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I have a 2008 ML350 W164 and it has started misfiring on cylinder 6 and running rough when under load. It does not misfire when idling or revving in neutral. I have a Snap-on Modis scanner giving codes P0300 and P0306 - Damages TWC.
If I clear the code while the engine is running, it starts firing on cylinder 6 and the rough running stops immediately. This makes me think it has to be an electrical fault.
I switched the coil pack and spark plug with cylinder 5, but the fault stays on cylinder 6.
What should I investigate? Injectors? Coil wiring harness?
This may not be related, but I also noticed that the 4 year old battery was low voltage this morning. After a 2 hour charge, it was showing 900 CCA, which fired the car up. May have been due to me troubleshooting, but maybe also a faulty battery.
Yes, I have that coil on plug setup, but I switched out the entire coil assembly and spark plug with a known good cylinder, and the fault stayed on cylinder 6, so it is not the coil, spark plug or coil boot.
Things I am thinking of are O2 sensors, injector, MAF sensor, crank sensor, engine mounts, coil wiring harness etc. I am not entirely sure how to actually test those components, as they are not flagging fault codes.
When I bought the car, it had a crank sensor code, but I cleared it and it has not come back.
Yes, I have that coil on plug setup, but I switched out the entire coil assembly and spark plug with a known good cylinder, and the fault stayed on cylinder 6, so it is not the coil, spark plug or coil boot.
Things I am thinking of are O2 sensors, injector, MAF sensor, crank sensor, engine mounts, coil wiring harness etc. I am not entirely sure how to actually test those components, as they are not flagging fault codes.
When I bought the car, it had a crank sensor code, but I cleared it and it has not come back.
I haven't had this problem on my ML350, but my F150 had a similar issue on a cylinder. I swapped the plug and COP, and it was still missing under heavy load. It ended up being the injector. It was an easy swap on that one. FWIW.
If you put a new plug in there and it's still misfiring, pull the injector and swap it with another cyl or replace. How did the plug that came out of that cyl look, usually a faulty injector will either be not firing fully and the plug will be super lean burning, or stuck open and the plug will be wet with fuel.
Misfiring only under load is normally ignition related but you could have an injector that's plugged and can't deliver full fuel at WOT.
It's normal for the car to shut that cyl down when a misfire is detected, then it comes back when you clear the code until the fault happens again. It does this to protect the catalytic converters from having raw fuel dumped into them and melting when the misfires happen. The M/E turns off the injector until the fault is cleared.
OK, so I put new spark plugs, a brand new coil on cylinder 6. I switched injectors between cylinder 5 and 6. The car ran great during a 5 mile test drive. Then this morning, as soon as I put it under load, check engine light came on, misfiring on cylinder 6 again. No other fault codes.
Could it be a burnt valve? I ran the Xentry DAS compression check, which came out OK, as they always do.
Thank you. I read both docs thoroughly and it gave a good explanation of why the CEL comes on in misfire conditions, although I am still unclear as to the exact cause in my case. However, the more I investigate, the more it looks like a burnt exhaust valve, because everything else seems to be working. I poked a borescope into the cylinder but couldn't really detect anything as I could not get a clear view of the valves. I will try and do a leakdown test today.
I went into the 'Check Smooth Running' guided tests in SDS, but they all require special diagnostics devices that I don't have. I have ordered a CIS fuel pressure tester, but I am not likely to obtain those specialist Mercedes diagnostics interfaces anywhere in a short time frame.
Are you referring to another test that I can do? The docs you attached do not provide guidance on how to run the test. I have WIS and search for some of the references in the docs, but found nothing. I am not a fan of WIS, I find it very difficult and counter-intuitive to use. I almost invariably find the answers with google and Mercedes forums long before WIS is useful.
The idle test shows everything seems to be in spec. However, the misfire does occur ONLY under load, as per the thread title. I will do a fuel pressure test when the gauge arrives.
Idle test values
However, I do get this error when I try to interrogate the Fuel Pump. I am not sure if this is normal or not, it is the same thing that happens when I interrogate other components that are not fitted to this car, eg Distronic.
Fuel Pump error
Last edited by spawnywhippet; 05-04-2017 at 04:10 PM.
This may be the Smooth Running test you mentioned? Under Motor Electronics 9.7 Actuations -> Mixture Adaptation:
I don't know how to interpret those values though, but I noticed that Cylinder 6 often has a higher value than the others, eg Cyl 1-5 fluctuate between -0.4 and +0.6, whereas Cylinder 6 is fluctuating between 0 and +1.2 l/S2
Last edited by spawnywhippet; 05-04-2017 at 04:23 PM.
That test is how much correction the M/E is applying to keep a smooth idle rpm. A cylinder with a larger variation has weaker/incomplete combustion, if it's adding to that one cyl all the time, it's not pushing down as hard on the crank as the rest of them.
At this point, you know something is wrong, it's more about deciding what route you are gonna take. Pull the head for exploratory surgery, drop in a used engine, or just drive it till it gets worse.
Yes, drivers side, rear most cylinder. It was a good point though. I also changed all the spark plugs and randomly allocated the coil packs back on the engine, except for cylinder 6 which first got the coil from cylinder 5, then got a brand new coil.
04-28-2017, 07:33 PM
