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Engine misfire cylinder 6 only at high rpm?

25K views 35 replies 9 participants last post by  megadamp 
#1 ·
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 ·
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.
 
#3 ·
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.
 
#6 ·
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
 
#7 ·
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.
 
#10 ·
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.
 
#11 ·
There are so many reasons/parts that can cause a misfire. You have checked none, but now you are ready to replace O2 sensors, because another member had a similar problem. What if I told you the vast majority of misfires are caused by either ignition wires, spark plugs or ignition coils, or a combination of.

If you don't mind spending $180 or so to find out that the O2 sensors did not cure your problem, then go right ahead and buy them and have them installed. All in all. what I am trying to get across to you is that this is not the proper way to diagnose or find a problem.

Here is an illustration of the location of the O2 sensors.
 

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#12 ·
Thanks for the feedback and image.

I am not sure where you got the idea I have checked nothing?

You also said:

"What if I told you the vast majority of misfires are caused by either ignition wires, spark plugs or ignition coils, or a combination of."

My independent mechanic did have all the plugs and wires and the #6 coil replaced already. He ran freeze frame with the truck on a dynamometer, told me he only saw a misfire on #6 and nothing else and was pretty confident that it was a valve train problem at around 5000 rom. Unless the new plugs, wires and/or coil are also bad, then I have to assume my mechanic is right about the valve train issue, but I am not completely convinced of that, since the way it occurs. Press the gas hard cel comes on immediately no lag.

I thought I mentioned all of this in previous postings?

The suggestion from mbenz80 that I switch the O2 sensors and see if the misfire changes to another cylinder bank is what I thought was a good idea. I am not planning to spend anything on replacing the O2 sensors at this time and never was. I am simply curious as to how difficult it is and what is involved. The image you posted helps.

However, since the truck is running the best it ever has and I have had it up to 4500 rom with no problems and no cel. I haven't tried to go above that and if I ever do and the problem reoccurs I figure that I really have only four choices:

1) Do nothing, turn off the engine and wait the cel to reset itself. Try not to drive
so aggressively.

2) If an O2 sensor is going bad I would guess it will continue to get worse then reveal itself, although mbenz80 didn't see that until 165k I have only 72k and at around 6k a year that will take about 15.5 years.

3) Find out what the actual codes are immediately after the cel comes on, see if new codes shows up.

4) Trade in or sell the truck and by a Japanese SUV. Actually I am considering a certified pre-owned ML from my local MB dealer and also purchase extended warranty and never own another Mercede$ Benz again without having coverage.

The truck is running the best it ever has so if some other problem shows up in the next few months #4 above will be my solution.

If there are any other suggestions please let me know, thanks for everybody who has replied so far.
 
#14 ·
My truck has 176K miles and I can say that in three and half years of ownership it is running the best ever since 130K when I got it.
I was playing "in the snow" today main streets empty and people stranded home(Chicago didn't see this much in years) and I was sure my CEL would come on because my RPM gauge was passing 5,000 every corner, orange triangle blinking and all systems working as they should. No problems at all.
Yes, I never say that you have to buy new sensors to find out if one is on it's way and if they are good, you don't have to switch them back because they are same.
The second one took me 10 minutes to replace including lifting up and supporting the front of the truck.
I could never exccelerate with my truck as I do today and I can post picture with all the original parts that I replaced before the sensors. I would of be very happy to pay $120 for those two sensors 3 years ago, if there would be any code pointing me to them.
You don't have to hate your truck or trade it in because of this, but it was bothering me as much as you. Even though I am mechanically inclined I could not get it and only last year I bought myself Actron CP9180 with live and freeze data to solve this problem.
You don't have to do what other members are telling you, but you can pick your best option and bring it up to your trusted mechanic and see what he thinks about it.
Bernard
 
#15 ·
First let me say until I got my ML I only opened the hood of a car to put in windshield washer fluid so I'm not much of a mechanic by any stretch.

I had the same problem on my truck for the past few months and it was getting worse, I went to a shop and got same kind of analysis (replace coil, wire, plugs, etc...).

I went ahead and got a scanner and read the codes myself the next time that CEL came up. I replaced all 4 O2 sensors (only 2 came up on the codes) but since my truck has 170K and I have no idea what the previous owner did i figure was the best thing to do. Also I had misfire in 3 cylinders and didn't have to replace plugs or any wires just replaced the MAF sensor and that fix the problem. Total cost $92.00 for Bosh OE sensors (x4) and 150 for Bosh MAF and I did all the work myself...Problem solved and the truck is running great

Just a note the left O2 sensor upstream is the only one that gave me a hard time to replace because of the location.

Good Luck!!!
 
#16 ·
Thanks for all of the feedback and suggestions.

The one thing that really bothers me is that my mechanic really believes it is a valve train problem. He based this on the fact that only #6 cylinder is having the misfire at around 5000 rpm. Since I haven't seen the data myself I can only assume he maybe right. I don't believe these is a code for a valve train problem so he is basing this on the fact that only #6 misfires. I would think he wouldn't say this unless he really believed it? Or that he simply doesn't know for sure and this is his best guess. He did not take the valve cover off that put a strobe on the valve train to witness a problem directly.

I am doubting his analysis only because the way it occurs. For example, floor it from a standing stop and immediately the cel comes on. I don't know much about these new cars with all of the computers and such but when I was a teenager I used to do everything even rebuilt a couple of engines. My experience would be that if a valve starting floating as to cause a misfire it would be a little more dramatic then what I am experiencing. I would expect backfires and other nasty symptoms, unless the misfire sensor (if these is such a thing) is triggered immediately and shuts the cylinder off in an attempt to save the engine. In this case it still doesn't mean that the valve floated it could be other causes.

Finally if my mechanic really believes that it is a valve train problem I would assume it will get worse, much worse over time and he would recommend that I have it fixed. This is not what he recommended instead he told me to stop driving like a teenager and it should be fine for a long time. I question him about this telling him I really don't drive like that but once in while I do want to jump on it, like getting on the freeway or uphill. He made it clear that as long as I don't go over the 5000 or so rpm the engine would be fine. This really doesn't sound like he believes the valve train problem just doesn't want to invest anymore time with it.

I still haven't tried to see if the problem is still there I am concerned it may cause a real problem if I do. Maybe I should invest in a scanner and check the codes myself after triggering the problem.

I do believe that if there is a problem either with a bad O2 sensor, MAF or valve train it will manifest itself over time but by then it might be too late :)

I guess for now I will just drive it for a while and see what happens. It is running the best it ever has since I bought it over 3 years ago. Hopefully it will continue to do so.
The trade-in value isn't very much so it is worth more to me to keep it and if something really bad happens I don't really loose that much.

Thanks again to everybody who offered suggestions, much appreciated.
 
#20 · (Edited)
There is an alternative explanation that will produce the same single-cylinder high-rpm failure scenario:

Consider the ignition coil rapidly producing spark pulses every second. At twice the rpm, it has to produce twice as many pulses per second and thus draws twice as much current. If the coil and/or wires are marginal, then they will work fine at lower rpms but fail at higher rpms. The spark failure will be very repeatable, probably much more so than a floating valve.

This is in fact what happened to my ML430. In my case, cylinder #3 would fail at high rpm. My mechanic verified that the coil and wires for cylinder #3 were failing at high rpms, and installing a new coil and wires (for cylinder #3 only) fixed the problem without running up a big repair bill. As a bonus, my engine runs stronger and my gas mileage improved.

DelJ




....The one thing that really bothers me is that my mechanic really believes it is a valve train problem. He based this on the fact that only #6 cylinder is having the misfire at around 5000 rpm.....
 
#17 ·
Let me ask you this about the valve. If it is floating it will make noise. Now in the truck
you may not hear it. Can you zip by a person that knows engines and see if they can hear
the valve? I have seen plenty of floating valves and every single one made noise.
By zip by a person I mean have them standing there as you drive in first at 5,000rpms.
 
#18 ·
I paid $80 for my scanner, used from a local guy on creigslist.
In recent couple of years I have rebuild seven 2.7L engines in Doge Intrepids, two of them had a water pump damage with floating valve after coolant and oil was mixed together, you could hear the tapping better and easier in lower RPM then in higher.
For 3 years, I couldn't dare to pass anyone from standing at red light and if I did, the next light I had to shut the engine off to reset the ECM, as you have to because there is no such a thing as misfire sensor, at least not yet.
The right code may eventually show up, as it did for me, if you still have your truck but don't give up on it, they are like a luxurious tanks!
Bernard
 
#19 ·
I tried today and sure enough as soon as it hit about 5000 rpm the cel came on. No clicking sound, cel comes on quickly and apparentely shuts the cylinder off, runs very poorly after that. Stopped the truck, turned off/on and ran very wel cel still on but I know it will turn off after several cycles.

I guess the next step would be to check the codes myself.

I would assume if something in the valve train is bad it wouldn't run so good but it runs great as long as I don't hit 5000 rpm, which in normal operation never appears to occur.

I guess my question is if it is a valve train problem wouldn't it be worse and even occuring at lower rpms or is this something I will see over time as it does get worse?

My mechanic said as long as I never hit the 5000 rpm it should never get worse, does this make sense?

All opinions are welcome and thank you.
 
#22 ·
Lets wait for the codes and see what they show now. I don't think the P0306 is there alone, if it is (and you rent a scanner to reset it) eventually it will come on together with other code that will finaly point you directly to the problem.
 
#23 ·
My mechanic already replaced all of the plugs, wires and #6 coil. The truck does runs significantly better but still has a misfire on #6 at around 5000 rpm according to the mechanic reading the codes. I guess it could be the replaced #6 coil is bad or marginal but I would think unlikely since it was obtained new from the MB dealer.

I took the car for a long drive for 200 miles, never hit 5000 rpm and it ran perfectly.

I plan to check the codes myself in the near future.

Does anybody have an engine layout to identify the cylinder numbers? I would guess it starts with #1 the engine front drivers side, #2 front passengers side, #3 middle drivers side, etc...

Thanks to all who have offered help!
 
#25 ·
And here is what I replaced to fix my misfire anytime I would pass 4,000RPM!
Now, I can't trigger the cel even if I want to and I did try few times already, since I have my own scanner, I had it over 6,000RPM a couple of times but after replacing those two front o2 sensors, never had a cel or code in 6 or 7months of daily driving. First real test was hard acceleration with 6,000 lbs. trailer hooked to it, all good.
I would not want to mislead anyone if I wouldn't have that experience and I think that is why we are all here at this forum, to help each other out.
Bernard
 

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#29 ·
the same problem

Was the problem solved?

I have the exact same problem on my ML320, 2000, with 163000 miles. It runs like this for at least last 30 000 miles. If I keep rpm-s above 4500 fore a few seconds it has misfire on cylinder 1, somitiems on 4 or both of them.

changed plugs, swapped the coils and it's wires.
And my car runs both petrol and CNG. the problem exists for both fuel, so injectors, pump and filter shouldn't be the problem. Tried 3 diffetent Maf and with no difference.
 
#30 ·
Since you have 2000 model, both, front and rear oxygen sensors are the same. Swap them out (without any cost) or buy new sensors for the front and from my experience, you will never see this problem again and you can accelerate as fast as you want to without RPM limit. I had the same problem, new plugs, coils, MAF, injectors, crank sensor, all OEM parts without help at about 155,000 miles and only misfires would show up as a code. After replacing from O2s with Bosch 13782 this problem NEVER returned and I'm at 236,700 miles on the clock. I just had to floor it the other day and I was pretty sure it will trigger out my check engine light, nothing, no pending codes and trouble free over 80,000 miles (knocking on the wood)!
Try it, Bernard
 
#31 ·
I have not had any problems for several years. I never "floor the gas" so I don't know if it would happen again, I have to assume it would. Maybe if I feel lucky I will try that again.

I have seen the rpm around 4500 but I have never had it over that.

They did replace the plugs, wires, one coil, MAF. I did replace the voltage regulator but that was a different problem.
The mechanic said something about upgrading the onboard software but I am not sure.
 
#32 ·
Bernard. You can lead a horse to water but you can't make him drink it. There has been much said by very experienced ML owner on here with some zero cost ideas to try and solve the problem experienced by the OP.
However he seems hung on it being a valve problem, and does not seem interested in getting the spanners out to try some of the very good suggestions. All in the hope that we as a forum can save other ML owners a few quid/dollars and not get ripped off by dodgy stealers.
There are many reasons for misfires, and without methodical tests can work out to be very expensive.
 
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