Lumpy running when warm - coils?

[donkmeister]

2004 Mercedes 5-litre V8 (M113 engine code). Two spark plugs per cylinder, one coil per cylinder.

When the engine is cold or warmish it runs smoothly... But once it's up to full operating temperature it feels like it's labouring, possibly running on 7 cylinders. Lots of vibration at idle and low revs, but increasing the revs a little smooths it out again. No engine warning lights, and it's not flagging any codes.

I'm currently leaning towards it being a coil on its way out... Does that sound about right? Anything else it is likely to be? A sensor somewhere giving the wrong value when warm?

Can an ignition coil be tested with a multimeter or does it need specialist equipment?

(On most cars I would just take the plugs out and look for a dirty one... But on this engine there are 16, and they're not the easiest to get at so the actual checking of coils and plugs is waiting for a warm, dry weekend in the future )

[catman]

It could be a coil. When the coil was going on a previous car, it was dreadful at low revs, but when I managed to get it up to speed, it drove quite well. trying to find out which one is dodgy will be the fun part!

[Clifford Chambers]

Should flag a code, which reader are you using?

My local German specialist will plug it into STAR in exchange for a £20 note, money well spent.

[Chris32345]

catman said:

It could be a coil. When the coil was going on a previous car, it was dreadful at low revs, but when I managed to get it up to speed, it drove quite well. trying to find out which one is dodgy will be the fun part!

If the exhaust manifold is visible on the car
An infrared thermometer on each tube of the manifold from a cold start is a good way to find the bad cylinder
As it will warm.up a lot slower then the others

[catman]

Chris32345 said:

catman said:

It could be a coil. When the coil was going on a previous car, it was dreadful at low revs, but when I managed to get it up to speed, it drove quite well. trying to find out which one is dodgy will be the fun part!

If the exhaust manifold is visible on the car
An infrared thermometer on each tube of the manifold from a cold start is a good way to find the bad cylinder
As it will warm.up a lot slower then the others

That's a good idea. Back in olden times with a single coil, you could simply pull off one HT lead at a time to find the dodgy cylinder. The one that didn't change the idle speed would be the culprit. Would probably toast the electrics now.

[Hereward]

O2 sensor issue? When the engine is running cold it's not using O2 sensor values but then starts to use them (or try to) once the car is up to temperature. I would have thought a fault code would be logged if that was the case.

Quick and easy to change (unless corroded threads).

No harm cleaning the MAF filament too.