C5 RS6 Misfire When Cold - Cylinder 6

[super-ron]

Hello everyone.

I have recently purchased a 2004 Daytona Grey Avant and it has a misfire when cold. It clears up and runs smoothly when hot. The car runs perfectly once its warmed up, no misfire, no noises, no hesitation at all, very fast!

With my VCDS i ran a scan and it shows:

17981 - Left engine mount solenoid valve (N144) - P1573 - 04 - open circuit
17983 - Right engine mount solenoid valve (N145) - P1575 - 01 - short to plus
16684 - Randome/multiple misfire detected - P0300 - 08
17983 - Cylinder 6 - P0306 - 08 - misfire detected

A bit of history on the car, it has done 119k miles, but for the last 4 years it has only done 1000miles a year so it sat around alot, outside. It has still been regularly serviced annually despite the mileage, and recently had the cambelt, chains and tensioners done.

I started with the basics and swapped the coil packs around, cleared the faults but it stayed with Cylinder 6. I then swapped the spark plugs, cleared the faults and still showed up on Cylinder 6. As it has been sitting so much I wondered about bad fuel and a clogged injector, so I ran 2 tanks of Shell Vpower through it, but again same result. To eradicate an injector I swapped cylinder 1 and 6 injectors, but the misfire still comes back to cylinder 6.

This leads me to believe there is something wrong with cylinder 6 and not a bolt on component. I have just fitted 8 new spark plugs out of due course, but no change.

I run the misfire detection screens in measuring blocks 14 15 and 16 (I think it was...) and all misfires detected are in cylinder 6. On cold starts it runs into the 100's and all 7 others are at 0. Air flow meter measuring blocks appear to be matching quite well and these were both replaced in 2013 with genuine items. There are no misfires detected or shown once warm.

At some point there has been a mouse in the bay chewing on all the rubber boots of the plugs, but with it being temperature related it seems unlikely to be a loom fault? I do have Elsawin so with having a look at the wiring diagrams for a douple check.

Any help would be much appreciated, I'm hoping it's not the engine, but I do plan on running a compression test at the weekend.

And help or pointers would be much appreciated on what and where to check next.

Thanks in advance

[Shoppinit]

Sounds like you've been doing all the logical diagnostics.

Did you swap the coil pack with an adjacent one or bank to bank? The RS6 only has a couple of knock sensors and it can make mistakes about which cylinder is misfiring. By swapping all the bank 1 coilpacks with the bank 2 ones, you at least eliminate that possibility. Same applies to injectors and plugs. A pain, I know.

Compression test is the next logical step. Could be a dodgy valve or something.

[Jayjay101]

id do a compression test as sounds like a valve problem and cylinder 6 seams to be the one that people get problems with when valves are bent or stuck.

i had the same problem with a misfire on cylinder 6 and did the same tests thinking it was a electrical problem even got a sparky out to check all my wiring.

turned out id bent all the top valves on the that side of the engine after my cam chain jumped 2 teeth after the pad broke,when doing a compression test it was fine on all but cylinder 6 it was only when i went to the engine builder and he showed me that they were all bent!

[Shoppinit]

You can also test your cam timing to see if you've suffered what jayjay did.

[super-ron]

Thanks for the quick replies

I'm no stranger to these engines in N/A form having just sold my C5 S6 that I'd owned for 4+years. I had one cam chain completely destroy its self, and a few years later caught the other one just before it let go, I had the dreaded oil cooler coolant link pipe crack, thermostat leak, you name it I had it it seems I was lucky not to have bent any valves with the tensioners letting go.

The previous owner of the RS6 did the timing belt and chains a few months ago, so I did check the camshaft timing coloration when I viewed it for my own piece of mind and it was showing 0.0 and -0.5 degrees so I was happy with that.
The question is now were they replaced for the schedule or because one failed.

The strange thing is it runs so perfectly when warm. I came home about 3 hours ago, the coolant temp is at 65degrees, just plugged in and started it up and nice smooth idle and 0 misfire detected on measuring blocks.

But stone cold cylinder 6 doesn't fire at all. I have read i few threads about headgasket failures.....

[Shoppinit]

What is your trim doing? Group 32 IIRC.

[super-ron]

What is your trim doing? Group 32 IIRC.

Adaptation (idle) bank1 sensor1: -0.4%
Adaptation (partial) bank1 sensor1: -0.8%
Adaptation (idle) bank2 sensor1: -1.1%
Adaptation (partial) bank2 sensor2: -0.8%

0 misfires detected at the moment (90degree coolant temp)

[Shoppinit]

Good trim, then. So much for that idea.

What about if you reset the trim next time it’s cold and see what they do. Or watch the live adjustment values.

I’m just thinking that if the mixture is poo when cold then a weaker cylinder (in terms of compression) might be the first (or only) to show misfires.

[Ajc]

You can also test your cam timing to see if you've suffered what jayjay did.

Nah , Jay Jay just chops through cables and does the diagnostics later

[Ben_r1]

Interested to see what this turns out to be.

Had a similar thing a few years back with a Clio and turned out to be an injector that was sticking when cold but worked perfect once warmed up.

Obviously you’ve already eliminated the injector though by switching around. Best of luck to you.

[Jayjay101]

You can also test your cam timing to see if you've suffered what jayjay did.

Nah , Jay Jay just chops through cables and does the diagnostics later

i have no come back for that!! ha!ha!

[Jayjay101]

I was lucky not to have bent any valves with the tensioners letting go.

are you 100% none are bent?as this really sounds like what i had

have you done a compression test yet?

[super-ron]

Looks like I probably have an answer. Got to the workshop and gave it a compression test hot

1. 170psi
2. 170psi
3. 170psi
4. 175psi
5. 180psi
6. 180psi
7. 180psi
8. 180psi

Nothing untoward there.

Left it all day and did a compression test cold

1. 200psi
2. 200psi
3. 200psi
4. 200psi
5. 200psi
6. 165psi
7. 200psi
8. 200psi

The test was done with all spark plugs removed and where the tester was in cyl 6 there was a clicking.

I had the camera down the spark plug hole and nothing looked any difference to any of the other cylinders, you could real the bore size and arrow on the top of all the piston, and none were noticeably wetter than the others. No signs of any valves touching pistons, although once warm and the its runs perfectly so the combustion would cover it up if that had happened.

So seems like a sticky valve when cold but fine when hot?! Still miffed how perfect it runs when warm?!

[ahoooga]

Sounds more like a piston ring than a valve

[Shoppinit]

Bad cam follower? Doesn’t seem likely.