W221 3.5 petrol Misfire at idle?? Had new coils

[widebodysec]

Hi guys and wizards, i have 06 plate w221 s350 petrol. Had a coil pack go down so swapped it, then another in a fortnight so replaced all with bosch so no issues here i hope. All codes clears started back up and have a misfire on idle. Seems smooth and fine over 1000/1300 revs and drives fine just chugs at idle. Swapped all plugs too- no change! Then every once in ten starts it idles and revs up to 1500/1700 with out a pedal touch. This could be seperate but odd any way! Ideas . Thanks in advance

 

[Uncle Benz]

Bad injector? Possibly hosing? Could account for the occasional rev up if it’s burning dripped fuel off maybe. Hard to diagnose remotely. What did the plugs look like?

 

[widebodysec]

I changed all plugs when put 1st coil on, so all brand new. I have just been out and swapped for the old ones i case i got a rare duff one or failing since played with all looked ok and no change at all from doing so

 

[widebodysec]

Nothing came up feom diagnosis comp after all codes cleared from new coil replacements. I started car and it then picked up the lumpy missfire that's all . No cylinder number just misfire drove home to change plugs.

 

[widebodysec]

The revving is maybe separate i think . I can start car and is fine touch the pedal for a blip a d it stays at 17/1800 or falls. Or i touch pedal againand back to noral. Like a sticking cable but not like that these days lol done it maybe 5 times in a fortnight can be sat at say 35/40 mph and the revs that at 1700 with foot off pedal then after a good few seconds, 10/20, maybe it will fall

 

[Botus]

when you say misfire what its like ?

and how long had you driven with the coil packs playing up - the CATS don't like life when they are playing up

I'd also do a CPS (crank sensor) they pay up all day long and are an easy fix. On mine it coughed at low revs now and then and stalled slowing for mini roundabouts

 

[widebodysec]

That first coil pack replacement is first issue i have had with electrics / engine in over year

 

[widebodysec]

Misfire- can i add vids here? Not tried

 

[widebodysec]

Coil pack was maybe the day till home and got replacement. Then the 2nd time drove to garage for further 2 then replaced the others within the week. To replace all of them with 6 complete set of new bosch ( 3 already and 3 extra as best plan ) Cant add a video clip - But just started it to record it and 5 seconds in engine management light appeared so back to garage & computer see that it says now! Fed up with this!! ‍

 

[Craiglxviii]

What computer/ code reader are you using? Is it Star?

 

[Botus]

CPS eventually throws the EML (but not always) - you should NOT have driven and or be driving the car with a misfire - its will damage the CAT which will already be rather sad at this mileage

Its quite common on older Merc's that the CAT collapse inside and block the exhaust totally

FYI older engine oil was high SAPS it was very durable and really protected the engine internals, but it has old world additives that clog up modern car's CATs - so they went to fragile super expensive oil with low SAPS where I believe the additives age and breakdown (hence greater need for max two year oil change)

Understanding Engine Oil: The ABC’s of ACEA Specifications

Ever wondered what A5/B5 meant or C1 engine oil? Read our in-depth news piece to find out why this information is so important when choosing an engine oil.

www.finol.ie

 

[widebodysec]

Yes i believe star yep . Was only couple miles home and was fine over idle so just nipped to change plugs.

 

[Blobcat]

Misfire- can i add vids here? Not tried

Upload to YouTube and post the link back here

 

[Uncle Benz]

If it runs cleanly on all six when you bring the revs up it’s unlikely the plugs or coils are the problem. With a misfire only at idle it could be poor valve sealing or a bad injector which is leaking fuel all the time. It will burn the excess fuel at higher revs, but flood the plug at idle. Injector faults aren’t common, but very bad ones can hydraulic lock the cylinder and bend a connecting rod, so be careful. It has been seen on m156 more than a few times.
If the problem is a poorly sealing valve a compression test followed by a cylinder leakage test will pinpoint it.
Going back to my college days, thirty five years ago now, my lecturer Doug Wragg would have pulled out a vacuum gauge. He could diagnose pretty much any top end fault with that one simple tool. I can still remember him diagnosing a sticking intake valve on a poor running engine. “See that boy, the gauge suddenly has two needles! What’s causing that?”. Sticking intake valve, I’ll never forget it.

 

[widebodysec]

Plugs were light brown ( happy ) and deffo not wet when i changed them one at a time to identify if it was a bad plug and ran mostly ok for a time then last time i went out to video it , was wuite lumpy like i just pulled a plug off!! So sort of intermittent ??

 

[Uncle Benz]

Mercedes are quite unusual in that if the ecu detects more than a certain number of misfires with a set time it will turn off both the spark and the fuel to that cylinder until a key off/key on cycle. I’ve seen seasoned mechanics become perplexed when pulling the wiring plugs off coil packs trying to identify which cylinder is misfiring. You cannot use this method with Mercedes. With a Star machine you can adjust the cylinder cut threshold to compensate for a worn engine with tired valve seats. In the words of Peter Sellers, not a lot of people know that.

 

[widebodysec]

Oki doki. Well will be on comp on monday see what flashes up initially then look again from there. Cheers for all ideas and thoughts etc.

 

[widebodysec]

Mercedes are quite unusual in that if the ecu detects more than a certain number of misfires with a set time it will turn off both the spark and the fuel to that cylinder until a key off/key on cycle. I’ve seen seasoned mechanics become perplexed when pulling the wiring plugs off coil packs trying to identify which cylinder is misfiring. You cannot use this method with Mercedes. With a Star machine you can adjust the cylinder cut threshold to compensate for a worn engine with tired valve seats. In the words of Peter Sellers, not a lot of people know that.

It did that on the day first had a misfire due to coil pack, then turned back on. And was fine for day or two then light came back on and first trip to garage and new coil pack fitted. Since then a few things have happened lol

 

[widebodysec]

It did that on the day first had a misfire due to coil pack, then turned back on. And was fine for day or two then light came back on and first trip to garage and new coil pack fitted. Since then a few things have happened lol

 

[Botus]

Mercedes are quite unusual in that if the ecu detects more than a certain number of misfires with a set time it will turn off both the spark and the fuel to that cylinder until a key off/key on cycle. I’ve seen seasoned mechanics become perplexed when pulling the wiring plugs off coil packs trying to identify which cylinder is misfiring. You cannot use this method with Mercedes.

I thought that …. but certain fault codes on my w221 specifically call out that it doesn't seem to cut the offending cyl. Coil packs being a good (poor) example ….

P301 misfire damages TWC (when you look up TWC its the CAT)