubbins. These also tend to pack up. Cost about £50 and five minutes to replace.
4) Carbon filter (forget the correct name for it) but sits in the right hand wheel well behind the wheel arch cover. These need replacing every 80,000 miles (I think) and can cause irratic idle and increased fuel consumption.
5) Lastly, the O2 sensor needs replacing every 80,000miles or so and can cause similar problems to those mentioned above.

My only concern with the above recommendations is that you have had the head off on your car and it could be something else related to that.......I would get them to check for a vacuum leak somewhere in the intake system, as this will also cause the throttle to surge and this will have been touched during the head gasket replacement.

Good luck.