Engine misfire cylinder 6 only at high rpm? - Mercedes-Benz Forum
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#1 (permalink) Old 01-24-2011, 11:08 PM
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Unhappy Engine misfire cylinder 6 only at high rpm?

This is my first Mercedes I bought used in Jan 2008 a 2002 ML320 with 56,000 miles it was a great deal and looked almost new.

Now My ML320 has 72,000 miles recently starting having a problem during acceleration like on the freeway or uphill. When I press the gas to downshift the CEL would come on and the engine would run rough until I turned it off then runs fine. After several starts/stop CEL is off and it continues to run fine, until next time I try to accelerate fast.

I took the car to an independent mechanic and at first they said replace all the plugs and wires plus they saw a misfire on cylinder 6 replaced the #6 coil as well. OK, problem still there, upgraded/recalibrated the ECU, problem still there.

The mechanic now say that there is a floating valve on #6 at around 5000 rpm
that is causing the misfire and the ECU is turning off the #6 injector and turning on the CEL. He states that as long as I drive less aggressively it should be fine and the bad valve won't be a problem.

I respect this mechanic and I think he honestly believes what he is telling me.
However, my gut feeling is that this is a different problem, possibly MAF which he said there is no code for this in the log and if it was the MAF it would misfire on more than one cylinder. He says this is only misfiring on #6 no other. Unfortunately I don't have the codes.

I really like this car however if it is going to be a continuing problem, then it is back to the Japanese cars for me, where I have had better experience.

Any ideas or opinions are welcome and appreciated.
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#2 (permalink) Old 01-25-2011, 12:13 AM
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Well a floating valve will never get better.

Did he have the valve cover of and confirm the valve is not being fully articulated by the
rocker? If so is the valve spring or keepers intact? is it the intake or or exhaust valve?
I look at at long term issues and that valve could chew up the valve seat or worse
drop and hit the piston.
If the valve really has started to float doing the repair now can save thousands later.
I trust your opinion of his honesty, but it is time to look into the #6 valve, spring, and lifter
assembly if he is sure that is what is causing it.
I would really want to see it in person once he had the valve cover off if it were my truck.
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#3 (permalink) Old 01-25-2011, 01:23 AM
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Thanks for the reply and I agree with your comments.

The mechanic made it clear that the valve only begins to float at around 5000 rpm and that is what caused the misfire and subsequent CEL. Driving less aggressively below 5000 rpm will be ok and the "bad valve" will not be a problem unless it begins to float.

No, he did not remove the valve cover and watch the valve to witness the float, furthermore his attitude that this would not be a problem as long as I kept the rpm below 5000. I asked can he simply replace the bad spring or other hardware to stop the floating?
No, he said that the ONLY solution is a complete valve job $4000-5000, yea right, although he doesn't want to do it because most likely there will be other problems with this ML320 since they have a history of problems. I wish I knew that before I bought it but I thought that Mercedes made good cars and like I said I really like this car (when it isn't having problems).

I guess I am doubting this mechanic after all, I am beginning to think he is good for some things but diagnosing ML320 codes are not his thing. I also feel he knows that he cannot discover the real problem so this is his way of sending me off on my own, just don't drive to aggressively!

My questions are is a floating valve the only cause of a misfire at high rpm, even I know that there could be other causes. Plus when this condition occurs the CEL comes on immediately, like press the gas and light is on. I don't even notice the engine doing anything but the CEL comes on immediately and the transmission doesn't shift correctly and the engine runs rough until I turn it off, then everything back to normal.

Is it true that a bad MAF would cause multiple misfires on different cylinders or could it be just one.

I guess I need to take it to the dealer who I really don't trust, the service advisor at my local MB dealer actually implied that if have have to ask how much I really shouldn't own a MB!

It's a nice car, handles well, quiet and small V6 gets decent gas mileage, what to do?

Thanks in advance for any help or advice.
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#4 (permalink) Old 01-25-2011, 02:46 AM
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If its at high revs it could be one of your cats is blocked...just a thought good luck!!
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#5 (permalink) Old 01-25-2011, 03:13 AM
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Thanks for idea, but how do you check for a blocked CAT?
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#6 (permalink) Old 01-25-2011, 10:07 AM
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I had this same issue for over a year, replaced many many parts with new oem (plugs, wires, coils, fuel rail with injectors, cps, maf) and still could not take off with it (accelerate) very fast, some times as fast as I would like to. Anything over 4,000 rpm will give me multiple misfire, or one cylinder misfire. Had to stop, turn key off, restart and continue driving as nothing happened and wait for the cel to go off on it's own.
I never have this problem any longer! Because finally the right code popped out P0150along with misfire P0306. After changing front oxygen sensor at 165K miles (bank 2 sensor 1) with oem Bosch 13782 car was fine for another 4 months till the other front oxygen sensor P0130, P0135 intermitent code showed up at 171K miles.
Replaced the other O2 front sensor (bank 1 sensor 1) and NEVER since then had the misfire show up again.
I'm hauling Dodge Intrepids home with my ML to fix timing problems on them, did 7 of them so far. I would take off with 5500 lbs. trailer as fast as it will go, floored it many times since new sensors were installed and no CEL, even if I try.
This past Saturday, I had to install new fuel pump, filter and lines. Took the truck for test drive on Sunday, passed 6,000 RPM to get cel (since I own good scanner with freeze data) or to see fuel starvation and nothing! Drives perfectly smooth.
If you believe me, please swap both front O2 sensors between each other and see, if that misfire will come up on the other engine bank (cylinder 1, 2 or 3).
This will cost you only 20 minutes of your time.
Even if you buy new sensor, it will be $70 compare to diagnostic bill for Mercedes dealer.
Good luck,
Bernard

1999 ML320 230K miles
2001 ML320 162K miles
2003 Lexus GX470 285K miles
1980 280SE W126 EURO 142mph top speed (sold)
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#7 (permalink) Old 02-01-2011, 09:16 PM
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Thank you your insight and experience.

I am somewhat mechanically inclined enough to be dangerous. Where exactly are these O2 sensors located and exactly how many are there? I thought they are near the catalytic convertor? One before and one after?
After replacing an O2 sensor does the car have to be adjusted for air/fuel mix or something like that or is this just a remove replace type of job?

BTW the truck is running the best it ever has since the mechanic replaced the plugs, wires and #6 coil. No rough idle and quick acceleration. I have seen the rpm go to about 4500 with no problems, I am afraid to try punching it or anything over 4500 rpm, although the CEL would eventually go off.

Thanks you very much, this may be worth a try.
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#8 (permalink) Old 02-01-2011, 09:29 PM
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It is just an R&R. If one was really bad it would show a code, marginal ones
will not. A scanner that can read the O2 voltage while the truck is being
driven can tell you how fresh an responsive the O2 sensors are.
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#9 (permalink) Old 02-01-2011, 09:34 PM
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where are they located and how difficult to r&r?
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#10 (permalink) Old 02-01-2011, 09:40 PM
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You said it, before and after the cats. The difficulty depends on how badly the threads
have gotten. Some come right out. Others make me want to throw tools.

As far as price autohausaz is pretty good on them. I would not buy all four. Get some live
data and see if there is a lazy one. I have never seen all four go bad unless the owner
ignored the vehicle for a substantial amount of time.
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