jwp2046

I have a 2010 ML350 Bluetek with 96000 miles. It has run great until now. While an a road trip it started running rough. I nursed it home. I have low compression in the #3 cylinder. I took it to a mechanic and at first he thought it was the injector. The injector was OK. After looking for a day he suggested I take it to the dealer 75 miles away from me. They claimed the problem was gasoline in the diesel and refused to touch it unless they change the diesel. (have you ever seen a diesel engine try to run on gasoline?) That was a huge waste of money and then they said it was the #6 cylinder. After 2 more days and me telling them it was the #3 they finally agreed but could not find the problem. Their solution---tear down the engine at a cost of $4,000. No way I said. It runs real rough under 1800 rpm and smooths out mostly above that. I was forced to drive it about 300 miles with this problem. I am now back to the original mechanic who has years on MB. HELP! has anyone seen this before. Plenty power above 1800 RPM. Real rough at idle.

Carsy

What did the dealers star diagnosing computer say, any engine lights?

Did they do a compression check on all cylinders.? If #3 is low ,I would assume that is your problem ie valves or compression rings.

Thinking aloud ..if its OK under way at load it is not air related eg turbo , inlet flaps, EGR etc.

So that leaves compression & fuel ( injector).

Interested to know what you find.

samaritrey

What were the compression numbers for all the cylinders? The dealer and the Indy should both have them so make sure they gave them to you.
How did they determine it wasn't an injector?
Glow plug sealed okay?
Did they check fuel pressure (doubt it is an issue)
Finally any start up noise that could possibly be timing chain?

jwp2046

The compression was first checked with the computer then checked manually on all cylinders. The compression was low and I did not write the number down. I was told it was 60% lower than the rest. Glow plug was pulled to do the compression. No start up noise other than a loud rattle / rumble. No issue on starting. This all started while on the road traveling.

Carsy

Sounds like a valve worn/chipped /not seating to me.

It it was a piston ring problem you would likely experience higher than normal blow by & sump ventilation flow.

I wonder whether it would be possible to insert a scope into #3 to view the valves condition?

jwp2046

Thanks for the reply. I having been wondering if that were the problem. My concern is that it happened suddenly while driving. Up to that time it ran fine with no hint of a problem. I follow the maintenance schedule.

Ausmbtech

I would be doing a leakdown test on the bank with the low compression cylinder. If the cylinder with the low compression has the same leak down as the unaffected cylinders then you may have a bent conrod. If the leakdown is higher on the low compression cylinder then you'd be looking for leaking valves or piston rings.

It's important to do these tests prior to pulling the engine apart.
When the engine first started making noises/running rough was to cruising or under a heavy load? Was it a sudden change in engine running?

jwp2046

Thanks for the reply. I was at freeway speed and it suddenly started running a little rough. I noticed it as being a little odd and at first thought maybe it was a rough patch of freeway. I pulled off at the next exit and when at idle it was real rough. I drove home (about 95 miles) with plenty power and at 70. Between trips to MB dealer and local independent I have put about 385 miles on it. Currently it is sitting parked while I figure out what to do.

Ausmbtech

Try not to drive it any further unless you need to.
It may have burnt a small hole or crack in the piston. It certainly shouldn't have occurred at such a young age.
Do you know if the engine has ever had any injectors replaced? Incorrectly coded or not recoding after a replacement can in some cases cause premature piston damage.

jwp2046

Thanks for the reply. It is not acting like a cracked piston or hole. Not sure how that would act on a diesel. When RPM get to 1800 it seems to smooth out. Still noticeably rough but plenty power. The first thing done was to change the injector with a new one and that did not help. Then the mechanic swapped injector from another cylinder and that one did not not work either. When put back in the original cylinder it worked. Also, I did not mention that the CEL did not come on until I drove a few miles at lower speed.

Carsy

A little light reading for you

"combustion chamber, there are only four places for the compression to escape: through a leak in the combustion chamber (cracked head or leaking head gasket), through a leaking intake valve, through a leaking exhaust valve or through a leak in a piston or piston rings. I find it easier to find where the compression went and in these cases, it is easy to check for pressure pulses in the intake manifold, the exhaust pipe, the engine crank case or in the engine cooling system. This process will require a labscope and a FirstLook sensor, but in most cases the process will be quicker and more reliable than having to remove parts and engine components in an attempt to gain access to a place to get an actual compression reading. - See more at: http://www.knowyourparts.com/technic....TBKkHf7P.dpuf "

Source :- http://www.knowyourparts.com/technic...art-diagnosis/

Here is some info on Borescopes. A large workshop should have one :- http://www.olympus-ims.com/en/standa...id-borescopes/

Interested to know what state you live in & what fuel you use ? Bio Fuel ?

jwp2046

Thanks,
I live in North Florida and I doubt there is Bio Diesel here. I use the recommended fuel. I have driven diesels since the time when you could get it a lot cheaper than regular leaded. I learned a long time ago to use a station that pumps a lot of diesel. This situation has me totally puzzled. I really appreciate all the help on this forum.

jwp2046

Well, here is the scoop on this problem. The mechanic (independent) took the engine apart and found the problem. (1) excess carbon in the intake (2) The exhaust valves in the number three cylinder are not sealing due to carbon build up. Therefore this cylinder has no compression. (3) One exhaust valve in the number 2 cylinder leaking (carbon) causing a drop in compression. (4) An O Ring on the cam shaft is about to blow out. Therefore the whole cam shaft has to be replace. If this O ring were to blow out it would be a complete disaster. AND of course you can't just put in an O ring. You have to replace the complete cam shaft. THANK YOU MERCEDES! I need to know if this is common with this engine,

I have driven MB diesels before for thousands of miles and never seen anything like this. I watch were I get diesel and follow the recommended maintenance schedule. I change my oil at around 5k miles. How does an engine like this build up this much carbon? I put 280,000 miles on a Ford diesel truck and never saw anything like this. Most of my miles are highway and freeway.

Is this a common problem with this engine and MB diesels in general???

Carsy

Disappointing after only 96,000 miles. It should be just run in. Sorry it was not an easier fix.

Cannot believe you have to replace the whole cam for a seal. Why is this so?

Interesed to hear the reason for the carbon build up.I wonder how clean the diesel is in the US?.

jwp2046

The cam shaft has a small "wheel" on it. That wheel has an O ring that has partially come out. It has something to do with the oil flow and pressure maintenance. Well, in Mercedes thinking you can't replace that one small part; you have to replace the entire cam shaft. If the O ring blows out it will cause a major failure. I am now wondering if this may be the cause of engine failures that I have read about in the forums. I too would like to know why so much carbon build up. I don't buy bio diesel and most of my miles are road miles.

samaritrey

Do you baby your car or do you drive it hard? Guess I need to check out my car at some point.

Carsy our diesel is crap compared to Europe but I have heard that it might be getting better but to be honest haven't looked into it.

Sadly the old ford engines could run on almost anything where modern MB engines and even the fords require very good quality diesel. I know someone that ran a 90's powerstorke on used trans fluid.

Ausmbtech

Yours would be the first time I've heard of carbon buildup holding an intake valve partially open. I've seen and heard of it a number of times with petrol engines but never a diesel and I've seen some pretty coked up intakes!.

The cause of the carbon in modern diesels is due to the very high use of EGR to control NOx emissions.

Unfortunately in markets with poor fuel quality EGR causes a lot more carbon in the intake than in other parts of the world.
I'm still surprised now that particulate filters are so common that they don't pickup the exhaust gas from after the filter rather than before it like they do in heavy vehicles which minimises the carbon buildup in the intake.

It's good to hear that it's not an engine failure but i'm sure it's still expensive none the less.

Something for us to bear in mind for future rough running problems.

I'm still not sure what cam seal you're referring to, the only thing coming to mind is the welsh plug in the end of the cam (they are hollow and filled with oil). Can you please get a bit more info or pictures of this seal?

jwp2046

This is a picture of the the cam shaft and if you look close you can see the o ring. If this o ring ruptured it would have been a disaster for the engine. According to MB there is no replacement for this part on the cam shaft therefore you get to buy a new one for a lot of money. It was the exhaust valves that are leaking.

Carsy

Thanks for the photo.
I am not an o ring expert but my core trade is a fitter & machinist .
O rings generally need to be slightly expanded then lifted off their seat to be removed.
Is it so in this case ?.
If so MB is taking its customers for an expensive ride.
If not the o ring must be a more complicated animal & the engineer designing it should be sent out to grass as there is generally no need for this expensive sophistication.

jwp2046

The part (wheel) that holds the o ring in cannot be removed. My mechanic tried and even contacted some tech friends that work at MB. If the o ring blows out it causes a major failure and if it "wears out" it has to be replaced which means the complete cam shaft. My mechanic has worked on Mercedes for almost 40 years and he states that everyday he sees things like this, on the new MBs that cost a lot of money for no real reason.

I hope someone with experience and knowledge can give us more information on this.

Ausmbtech

That o-ring failing wont cause an engine to seize but I think you just found the cause of the excessive carbon buildup.

That device on the end of the cam is a centrifugal oil separator. It helps minimise the amount of oil escaping via the crankcase breather system. Early engine only have one, I think later 642's have 2.
The o-ring failing wont release lubrication oil pressure but it will make the separator less effective and more likely allow a much larger amount of liquid oil to escape up the ventilation tube. The larger quantity of oil in the intake combined with EGR makes a much bigger than usual sticky mess post EGR valve.

I'd try and repair it, failing that you could probably try and seal it with epoxy.

I think these early 642's are getting to the age now where a seperate oil catch can might actually be a necessity in the aftermarket rather than just a waste of money.

jwp2046

Thanks for the information. That makes a lot of sense and would be the cause of excess carbon build up. Is it possible to repair or rebuild the centrifugal oil separator? All the diagrams I have found seem to indicate that it can't be removed. I guess all the money for a new cam shaft will be worth not having another valve problem due to carbon.

nelbur

I feel sure that that o-ring can be installed the same way it was originally installed. I know nothing about this engine but I would work the o-ring all the way from the small end of the cam shaft and if I was successful at getting it up to the old ring, I would swap it with the old one and work or cut the old one off. O-rings are fairly stretchy and can be worked over things.

Ausmbtech

If the plastic centrifuge at the back of the cam doesn't side off easily (don't force it too much, it's likely to break) I'd just seal the gap between the camshaft and the separator with a thin layer of high temp oil friendly engine sealant. It only has to seal air getting through, there is no pressure on the joint. It's no ideal but without the plastic part being removed from the cam I doubt you'll get the oring back in place properly.

jwp2046

Thanks for all the help and information. All the options on the o ring are good. but the speed of the cam shaft negates most if not all suggestions. After talking with techs at MB and other mechanics I considered the risk benefit and replaced the cam shaft with a new one. THERE is no way to fix the o ring. You just get a new o ring on a new cam shaft. ALL of this for a EGR system to control diesel pollution.

The problem with the rough idle and no compression on #3 was carbon preventing the exhaust valves closing. The same on the #2. We never had these problems with diesels before all of this. The head was pulled; the cam shaft was replaced and the valves were cleaned and now it is running again.

HOW DO I PREVENT THIS FROM HAPPENING AGAIN. There has to be a way to prevent this from happening.