W124 320CE Idle/Misfire

[Y15HAL]

Good Morning All,

I have a 94 320CE with 104k on the clock.

I bought original service parts and gave her a service a few weeks ago, started her up and all seemed well.

When driving her properly recently i noticed a slight flutter in the revs, not much at all, but it is there.

It could be a coincidence that this happened after i serviced her, or could be something i’ve done wrong

I double checked that the plugs are in secure as well as the leads/coils.

The MAF is connected as well as the air intake sensor.

Any ideas why this may be happening?

Thanks in advance,
Vishal

 

[neilrr]

Check the wiring harness, if it hasn't been replaced it will probably be disintegrating.

 

[Y15HAL]

Check the wiring harness, if it hasn't been replaced it will probably be disintegrating.

Harness has already been replaced

 

[grober]

check the plugs for age and appearance for misfire deposits--best case scenario coil extenders-leads worst case start of a head gasket problem and a few things in between---slight fuel contamination etc

 

[GP801]

As above possibly coil packs. Our 280 had a slight misfire due to faulty coil packs. Unfortunately they also fried the ECU!

 

[Y15HAL]

Thanks everyone.

Sparks were changed already with new ones direct from the dealer.

Potentially could be coil packs

 

[grober]

Thanks everyone.

Sparks were changed already with new ones direct from the dealer.

Potentially could be coil packs

WHICH PLUGS MAKE AND NUMBER-should be non resistor type--many MB main dealers are unfamiliar with these models now

 

[Y15HAL]

Mercedes part number A 003 159 68 03

Google search seemed to point it to the correct car.

 

[Gbrowncls55]

I had the same on my car. Aftermarket plug leads had broken down. Fresh MB ones and all is well other than my wallet! The other cause I have had in the past is a disconnected vacuum pipe somewhere . Mine was on the seat lumbar support (560 sec)

 

[grober]

looks like the BERU non resistor plug A 003 159 68 03

Beru 14F-8DU4 Spark Plugs for M104? - PeachParts Mercedes-Benz Forum

It looks like F8DC4 plugs are NLA from Mercedes and they have superceded to 003 159 68 03 when I tried to order 003 159 67 03. I just got the plugs

www.peachparts.com

I favour NGK BCP6ES myself

 

[Y15HAL]

I had the same on my car. Aftermarket plug leads had broken down. Fresh MB ones and all is well other than my wallet! The other cause I have had in the past is a disconnected vacuum pipe somewhere . Mine was on the seat lumbar support (560 sec)

Was it just pot luck that it was the leads? I don't fancy changing the whole lot, if not needed

 

[Y15HAL]

looks like the BERU non resistor plug A 003 159 68 03

Beru 14F-8DU4 Spark Plugs for M104? - PeachParts Mercedes-Benz Forum

It looks like F8DC4 plugs are NLA from Mercedes and they have superceded to 003 159 68 03 when I tried to order 003 159 67 03. I just got the plugs

www.peachparts.com

I favour NGK BCP6ES myself

Shouldn't (in theory) cause said issue though should it?

 

[grober]

Shouldn't (in theory) cause said issue though should it?

Theoretically no-- assuming they fitted the PLUGS as per invoice--- only true check is visual inspection-- sometimes the resistor plug leads get damaged when they are removed during plug changes and this can cause a misfire also. Best thing is to get the car on an old-fashioned oscilloscope based engine analyser and examine the firing lines for every plug-- more likely to find mechanics with experience of this vintage of engine technology at an MB specialist rather than a main dealer

 

[carat 3.6]

Harness has already been replaced

Did that include the throttle body rewiring?

 

[Y15HAL]

Soooooo!

Luckily, i had a spare set of plugs, swapped them out and that has done the trick!!

Upon close inspection, one of the plugs had a visible smaller gap than the others!! Will give them a clean, and gap them all properly!

Thanks for everyone's input.

Regards.
Vishal

 

[neilrr]

I think in 'what could be the problem?' posts such as this it would be helpful to be told up front what has already been done - parts already replaced, parts already tested etc.

 

[Gbrowncls55]

Was it just pot luck that it was the leads? I don't fancy changing the whole lot, if not needed

well my indie thought that was the most likely cause and it turned out to be the case. But as you are discovering there are a very large number of things it could be and that is but one.