M275 Idle misfire (or return to idle misfire)

pillow

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Hi all,

Does anyone know what can cause a misfire during idle only, or that can be provoked by poking the throttle pedal and letting the car roll back to idle (it then stumbles a bit when it hits idle rpm).

Codes aren't being set but they can be felt, and are detected by the fault counter. One cylinder (nr. 5) is notably worse than other but it occurs across about 9-10 cylinders. Cylinder 5 has read up to 5 faults on the counter before it resets.

When being driven under any conditions there is nothing registered on the fault counter under any driving style. This only happens at idle.

Any ideas?
 
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pillow

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Vacuum leak?

It does feel a bit like when you have a vacuum leak and you just close it up. Idle drops/stumbles in a similar way.

Got a smoke tester en route so hopefully that will identify any leaks.
 

alexanderfoti

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You will probably find loads of small leaks, and do check the one way valves are still functioning.

Also, dont rule out engine mounts etc, as these often feel like a misfire/rough idlle.
 
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pillow

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Did engine mounts last Christmas. Bit of a fiddly one.. I'd hope they last longer than 6 months!

In poking and prodding various bits the purge line to the manifold snapped while I gave it a little twist right at the manifold and now I have a rather large and obvious vacuum leak. I've got a new line and new PCV to intake manifold line ordered from MB so I'm hoping that was the main issue (I had to twist the purge line put of the way when I removed the passenger side cam cover. It didn't break at the time but I'm hoping now that it perhaps cracked and that is the cause of my issue)

As you say smoke test will probably reveal more leaks. I'll probably sort out any other small leaks that show up over the summer.
 

alexanderfoti

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If they are MB oem, then unlikely they are short by then! Some of the cheaper aftermarket ones though....

But yeah, those lines will do it every time, they go hard and brittle, as does most of the rubber connectors.
 
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pillow

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So I've checked the lines highlighted in green and they held vacuum indefinitely.

I highlighted the boost line in blue because that doesn't draw on manifold vacuum. So have ignored it.

I can't check the remaining lines without pulling it off the manifold and that's not easy to do without dismantling a fair bit.

Replacing the evap hose and pcv line helped a bit but still cylinder 5 and 12 are the main two that have any issue.

Smoke test should be here in this coming week so we'll see what that turns up. If it doesn't turn up anything I may have a look at the injectors.
 

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Uncle Benz

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That's very interesting! Do you think it was allowing a vacuum leak, or something else?
 

alexanderfoti

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That's very interesting! Do you think it was allowing a vacuum leak, or something else?
I do wonder if the increased opening was preventing the throttle body from making very small adjustments near 0% opening.
 
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pillow

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Really not sure as others have reported no issues when using the larger gasket.

I haven't been able to find any discernable leaks and have smoke tested which would show up leaks that would affect idle. I did suspect I may have an absolutely tiny vacuum leak inside my brake booster but this will be harder to prove until I can pull the damn vacuum line off (it's a stubborn one) and use my vacuum gauge on the booster.

I have induced a vacuum leak by unhooking the PCV line and it causes issues mainly with cylinder 11. Not sure why but this throttle body gasket caused an issue only with cylinder 5.

In any case no more misfires in the counter at idle so I'm happy.

Now I just need to muster up the strength to put the new gearbox mount in!
 

alexanderfoti

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Really not sure as others have reported no issues when using the larger gasket.

I haven't been able to find any discernable leaks and have smoke tested which would show up leaks that would affect idle. I did suspect I may have an absolutely tiny vacuum leak inside my brake booster but this will be harder to prove until I can pull the damn vacuum line off (it's a stubborn one) and use my vacuum gauge on the booster.

I have induced a vacuum leak by unhooking the PCV line and it causes issues mainly with cylinder 11. Not sure why but this throttle body gasket caused an issue only with cylinder 5.

In any case no more misfires in the counter at idle so I'm happy.

Now I just need to muster up the strength to put the new gearbox mount in!

Ah fair enough, if it runs fine then leave alone!

GB mount is easy, takes longer to jack the car up than to change the mount :)
 
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pillow

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Ah fair enough, if it runs fine then leave alone!

GB mount is easy, takes longer to jack the car up than to change the mount :)

Do you know if it can be done without removing the cross member?

As long as it isn't as bad as trying to do the engine mounts. That was a right faff when I tried it.
 

alexanderfoti

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Do you know if it can be done without removing the cross member?

As long as it isn't as bad as trying to do the engine mounts. That was a right faff when I tried it.

Engine mounts are a far far larger job.

It can be, but it's 5 mins to take out the bolts and remove the crossmember. Either way, gearbox needs to be supported to get the new mount in.
 
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