C240 Lumpy Idle / Misfire

[seymansey]

In the past three days, my otherwise perfect C240 (2.4) has developed a bit of an intermittent misfire complete with lumpy idle.

The fault just occured, no hints or signs. I first noticed it when there was a momentary loss in power when pressing the loud pedal followed by one or two small bursts of lumpy idling when sat at junctions. Once revs were past 1500rpm things were fine.

Today things started off fine when I went to work, small amount of lumpy idle at traffic lights, but other than that off to work fine. On the way back today the car started fine, I drove a mile then suffered a fair bit of loss in power along with a misfire all the way. Seeing as i had no option but to get home (50 mile journey) i persisted and mad it home albeit on what sounded like 5 cylinders.

When I got home, the misfire was still there as i reversed into my parking space. I turned off for about 40 seconds, went in the house to do stuff, came out, started her back up and everything was fine, all back to normal. I've done 3 separate 1 mile journeys in the last couple of hours to check her out and has been fine again.

Any suggestions as to what this could be? My gut reaction is the wet weather we have been having and i've done a fair bit of mileage (Swansea > Brighton, 50 miles in between then Brighton > Swansea) in the past 5 days and i've gone though a good amount of large unavoidable puddles in the roads, etc.

Coil packs have been checked, connections reseated, MAF has been checked, etc. Does anyone know the location of the ECU, is it the black box where the diagnostic plug is situated under the bonnet?

Thanks for all your help.

 

[Dieselman]

It won't be the ECU as that uses low voltage. It's most likely an HT short.

 

[BlackC55]

How did you check the coils?

 

[seymansey]

How did you check the coils?

Only visually so far, removing / reseating connections and HT leads. They're easy enough to remove, but suprisingly it was chucking it down in the costa del Swansea!

 

[BlackC55]

You many find that your air mass sensor may be acting up but more likely is the coils or leads have broken down.

 

[sweeper]

Most likely a coil pack, these often appear ok on start up, then break down as they warm up, similar problem occured on my jag, seemed ok above idle although was only running on five, very lumpy on idle especialy once warmed up (probably due to lambda sensors coming into play, causing all 3 cylinders on that exhaust to play up).

 

[beaucostil]

interesting post , ,got E240 (2002) with 63000 mls all dealer maintained, ,runs well & is,nt hesitant when gassed but seems lumpy when at idle ,now starting to annoy me so had been thinking of taking a gamble & having MAF replaced but now not so sure
have heard of coils but how many are there?how much are they? & is it a hard job to replace them

whats the vote?

 

[lynall]

Misfire on my e430 was the plug leads it was very subtle but the new leads cured it perfectly.
I measured the old leads ( removed from the vehicle) with a multi meter set to 20k ohms and the the reading should be around 2.00 ohm it says on the cap what the reading should be, i had one totally duff one and two others partly duff.



Lynall

 

[E320Trip]

I had maf sensor changed at Mb specialist. None to 5% difference. They then said it was engine mountings. Changed garage, who used diagnostic machine which diagnosed misfire on cylinder 5. Plug leads changed for that cylinder. Engine ran 85% smoother. Recently had throttle body cleaned, little difference noticed to my usually sedate driving. One year after plugs, I am about to have the engine mountings changed, which should sort out a vibration problem.