4.2 Intermittent rough idle, only misfires detected - ideas?

[trancytimbo]

Hi All,

Have had a good search but nothing seems to fit the exact issue I'm experiencing.
A somewhat annoying issue as it only seems to occur once in every say 15-20 times the car is idle.
Does not matter if the car is cold or warm but every now and again whilst sitting at the lights or stopped the idle will be really rough - car shakes around a little, small RPM fluctuations.
If I do a quick bump to > 1500 RPM and back to idle, the problem goes away.. until next time I pull up at the lights. It might be me, but I notice it occurs far more often when I'm slowly reversing into a parking lot!

Started off reporting 2 cylinders, then over the course of a couple of months a few more appeared. After a reset, initial two have reappeared.

I've been lucky enough to have my chain guides crack which lead to fairly nasty 8 bent valves and a huge hole in my wallet. Most of the timing and associated parts have been replaced.

Coil packs were replaced under recall, have had them all swapped around with misfires still being reported on same cylinders as before.

When it's idling rough, I hear a noticeable 'tick tick tick tick' sound but might be related to the additional vibrations introduced.


##Late last year##
Address 01: Engine Labels: Redir Fail!
Part No SW: 4F0 910 560 D HW: 4F0 907 559
Component: 4.2L V8/5V G 0010
Revision: -------- Serial number:
Coding: 0005772
Shop #: WSC 02335 785 00200
VCID: 234FFF74E234272660-8076

3 Faults Found:
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 001 - - Intermittent
16691 - Cylinder 7
P0307 - 001 - Misfire Detected - Intermittent
16686 - Cylinder 2
P0302 - 001 - Misfire Detected - Intermittent
Readiness: 0000 1000


## Feb 2016 before clear errors ##
Address 01: Engine Labels: Redir Fail!
Part No SW: 4F0 910 560 D HW: 4F0 907 559
Component: 4.2L V8/5V G 0010
Revision: -------- Serial number:
Coding: 0005772
Shop #: WSC 02335 785 00200
VCID: 234FFF74E234272656-8076

5 Faults Found:
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 001 - - Intermittent
16691 - Cylinder 7
P0307 - 001 - Misfire Detected - Intermittent
16689 - Cylinder 5
P0305 - 001 - Misfire Detected - Intermittent
16685 - Cylinder 1
P0301 - 001 - Misfire Detected - Intermittent
16686 - Cylinder 2
P0302 - 001 - Misfire Detected - Intermittent
Readiness: 0000 1000


## Feb 2016 few hours after cleaning errors ##
Address 01: Engine Labels: Redir Fail!
Part No SW: 4F0 910 560 D HW: 4F0 907 559
Component: 4.2L V8/5V G 0010
Revision: -------- Serial number:
Coding: 0005772
Shop #: WSC 02335 785 00200
VCID: 234FFF74E234272656-8076

3 Faults Found:
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 001 -
16691 - Cylinder 7
P0307 - 001 - Misfire Detected
16686 - Cylinder 2
P0302 - 001 - Misfire Detected
Readiness: 0010 1101


Any patterns with the reported cylinders reported? Are they all connected to a similar piece of hardware, or loom?

Cheers,

Tim

[Quattrocentric]

How many miles since the valve job? If it's a fair amount, a carbon cleaning may address this. I had a similar issue with my 3.2, ( intermittent misfires on the same two cylinders) and the cleaning seems to have worked.

[FormulaElement]

He has a non FSI 4.2 which doesn't suffer from carbon buildup. Cylinder 2 and 7 have different fuel rails, and wiring harness's. They are directly across the cylinder block from each other though i believe.

OP All i can think of is injectors being clogged or going bad.

[SinCityA6]

He has a non FSI 4.2 which doesn't suffer from carbon buildup. Cylinder 2 and 7 have different fuel rails, and wiring harness's. They are directly across the cylinder block from each other though i believe.

OP All i can think of is injectors being clogged or going bad.

+1 here. I am thinking injectors as well. Rough idle/misfires is caused by only three things...Lack of fuel, Lack of spark or bad timing. Timing was resolved with the major repair work you did on the valves, etc. You have potentially ruled out spark by swapping the coils (what about the spark plugs). That leaves fueling; and since it is not consistent across all cylinders that will rule out a failing fuel pump. You can test the injectors by moving them to a different cylinder and seeing if the misfire follows.

[Markie93]

Tim, Did you ever figure out the cause? I have been noticing my 4.2 loosing a bit of its power... when aggressively accelerating, it doesn't want to increase rmps as quickly as usual. I currently have the exact cylinders misfiring, 1,2,5, and 7.

I would be curious to know more about your findings.

Thanks,
Jake

[ked]

I am having the same issue in my '05 4.2. i replaced all ignition coils, spark plugs and one fuel injector. The problem is still there, so I'm going to replace all the fuel injectors and hope the issue goes away. I will say that after having the work done, there is a noticeable increase in power and I'm getting around 18mpg rather than the 12-14mpg i was getting before.

[FormulaElement]

Besides for coil packs, spark plugs, the only other thing that would make sense is the intake manifold. A flap could be stuck in the wrong position causing not enough air to get into the cylinders, thus misfires... Usually i thought that this only affected one side of the engine or the other though so I'm not really sure about the possibility of misfires on both sides.

Try checking all of the vacuum lines on the of the engine to see if one broke. Mine form my 06 were very brittle and pretty much disintegrated in my hands.

[Sofreshs5]

I have an 2012 s5 and I had multiple misfires on cylinder 7. I already had the car serviced with new plugs and my coil packs were good. The dealer stated that it was caused by the fuel injector rail. So it's getting repaired as we speak. Hopefully this takes care of the issue. I hate dropping $1400 into things like this but it has to be done.

[Augustas]

Hello! did you managed to solve the problem? :)