When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
It started with misfires upon romping on the throttle. In short: Misfires cylinders 11 and 12, codes: P0311 P0312, eventually P0300 (general misfires).
I just pulled the coils and noticed a bunch of oily crap everywhere. I also pulled the spark plugs, and Holy Moly was there a lot of oil on them, with no obvious oil leak. Spark Plugs for cylinders 11 and 12 were drenched in oil like I’ve never seen before.
The previous owner, a mercedes tech, did the valve cover gasket. It doesn’t seem to be leaking. The source of the oil remains a mystery.
I just fired the spark plugs with the torch and they’re sitting in household ammonia removing the carbon deposit ****. While they’re off, I can see inside the cylinders, and the tops look *covered* in sooty-****. The car has a Renntech R3 full tune. I’m in the process of furnishing an air compressor, and them I can clean out the spark plug wells, as well as the tops of the cylinders. I’m thinking sea-foam or carb/choke cleaner. Something is unacceptably off here, and I don’t like it. I had a problem with the secondary air injection system (P0410), but I believe replacing a relay solved this issue. In the course of pulling off the coil, I noticed some broken hose lines that I will also replace.
I’ve managed to peak into the intake, and it looks flawless. I plan on replacing the fuel filter and run some liqui-moly pro engine fuel system flush through it once I have it all back together. I first need to clean the insides of the cylinder, and find out where the @#$% all this oil is coming from. I’m also hoping and praying my coils were spared from this mess. Right now I’ve only messed with the driver’s side coil, but I thinK i’m also going to pull and clean the plugs on the passenger side.
A similar thing happened to me about 8 months ago on my '05 cl65. I do not know if this is the same for you but worth a check.
My plugs on my left side bank of cylinders were oil drenched. It literally seem to happen overnight. Drove car home from work, parked in driveway, ran fine. Started up in the morning and a huge cloud of blue smoke billowed out, but only from the left exhaust, pipe was oily wet. Right hand side was find, no smoke nor any oily residue at the tail pipe. Car would run while it was cold and idling high (with lots of smoke) but once dropped to a normal idle would stumble and die.
Finally tracked it down to the pcv system, the left side has what amounts to a small vacuum pump driven off of the cam. It is the black plastic circular thing at the front of the cam cover. The diaphragm inside was cracked and it was allowing oil to be sucked back into that side of the motor. I replaced that part and the problem stopped, no more smoke and it ran smoother than before.
I was going in circles trying to figure this out. I ended up scoping the bores which looked fine, scoped the intake manifold where the line from that pcv unit connects to the center of the intake manifold at the front. No oil to be seen, so I discounted the pcv at that point. Nothing else was making sense so I took off the pcv unit anyway and found the broken diaphragm. I also found the part manufacture date was about 5 yrs after my car was built, so it had already been replaced once before. Car just turned over 80k miles this week.
One thing that does seem to be different is the amount of oil that is on your plug socket, implying the recess has filled with oil. I did not have this. Other than the valve cover gasket I can not think of where that oil could be coming from.
Quick update: my plugs looked almost exactly as described here: https://mbworld.org/forums/s-class-w...ml#post7479486
I just cleaned them extensively, and found three of them had higher resistances, two at 1.5 kOhm and one at 1.7 kOhm. All others around 1.3 kOhm. I’m debating whether to replace all spark plugs or not.
Dave, the front spark plugs look fine, it’s the rear ones that were dripping in oil (also misfires on cylinders 11 and 12). It's the spark plug well that is/was drenched (not visible in the picture due to the spark plug socket).
Originally Posted by chassis
Oil can only get into the combustion chamber, where the spark plugs are located, by three means:
- from the crankcase, past the rings
- from the cam chest, past the valve guides
- through the intake, from PCV as noted above
Bad rings may be detectable by a compression test. More careful observation and diagnosis is needed for PCV and valve guide sources.
How would this oil then get into the spark plug well? It would have to travel up the spark plug threads and into the well... are these not sealed?
Last edited by Fried Chicken; 02-03-2020 at 02:37 AM.
Just pulled and cleaned the spark plugs, and removed the oil. Replaced three spark plugs that showed out of spec resistance values. The new spark plugs have the same value as the other nine (~1.3 kOhm).
I ordered a driver’s side coil pack from Clark over at v12icpack.com. Hopefully the diagnosis of a failed coil pack is correct. If the coil pack doesn’t do it, then my only guess would be a problematic fuel injector.
Presence of oil in the spark plug well could come from above, or from below.
- from above needs to be checked out visually, I am not familiar with the details of this engine. The oil could have come from a spill during an oil change.
- from below would mean oil is in the combustion chamber, root causes are possibly the valve guides, cylinder head gasket or rings. If the oil came from below, and was found in the spark plug well, it suggests a failed spark plug sealing ring (washer), or spark plugs installed with too low a torque. Low torque may also cause misfire.
How long have you owned this vehicle, and how many miles were on it when it came into your ownership?
Presence of oil in the spark plug well could come from above, or from below.
- from above needs to be checked out visually, I am not familiar with the details of this engine. The oil could have come from a spill during an oil change.
- from below would mean oil is in the combustion chamber, root causes are possibly the valve guides, cylinder head gasket or rings. If the oil came from below, and was found in the spark plug well, it suggests a failed spark plug sealing ring (washer), or spark plugs installed with too low a torque. Low torque may also cause misfire.
How long have you owned this vehicle, and how many miles were on it when it came into your ownership?
I have searched extensively. The previous owner claimed to have done the oil cooler lines, the ignition coils, the spark plugs, and the valve cover gaskets. The oil definitely comes from the spark plug well, although I don't see any leaks from the valve cover that would explain this amount of oil (drenched!). My suspicion is that the valve cover gasket was done after the spark plugs and coils were (allegedly) replaced. I'm going to inspect further, as the oil seems to be localized to the rear end of the engine.
The M275 doesn't have spark plug rings. The spark plug torque value for this car is a very paltry 25 NM. It seems low, but unless someone compellingly suggests otherwise, I will stick to this. I did use a dab of copper anti-seize. Hopefully the misfire goes away with the new coil pack, if not I will be both relieved and pissed.
I've run into this problem when I made a mistake re-doing the valve cover gaskets. Did the previous owner, perchance, make the same mistake that I once made 2.5 years ago and put RTV on those gaskets, "to make 'em seal better"? It's possible, and if so, that's likely the cause of that leak back there. Yes, I re-did the valve cover gaskets (bought new ones) and cleaned everything thoroughly before putting them back on again. I did use Genuine-MB gaskets as well.
I've run into this problem when I made a mistake re-doing the valve cover gaskets. Did the previous owner, perchance, make the same mistake that I once made 2.5 years ago and put RTV on those gaskets, "to make 'em seal better"? It's possible, and if so, that's likely the cause of that leak back there. Yes, I re-did the valve cover gaskets (bought new ones) and cleaned everything thoroughly before putting them back on again. I did use Genuine-MB gaskets as well.
Not that I can tell. Did you get localized leaking at only the rear of the gasket that focused mostly on the rear spark plug wells? I really don’t want to redo a valve cover gasket *smh*.
Yep. In my case, the valve cover gasket was leaking in the rear, where I couldn't easily see it. Cylinder #12's spark plug wells were pretty full of oil, much like yours.
The V12's don't have the crankcase breather system in the same spot as the V8's, so about the only place I can think of that it would leak like that is either the rear of the valve cover, or possibly the camshaft position sensor. Perhaps the O-ring on the camshaft position sensor has gotten brittle and is allowing oil to leak out.
Yep. In my case, the valve cover gasket was leaking in the rear, where I couldn't easily see it. Cylinder #12's spark plug wells were pretty full of oil, much like yours.
The V12's don't have the crankcase breather system in the same spot as the V8's, so about the only place I can think of that it would leak like that is either the rear of the valve cover, or possibly the camshaft position sensor. Perhaps the O-ring on the camshaft position sensor has gotten brittle and is allowing oil to leak out.
I will see what I can find probing with my handheld mirror once my replacement coil pack comes in.
I did recently add stop-leak (liqui-moly) to the engine, and I think that alleviated a lot of oil leakage that I was seeing (suspected from the rear main seal). The stuff really works. It's possible this has alleviated the leak. Unfortunately it's not easy to check for oil leak buildup because this engine sits in the engine bay like the mechanics in a switch wristwatch.
I may just go all out and do everything. I've been avoiding removing the intercoolers, but if it comes to it I may just go for it and while I'm in there replace the crankshaft position sensor, remove whatever sealant might be on the valve cover gaskets, fix any vacuum leaks that currently seems to stop the diaphragms on the turbos from actuating, and deal with whatever else I may find.
Then I will once again curse mercedes for building a mechanical princess.
I ****ing screwed up when re-installing: left the shop towel in there, and the turbo proceeded to chew it up and **** it in the intercooler.
Took a good hour of shop vac + fingering to get as much as I could out. Thank Heavens the intercooler collected all the pieces. I hope and pray that the turbo blades didn’t suffer from this bout of abject stupidity. Maybe they got cleaned, who knows.
02-02-2020, 07:43 PM
