M104 Misfire (1996 Mercedes S280)

[richard300]

I always felt my car had a slight misfire (barely detectable, to the point that i thought i was imagining it) Today though, after a 15mile motorway journey i pulled to a stop, the revs dropped and the car has had a very notable missfire since.

I changed the plugs 3 months ago, so (currently i am ruling them out) i had a spare (new) coil and swapped them over one by one - That made no difference.
Removed the MAF and that made no difference.

I did notice that the hose from the MOT valve to the engine had come off, but i re attached that and nothing changed.... on the subject of the MOT valve (or whatever its called) should i hear this making a constant clicking noise? because i cant.

I have ordered a set of 3 leads, which will hopefully be here tomorrow. But when i pull every lead/coil away from the spark plugs i do hear a spark/arcing from all six of them, so not sure this will help.

There is also a tapping/hissing noise that i can hear - cant make out if its from the rear of the engine, the rear exhaust manifold or the front section of the exhaust as i can hear it under the car. However, someone told me that was un-burnt fuel in the exhaust system as a direct result of the misfire.

I live in Australia (moved from the UK in April) and nobody wants to touch it, and the nearest (alleged) specialist is 50km away....

The wiring loom seems to be in good shape, it's either a car with the later loom or its been replaced.

 

[bolide]

There are three outputs from the ECU to the coils and the ignition uses a wasted spark system where two plugs are fired at the same time from one coil

Each coil pack sits on an extender and has a cable to the #2 cylinder. If you replace the cables and the extenders, and the coil packs are all good, then you either have a loom and / or ECU problem or there's a fuel problem

I'd check the plugs to see what colour they are and to check they are the correct type for the engine. Also check the plug recesses for water or oil contamination

Old-fashioned diagnostic systems like the Crypton unit used to have a clamp to go round the HT lead to check the spark timing. If you can check the three feeds & get some idea of the health of the voltage that'd be a good idea

You could measure the HT voltage feed with an analog multimeter with a needle. I have no idea what voltage the ECU outputs to the coil but checking they are all the same should give you a clue where the fault lies

The meter will under-read due to the short duration of the pulse but that doesn't matter for this test

Nick Froome

 

[hotrodder]

Nick's covered the ignition system, other things that jumped out...

Removed the MAF and that made no difference.

While it's obviously not proof that the MAF is dead (especially if the engine is missfiring rather than idling badly) unplugging a working MAF typically makes a noticable difference

I did notice that the hose from the MOT valve to the engine had come off, but i re attached that and nothing changed.... on the subject of the MOT valve (or whatever its called) should i hear this making a constant clicking noise? because i cant.

The regeneration switchover valve (MOT) is explained here http://www.w124performance.com/service/w124CD1/Program/Engine/LHIS/47-0200.pdf It's not something that's easily audible but when it's working you should be able feel it 'ticking'. A dodgy valve can cause idle weirdness, with the hose to the inlet manifold disconnected you have a significant vacuum leak. Similar deal if it's stuck open, temporarily blocking the hose to the inlet manifold is one way to eliminate it

There is also a tapping/hissing noise that i can hear - cant make out if its from the rear of the engine, the rear exhaust manifold or the front section of the exhaust as i can hear it under the car. However, someone told me that was un-burnt fuel in the exhaust system as a direct result of the misfire.

Dunno how sensitive it is but IIRC the HFM-SFI (hot film management-sequential fuel injection) management will/is supposed to switch off injectors if a significant missfire is detected in order to protect the cat

Getting/checking for fault codes may narrow things a chunk

 

[richard300]

Ok, so the new (aftermarket) leads arrived today - the two shorter leads were to short, but i used the long one to swap them all over one by one - Nothing changed.

I removed the plugs - It wasnt great! When i removed the previous plugs (have no idea how long they were in the car for) they all came out looking in good shape. The 3 month old plugs that i removed this evening were in less than satisfactory condition.
The front 3 plugs were all dry and black and the rear 2 plus were all wet and black. I gave them a clean up as best as i could but had to put them back in the car... I'll buy new plugs, the current plugs are NGK BCP6ES (non resistance) if i remember rightly.

Other things perhaps worth note: when playing around i found that if i removed the lead from cylinder 1 the misfire pretty much stayed the same. Then i but lead 1 back in and removed the lead from cylinder 3 this caused the engine to stall. But then whilst further playing around i found that if i pulled the coil from cylinder 2 (with all the other leads/coils in place the missfire remained the same. re inserted the coil and them pulled the lead on cylinder one (that previously made little difference) and this time the engine stalled..... so i wasn't able to narrow anything down.