Occasional Rough Idle

[MakaveliX]

2002 Mercedes C240 ( 32,000 miles )

Every now and again at lights when stopped in drive, foot on brake, I get a very minor rough idle - usually the revs sit at 500rpm and feels fine, but now and again it drops just below that and feels a bit rough.
If I put it into park, revs increase and it disappears. It only does this now and again.

Air filters have just been done. The fuel filter is over due ( I got the car off a family member who barely drove it )
Could it be that ?

I will be cleaning the MAF sensor when I get a chance and hopefully the spark plugs too.

Any ideas ?

Edited by MakaveliX on Friday 14th June 19:43

[GreenV8S]

Worth seeing whether the idle air control valve is accessible for cleaning too.

[hedges88]

MakaveliX said:

2002 Mercedes C240 ( 32,000 miles )

Every now and again at lights when stopped in drive, foot on brake, I get a very minor rough idle - usually the revs sit at 500rpm and feels fine, but now and again it drops just below that and feels a bit rough.
If I put it into park, revs increase and it disappears. It only does this now and again.

Air filters have just been done. The fuel filter is over due ( I got the car off a family member who barely drove it )
Could it be that ?

I will be cleaning the MAF sensor when I get a chance and hopefully the spark plugs too.

Any ideas ?

Edited by MakaveliX on Friday 14th June 19:43

Have you checked for any fault codes with a scanner or done any basic sensor readings? It's a good place to start to try and see what's worth investigating. My W169's coil pack leads had gone brittle and once the leads were replaced and the boots greased up I fixed it. Similar symptoms to yours but I had no codes or anything. Also my W169 had a rougher idle on 98RON fuel.

[MakaveliX]

Thanks for the replies.
After running some Redex and changing the MAF sensor, the rough idle seems to have disappeared and no longer drops below 500RPM when in drive