M112 engine recurring misfires on middle cyls under hard acceleration - PeachParts Mercedes-Benz Forum

Can't Know · #1 09-04-2010, 03:41 AM

Hey, folks.

Newb to the PP forum but been on some others in the past. You guys seem to have a few more technically oriented members so I thought I'd roll this past you. I got a problem that I can't get to the bottom of and don't really want to drop lots of cash with a shop letting them guess at it. Plus I like DIY, not just because I like to save money.

I have a 2001 E320 with the V6 used in so many other models. Currently about 120,000 miles, all maintenance and repairs are current. Also replaced all the cats for the rattle deal but paid out of my pocket (!). For the most part I do my own repairs, not a fan of running to the shop for things.

I'm pretty mild in the way I drive, got the hotrodding out of the way when a kid. However, I do get on it every now and again, which is when the other problem crops up. I don't know if they are related, just these three problems.

The only symptoms I have are these. When the engine is fully warmed up, there is a very gentle vibration that can be felt at idle in gear. (No, I already changed the mounts). It can be felt when the belt is off and goes away if you simply step on the gas a tiny bit and raise the rpm a hundred or so. (I have a feeling it would be there with the engine cold, except that the rpms are up higher, so you can't feel it.)

But this is really the most bothersome problem. Under hard acceleration, as it gets up around 4,000 rpm, the CEL flashes and the ECU shuts down the middle cylinder on each bank. Well I assume it is shutting down those two because of the resulting rough running, stumbling idle (as though two cylinders were shut down, that clears after a restart) and the only codes that show up are P0302 and P0305 (and of course 0300). It doesn't have to be full throttle, but it has to be more than moderate acceleration.

Last, the mileage is not quite what it once was, I get about 28 on the highway and used to get 30-31.

I don't know if these three niggling things are related (I almost suspect they are but probably that's just wishful thinking). I have read some threads about a miss over 4,000 rpm, but that doesn't seem to be the same as this one.

Engine-related things I've done (mostly as regular maintenance, not trying to chase this): MAF (genuine Bosch, not the insert piece), plugs (iridium), wires (Bosch), KPS, cleaned the EGR tube and valve, breather hoses, motor and trans mounts, cats (all four), O2 sensors, air filter, fuel filter. I've run seafoam and techron through the tank (up to two cans/bottles per tank).

The only thing I sort of wonder about is carbon buildup in the combustion chamber because I don't drive it hard (and really can't now anyway) as there have been a few threads about that causing an idle vibration. But I can't think how that would cause the miss/cylinder shutdown on hard acceleration.

I haven't tried swapping coils or injectors, since it's always those two middle cylinders and it doesn't make sense both of those would act up at the same time and every time. I think about fuel delivery, but I don't really want to just throw an expensive fuel pump at it. There is plenty of pressure at the rail, I didn't measure it but it sprays out good.

Anyway, that's about it. Any useful suggestions are welcome, particularly since with this economy I may decide to start running booze or something and you can't get away if you can't floor it.

I appreciate you taking the time to read my tale, have a good long weekend, hope you have some good plans.

Can't Know · #2 09-06-2010, 02:09 PM

Just a bump since it's off the first page and it's been a couple of days. I think maybe I feel good that no one leaped in right away with an idea (be sad if it was something that obvious), but I'm hopeful someone will.

david s poole · #3 09-06-2010, 02:45 PM

first thing comes to mind is idle vibration cured by an extra 100rpms.that screams vacuum leak.i would check the hoses around and underneath the mass air flow.found one off there once before and it did not appear to be that way until very close inspection.this fuel injection system will shut down fuel to cylinders if it detects lean missfire and i think something changes in the intake at about 3750.just rambling hope it gives ideas.

TimFreeh · #4 09-06-2010, 03:18 PM

Several members that had cars that were throwing random misfire codes during full throttle acceleration had problems with exhaust system back-pressure. You mentioned that you had changed the cats because of a rattle, were the replacement cats OEM or aftermarket?

Have you checked the exhaust system for excessive back-pressure?

Whiskeydan · #5 10-04-2013, 12:22 PM

I wonder if the OP ever sorted this out?

I have a very similar problem with a 2002 E320. Rough idle that seems to be a random misfire. Worse when the engine is warmed up.

About a 1K miles ago I had a CEL. Code indicated drivers side upstream O2 sensor. Although, I didn't notice the rough idle at that time and the car seemed to run fine.
Prior to replacing it I noticed the idle degrading.

I replaced the O2 sensor and reset the CEL, rough idle remained. The car has roughly 142K and not knowing when the last time the plugs were replaced I figured it would be a good time.

I used NGK 7090 plugs which appear to be very close to the original plugs I removed. The old plug electrodes are well beyond their useful life. All twelve had the same appearance, no fouling, same brownish grey color. I also checked all the plug wires for proper 2K ohm resistance plus checked them for leakage from center to ground as I have seen this before in the ML55 we own.

With everything back together the idle is slightly better but still not right. Revving the engine to 3-4k gave me a CEL. This time P0305 cylinder 5 misfire. It must be right at the threshold of fault range. Cleared the code and tried revving it again. No CEL this time but still has an obvious intermittent miss at idle.

Not sure where to go at this point.

Whiskeydan · #6 10-05-2013, 12:34 PM

No CEL yet but it's still not idling as smooth as it used to.

I may take an O-scope and look for a missing pulse near the coil wires and see if I can verify which cylinder it is. I'll start with #5 since that was the recorded miss fire.

Edmund · #7 01-29-2014, 01:25 AM

Did you ever resolve this idling issue?
I have exactly the same issue even on the same cylinder.
I checked the sensor values with an OBD reader and all seems normal except for bank 2 Lambda sensors. I used OBD car doctor that draws a graph of the values. Everytime I feel the 'missfire' in the car the voltage levels on the 2 sensors drop to about 0.15V indicating almost no burnt or unburt fuel indicating a problem with the injector.

My next step is to connect the scope to see the waveform sent from the ECU, but if the ECU can just turn this signal off because it detected a missfire, the problem can be elsewhere...

Any advice?

Vehicle : Mercedes WDB2100622Bxxxxxx E240 2001