I see many postings of this problem, but no final solutions. I'll post the solution when I finally succeed!

1999 ML320, 200k miles, began registering misfires on ALL cylinders, all of a sudden. I saw previous sloppy work on the RIGHT side catalyst, so I replaced from the exhaust manifold to the muffler with a Walker replacement. Used existing O2 sensors -- but success! -- on the RIGHT side at least!

So the cylinders 1,2,3 misfires are now gone. But cylinders 4,5,6 misfires (and the random misfire code) remain and persist. Assuming the catalyst, I replaced the entire LEFT side exhaust from the exhaust manifold to the muffler (again, a Walker replacement), and I replaced both O2 sensors on the LEFT side. It now sounds great -- and will idle with no codes -- but when you hold throttle anywhere above idle, it'll throw cylinders 4,5,6 and random misfire codes, and proceed into LIMP mode. UGH.

No way all spark plugs and/or wires and/or coil packs went out at once. So what next? Intake manifold leaking on that one side? Surely it is something common to the LEFT side. Could it be a bad O2 sensor, though they are new Bosch? Is there any way to diagnose them by viewing "Live Data" on a common OBDII code reader? Any other possibilities?

Thanks for any help offered. And again, I'll post the steps to success when I achieve it!

 

Follow-up - I sprayed brake parts cleaner on the intake manifold at the head and it had zero effect on the way the motor ran (sprayed while idling).

Please - what else to try?

 

BTW, I should note, the "misfires" are never heard nor felt - engine sounds excellent until the computer puts it into limp mode.

 

I had a similar problem. Catalytic converter crapped out. Do you hear a rattling from underneath?

 

nope, never heard rattling. but it did sound like a tractor on the right side from a sloppy weld earlier in its life!

 

Random misfire codes often mean that you need to replace the ignition wires - they lose conductivity over time. It's a cheap and easy fix, and therefore a good place to start. I would replace the spark plugs at the same time.

When one cylinder bank starts giving you problems, it does suggest to me an exhaust problem. This is quite likely on this vehicle at that age and with that mileage.

 

yep, but I have replaced the entire exhaust on both sides now. plug wires and plugs, not yet. just seemed strange to me they would all three go out at the same time. plus, I do not hear nor feel *any* misfires until the ECU puts it into limp mode - and even then the effect is not a misfire. thanks for your help!

 

EXCELLENT - I am now running code-free! I wish I had done these things one at a time, and tested after each one - but I did not. I'll not repeat previous posts - I'll pick up where I left off - Mercedes Misfire Solution...

I'm not sure which of these did the trick. 1) I changed the oil and oil filter - gerbils are much quieter now when cold. Mann filter, Mobil1 0w40 synthetic. 2) I replaced the plugs on the RIGHT side (which was not throwing codes) because I felt unbalanced with only changing the left. The right side plugs were in the same shape as the left - nice ceramic but worn electrodes so a big gap. 3) I did exchange plug wires between left and right banks and tested - still threw the same codes - so I knew wires were not the problem. 4) I noticed a small oil leak on the valve cover on the right bank above cylinder 2 - oil had leaked into both plug chambers - wasn't causing any issues though - but I cleaned off the oil and gently snugged up the valve cover bolts. 5) I took BOTH new Bosch O2 sensors off the left side and put the old ones back on (Mercedes branded - not sure how old). This was just a notion - there has to be a way to test those things prior to installation... Now all 4 O2 sensors are the old ones. 6) I took off the Transmission 13 pin connector and found it completely swamped with oil - in fact, that connector leaking was the source of oil all over the tranny pan. I have pics if anyone wants to see. What a mess. I sprayed the plastic harness with brake parts cleaner and quickly swabbed and blew air to clean. I did the same to the connector "port" on the tranny. 7) I did all of the above (OK, except #3) with the negative battery cable undone (1st time I've disconnected the battery in this whole process).

And, success! I drove it around for 15 minutes *trying* to get it to throw codes, but it did not. Then, my daughter drove us both (it's her car...) to the DQ for Blizzards (20 minutes away) - no codes!

However, I will end my long story with a sad admission of guilt - I spilled my Blizzard - the entire mini-Blizzard - right between the seat and center console...

More folks helped on the benzworld forum - check out the discussion there, same thread title. And, please feel free to offer enlightenment on which steps or combination thereof you think did the trick!

 

Good to hear that it's working for you now. Did you replace the the tranny connector? If there's oil in there, it means the O-ring has failed, and the problem will persist. I've done this on three vehicles now with these 5-speed trannies. It does require changing the ATF, but that's a good idea anyway.

 

Marc, I did not replace that tranny connector - know of a thread or video that shows how?