Help - Misfiring and Hesitation

[Bennyd]

Hi,

I been having a problem with my car. Basically the car won't idle properly, it's like its misfiring at idle and its even worse when moving. When moving from standstill with a small amount of trottle she hesitates/misfires, but when at constant speed everything is fine but when I give her a little bit of trottle she hesitates again and then some times she slowly starts gaining speed then all of a sudden she takes off(as normal). If I give her alot of trottle there is not as much hesitation but there is still a bit. Also when comming to a stop, the revs drop to about 500rpm briefly and then to 750ish rpm, I keep thinking that shes going to stall. I have changed both MAFS, 2 fuel pumps(one failed not so long ago) and all the vacuum hoses have been changed.

The faults I get are as follows:

5 Faults Found:

17628 - Fuel Injector for Cylinder 8 (N86)
P1220 - 001 - Short to Plus - Intermittent
17535 - Fuel Trim; Bank 1 (Mult)
P1127 - 002 - System too Rich
17537 - Fuel Trim; Bank 2 (Mult)
P1129 - 002 - System too Rich
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 008 -
16692 - Cylinder 8
P0308 - 008 - Misfire Detected

I have searched the forum but and checked the usual bits but I cannot find anything wrong.
I also did some logging, but I don't know what to look for in the logs. One thing in the 1st log is at line 390, you can see the revs drop to 360rpm as I come to a stop. And in log 2 there are alot of misfires.
The weird thing is I know how to fix it... if I plug one of the MAFs out(anyone) it runs as normal, this is really confusing me and my mechanic.
If anyone can have a look at my logs or recommend something it would be appriciated.

Thanks,

Benny

Also, if I clear all the fault codes, disconnect out the battery for about 5 minutes and reconnect, the car runs fine for about 50miles and then starts acting up again.

[adsgreen]

O2 sensors?

[MikeFish]

Sorry I don't know a lot about the C5 RS6, but don't they suffer from the usual VAG coil pack problems?

[Ryan_Pestell]

I would say if it runs properly with them unplugged then it's the MAF sensors, you could log them to see, reason I would imagine the car runs fine when codes are cleared is your probably clearing the learned fuel tables that the faulty MAF's have created or possibly a leak around the seals where there mounted. Is the idle iratic?

[Shoppinit]

MAFs, MAF seal or other vacuum leak is probably causing most of your problems. You say you have changed the MAFs, but are they seated properly? The airbox is a biatch to put back on, and if it's remotely springy at the back, it's not on properly.

You say you have changed the vac pipes. Have you checked the main vac hose going to the brake servo? What about the hoses under the inlet manifold? Going to SAI?

The injector problem needs looking at. That is not a DTC than is likely to be caused by anything other than a bad injector or a faulty connection. The misfire on cylinder 8 backs up the idea of a problem with an injector.

[Shoppinit]

You can test the resistance of the coil on the injector. It should be 13-16ohms when cold.

[Bennyd]

O2 sensors?

I hope not!!

[Bennyd]

Sorry I don't know a lot about the C5 RS6, but don't they suffer from the usual VAG coil pack problems?

The C5 RS6 coil pack are pretty good, not to many problems.

[Bennyd]

I would say if it runs properly with them unplugged then it's the MAF sensors, you could log them to see, reason I would imagine the car runs fine when codes are cleared is your probably clearing the learned fuel tables that the faulty MAF's have created or possibly a leak around the seals where there mounted. Is the idle iratic?

Ya, the idle is iratic. I have logged the MAF's and they seem to be OK. The problem with the seals is you just can't tell if they are sealed properly or not when I put the airbox back on.

[Bennyd]

You can test the resistance of the coil on the injector. It should be 13-16ohms when cold.

I'll do a bit more testing at the weekend.

[Shoppinit]

Odd logs. Did you do them while driving? If so, why do you not go above 2500 rpm?

The MAF values seem low to me, but the car is not making the mixture richer, it's making it leaner. So not like there was an vacuum leak. More like there was a boost leak.

I noticed your turbos weren't making specified boost, either, but since you rarely got above 2000rpm, I don't think it's indicative of anything.

[Shoppinit]

Had a similar problem with my Rangey recently. Turned out to be a perished diaphragm in the fuel pressure regulator letting fuel into the vacuum system.

[Bennyd]

Ya, logs were done while driving, I don't like to push her when shes acting up as she does. I'll try and get the MAFs to seat properly to illiminate them and do a few more logs.

[adsgreen]

I would have thought with a vac leak it would be lean under vacuum and rich under boost.

[Shoppinit]

It's difficult to say for sure what the mixture is doing because the OP reset the trim values before logging.

You can see in the logs that the car is almost not boosting at all, and yet the ECU weakens the mixture to the max almost immediately, but only the multiplicative values, which makes me think that maybe it's not a vac leak after all.

I was thinking that maybe that n°8 injector was stuck open, causing the rich mixture overall, but lean running in the other cylinders when the ECU pulls the fuelling, but that doesn't explain why the other bank is showing "too rich" errors, too.

In the logs, the MAF values rarely go above 40g/s, but then, he's feather-footing it, so difficult to say.

The more I think about it, the more it looks like a fueling problem.