Intermittant misfire cylinder 6

[Lee20k]

Hi all,

I have a 5.7 VXR that has an intermittant misfire on cylinder 6, this has been going on for ages now and has got worse.

Swapped coil and lead to another cylinder and also replaced all of the spark plugs but still shows cylinder 6.

My mechanic has tested all of the wiring and said that when it is cold there isn't a spark signal being sent to that coil but when warmed up it does so he thinks engine ECU issue?

Any suggestions regarding this?

Thanks in advance.

[Lee20k]

Thanks for the reply,

forgot to say new injector put in as well. mechanic checked the fault codes and it kept on saying cylinder 6 even after clearing.

Also once it has heated up and even whilst driving it clears itself up and runs fine.

[Lee20k]

I live on Walney Island just the other side of Barrow-in-Furness in the NW so no-one local to me as far as I am aware.

I have seen people advertising on ebay about sending them your ecu and they will look into it and repair it for a fee?

[Lee20k]

mfp4073 said:

As you are from Cumbria you are not a million miles from AAS in Newcastle upon Tyne.
Give Stuart a call as he might have a few more suggestions, at least better than mine.
If you are still struggling he’s the man who will have a good chance of fixing it assuming you can get your car there.
The down side is he’s always very busy.

Many thanks for your suggestion and advice

[Lee20k]

mfp4073 said:

I wouldn’t be sending your ecu away unless you are as sure as you can be they are genuine and recommend.
As far as I’m aware there was a guy on here called …speed for 4 you?, he had a few scrap Monaro’s he was breaking, I have bought some used parts from him in the past which were fine.
I’m not on face book but I believe there are some knowledgeable HSV owners on there that may be able to help

Yea I have bought quite a lot of stuff off of Brett in the past, will keep him in mind.

[Lee20k]

Lincsls1 said:

Assuming you trust the abilities of your chosen mechanic, it seems he has found the fault.
If no power is being sent to coil pack no.6 when cold then it is either the ecu or the wiring between the ecu and CP6.
Perfectly feasable for a component within the ecu to fail to do its thing when cold only to start working when the heat from the engine bay warms things up!
Could even be as simple as a dry joint.
As John above as posted, I'd be looking to make a trip to AAS and leave it with them.
Remember if you are looking at getting another 'used' ecu, the mapping of the 5.7 VXR is different to that of the CV8 and although I assume it would run just fine, you might loose 50 bhp or so!

I trust my mechanic seeing as though he is my step son with his own garage lol.

Will keep AAS in mind.

Don't fancy losing any bhp thank you lol.

[Lee20k]

stevieturbo said:

It is extremely unlikely an ecu fault.

Exactly how has he been testing for the spark or trigger to the coil ? and is this misfire present at all times, or at least often enough to catch it testing ?

Is that injector firing ok ?
What is compression like on that cylinder vs others ?
Any odd noises etc ?

I think he has checked the voltage for the signals not 100% sure how he tested it but said the signal for a spark on coil 6 only sprang into life when it was warmed up, misfire is when cold then can suddenly spring back to life whilst driving.

New injector.

Not done compression test yet and no odd noises.

Thank you all for your replies, much appreciated.

[Lee20k]

stevieturbo said:

Without knowing exactly what and how it was tested, cannot give any help as to whether the tests/results are valid.

Why was the injector replaced ?

For an ecu signal to drop out and randomly appear, doesn't make sense.

Scope both the injector signal and coil trigger for that cylinder, or use a scope/current clamp and monitor all 8 coils to see if anything from the ecu is actually dropping out.
they're about the only tests that will give any real picture of what is, or is not happening

Hi Stevie,

I asked him to change the injector just to see if that was the issue.

Thanks for the advice I will pass this on to my step son to look into .