Stall under high acceleration C240.

Custador · May 29, 2012

Hello! I've got a lovely little R-Reg C240 Elegance which has had a few issues around misfiring - As in, it was a bit like driving an underpowered tractor. I've resolved them by replacing the plugs, the MAF and one of the injectors, however there now seems to be a new issue:

I got tucked-in behind a truck this morning on the motorway and decided to give the V6 its legs now that it's fixed, so floored the gas (I was doing about 60MPH at this point) to pass. It cogged down a gear, revved up to maybe 4500 to 5000 RPM, made a noise very much like valve-bounce for a second or two, then stalled. I coasted it in, restarted, and all was fine. I tried it again on a quieter road just to see what would happen - Same thing. Floored it, roar, valve bounce, stall. Restarted, ran fine.

It was on a diagnostic before I did the MAF sensor, which told me the injector was faulty (I did the MAF just because I happened to have the engine exposed for the injector and figured, what the hell, it's bound to need changing soon anyway). It showed no other faults.

I'm thinking maybe I need to de-coke the engine (which would surprise me, I use injector cleaner every thousand miles or so), or my coil-pack is on the way out. Anybody have any better ideas or has experienced this before?

marvinstockman · May 29, 2012

Remember, every cylinder that's misfiring is shut down to prevent damage to the cat. On restart, they are turned back on, until a misfire is sensed. If you have 5 cylinders misfiring, all 5 are shut down.

Custador · May 30, 2012

True, but multiple misfires would show in the ECU memory on a code-reader. The only misfire was on cylinder 4, and that's now fixed. I have no idea why the rev limit should suddenly be 2500rpm less than normal, or why the car would go into safety mode at high revs. I'm going to disconnect the battery for an hour and reset the ECU, see it that's the issue :-/

marvinstockman · May 31, 2012

I can't tell you what's wrong, but the rev limiter won't stall the car. The car just kind of 'bangs' against the limiter.

benzboyz · May 31, 2012

you can never have 5 misfiring cylinders at the same time..max is 2 at a time, but it will be stored on the control unit

benzboyz · May 31, 2012

you might have a faulty ignition leads or ignition coil which fails during high loads

Custador · Jun 1, 2012

That's where I'm going with it, since now when I ease off the misfire continues until I turn-off and re-start the engine. I'm assuming that the ECU is detecting one or more misfires and turning those cylinders off. Like I said, it *was* cylinder 4. So I figure I'll do the ignition leads first, and if that doesn't work I'll change the coil pack.

Custador · Jun 1, 2012

Incidentally, I've never done this job before (I don't even know how many coil-packs there are on the car), so if anybody knows of a decent how-to guide, I'd really appreciate a link!

Custador · Jun 2, 2012

Aha. There are six. So, Plan A is to get some live data - Find out where the misfire is, swap over the ignition leads with another cylinder and look again, if need be swap over the ignition coil with another cylinder and look again, and hopefully that should tell me what I need to replace. What a colossal pain in the arse!

benzboyz · Jun 10, 2012

clearing the fault memory doesn't fix the problem, you have a intermittent fault which you need to find, if the fault happens again read off the fault memory and concentrate on that cylinder. do interchange items at a time so you can find out which parts is giving you this problem.

Stall under high acceleration C240.

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