Rough Idle ( Sometimes )

[MakaveliX]

Hi guys.
Got a C240 ( W203 112 Engine ) 2002.. About 32K miles.

Every now and again I get a rough idle when at lights if the car is in drive and foot on brake pedal. If I put it into park it stops rough idle and feels smooth.

Air filter/ Oil filter etc has been done.
I have cleaned the MAF sensor properly. And even put some redex in the tank for good measure to help clean the injectors.

What seems to happen is the revs drop below 500RPM and then it goes rough until I put it into park or pull away.

Any ideas ? The fuel filter may not have been changed in a while ( perhaps around 2019, but the car has hardly any miles on it )

Edited by MakaveliX on Tuesday 18th June 17:30

[trevalvole]

Presumably the engine is under a small amount of load when in D, but under less load when in P?

I'd very surprised if the fuel filter caused problems at idle and not at any other time, as fuel demand must be at its lowest at idle.

[MakaveliX]

trevalvole said:

Presumably the engine is under a small amount of load when in D, but under less load when in P?

Car idles about 500rpm in D, but slightly higher in P ( always )

[MakaveliX]

trevalvole said:

I'd very surprised if the fuel filter caused problems at idle and not at any other time, as fuel demand must be at its lowest at idle.

Yeah good point. No issues starting the car at all, so probably not the fuel filter.

Done some googling and I am not alone, however the causes have been different for different people.
Ranging from the EGR valve, MAF unit itself, Throttle body being dirty, even the Cat converter.

So it is a difficult one.
No error codes, and car is perfect otherwise ( especially for a car from 2002 )

[trevalvole]

MakaveliX said:

trevalvole said:

I'd very surprised if the fuel filter caused problems at idle and not at any other time, as fuel demand must be at its lowest at idle.

Yeah good point. No issues starting the car at all, so probably not the fuel filter.

Done some googling and I am not alone, however the causes have been different for different people.
Ranging from the EGR valve, MAF unit itself, Throttle body being dirty, even the Cat converter.

So it is a difficult one.
No error codes, and car is perfect otherwise ( especially for a car from 2002 )

I suspect you should focus on why it is doing it in D and not P. Again, it seems unlikely that many of the possible causes you have found by Googling would present themselves at idle and not when the engine is at higher load i.e. when the car is moving. As I recall this car was standing for some time, so presumably it had an MOT recently - if so, what did the emissions data look like as that might give a clue?

[MakaveliX]

trevalvole said:

I suspect you should focus on why it is doing it in D and not P. Again, it seems unlikely that many of the possible causes you have found by Googling would present themselves at idle and not when the engine is at higher load i.e. when the car is moving. As I recall this car was standing for some time, so presumably it had an MOT recently - if so, what did the emissions data look like as that might give a clue?

Thanks for the info. It is just info I found on other forums regarding older Mercedes' owners.

MOT was done before I owned the car ( it was a family members ) so I have no idea about the emissions unfortunately.

Don't really want to go throwing money about unnecessarily trying to fix this minor issue unless I can definitely pin point it. Just concerning some times as earlier it felt as if the car was going to completely stall. Most days it happens a couple of times, it is not regular.

Gearbox feels very smooth, no issue shifting gears etc.

As said, when in Drive the revs drop and that's when it is prone to happening. So maybe it is related to the revs ?
I was always under the impression that a bad fuel filter can cause rough idle

Edited by MakaveliX on Tuesday 18th June 19:24

[trevalvole]

MakaveliX said:

As said, when in Drive the revs drop and that's when it is prone to happening. So maybe it is related to the revs ?

The lower revs may be the symptom, not the cause, as if you put more load on the engine (which I understand happens when the transmission is in D compared to P), but don't supply it with enough additional fuel, then the revs will drop.