R36 single cylinder misfire and potential coolant leak

[JaysBlueR36]

Hi all,

I've got an R36 which has become a bit of a pain. There are no fault codes yet.

It is leaking coolant somewhere very slowly and it's not obvious were. Pressure tested and pressure held. All usual places checked - no signs of a leak. We wonder if it's burning off as soon as it leaks out.

It has also got a single cylinder misfire which isn't very obvious anywhere else but on idle. It's most prominent in PARK but seems to smooth out under load in DRIVE. You can feel the vibration in the cabin particularly on start up from cold. We've changed the ignition coil and spark plug. This improved it for a bit but 24 hours later it was missing again.

It developed a hot start issue briefly when I first had it and on about 2 or 3 occasions struggled to start and then once started revs would only be held at 200-600rpm it would then come up to normal idle speed. This has now stopped happening and would normally only be the case after shortish journeys.

It had a new coolant temperature sensor and the throttle body cleaned out recently. We do not think the head gasket has gone as misfire is only in one cylinder and there is no white smoke out the back. It's possible there is a crack in the head or block but again wouldn't this be unlikely?? Timing is fine, there is about 3 degrees between cam intake and cam exhaust. No rattle on startup.

Are there any obvious things I'm missing.

Thanks in advance for the help

[MIG]

How many KMs are on the car? Have you checked what cylinder is having the misfire via a scan tool?

Getting a bore scope and checking each cylinder and the intake valves for carbon buildup might be a good idea; you can also see if water is getting into the combustion chamber.

[JaysBlueR36]

Car is on 111k it's been suggested as its direct injection that it could be carbon build up on the valves.

VCDS just shows cylinder 2.

No water in chamber. Plugs not wet with oil either.

[MIG]

Carbon build-up does cause idle issues and misfires under load when cold, but I had misfires on all cylinders.

See this for what my valves looked like after 140,000km B7 Retrofitting - MIG's Passat


If you have checked the spark, air and fuel are the other two things to look at.

A bad injector etc, could cause that issue too.

[JaysBlueR36]

So it's been diagnosed as something mechanical not sure what yet.
It's definitely not vibration being caused by a misfire. It's more like a judder that's only present in PARK and NEUTRAL. Under load it's smooth. DSG gearbox works really well and is nice and smooth for an old school DSG otherwise I would agree with many people who say the Dual Mass Flywheel.