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HELP warm idle knock. Black dots on cylinder, spark plug bone dry back.
Hi everyone,Looking for some advice/diagnosis on my "new" 2010 ML350 (M272 engine) with 80k miles. I am worried.
Its making a quiet tapping noise, but only after the engine warms up.I scoped the cylinder on the passenger side and noticed black dots/spots on the cylinder wall. Also large black dot on piston.Spark plug from that same cylinder was totally black, covered in dry soot.I changed the spark plug, but there is absolutely no difference in how the engine runs or sounds.
Intake manifold has a lot of leaks all around I am planning to replace it but need to know if engine is dead or not :p
Firstly, nothing in those pictures that some basic maintenance won't sort out and prevent😉
The little black spots are carbon from the fouled plugs that has been dislodged as you removed the plugs. The larger patches are indicative of an engine in desperate need of a proper service.
That engine would benefit from being serviced.
Replace ALL the plugs and change the oil (8 litres/quarts of decent 5w40 to MB 229.5) and filters and clean out the crankcase breather system. Replace anything in the breather system that's past its best... ie rubber hoses gone hard/blocked up oil separator etc. Also, if you know the intake needs attention, do that. Any issues aren't going to help the fouling.
It wouldn't hurt to do a couple of 500/1,000 mile oil and filter changes afterwards.
Then run a couple of tanks of Shell V Power through it with a can of injector cleaner in each tank.
Get it up to temperature and drive it hard to burn that crap off.
Once you've done that lot, pull that same plug for a like for like comparison. You'll appreciate the difference.
The moral of this tale is to service the car properly and if you regularly use crap fuel, try and use a fuel cleaner in each tank and treat it to some decent fuel periodically. Also if you only ever use it for short journeys, take it out for a decent blast of at least 50 miles at highway speeds once a week.
I'd add to that advice maybe do a compression test and see if you have a cylinder specific issue. A before and after (as per advice above) would be interesting.
One area you might want to investigate is the fuel injector on that cylinder as well. Seems unusual that you have only one unusual cylinder based on what you've said.
Last edited by BlackML550; Feb 24, 2026 at 03:31 AM.
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