slightly "rough" idle on C280
 

I have a 1999 C280 (V6) which idles slightly rough. Its not a constant rough idle. It will idle smoothly for say about 5 sec and then it will shake slightly- idle smooth, then shake. The rev counter only moves slightly, maybe 100 RPM, if than much - but it definately does have movement.

I already replaced the MAF sensor, due to stalling problems. The Benz technicians also say the car does not give any fault codes. They suggest either new plugs, or engine mounts. I don't really have confidence in their abilities and dont want to replace something if it wont cure the problem (already done to much of that)

Any suggestions??

I do have some question for the guru's:

1) Can it be motor mounts if the rev needle moves slightly during the "rough" idling?
2) If the problem is spark plug related, shouldn't it give an error code?

Something else: it does not make a difference whether the transmission is in P,N or D.

Thanks for helping

How many miles on the car? Are the original spark plugs still in it?
Dan

It has about 150 000 km and the plugs were changed at 90 000 km.

Does your model have coil packs? My 1997 C280 (I6) has a coil pack for each two cylinders and I have had to replace two already, the second just last week. (87,000mi.) When mine went out there was occasional backfiring and sluggish acceleration along with the check engine light going on. Simple to replace and with discounts cost approx. $90.00 each at the dealer.

Tim

Well I don't know the first thing about engines, but I supose it will have coil packs.

As I said, the idling is not really rough - will rather call it uneven. And I dont get any CEL lights or error codes.

Anyone care to comment on my two questions in my first post. This problem is really irritating me. Especially because the Benz mechanics don't know for sure what the problem is, and want to solve it by throwing parts at it.

Will really appreciate it if someone can comment on the questions in my original post.

I've since realised that the roughness is not only on idle. It also misses if I increase the revs slightly to 2000. Aslo, when I really push the car, it has a hick-up / misses (only for a instant) at about 5000 RPMs - mostly only in 3rd gear.

nobody?

Sounds like the AC engaging on and off with the auto climate control. Did you try setting it on economy mode? Mine also did this when the water pump started to go bad (seizing bearings), but this was accompanied by a squealing belt. I have a 98C280, same V6 engine.
MarkC

Thanks for the advise Mark. I checked, the roughness remains whether the A/C is on or off. Also no squealing or grinding noises.

Any other suggestions?