Mis-fire

[AndyP]

For the past 6 months I’ve been trying to find the cause of a high boost mis-fire on my car, so far I have either replaced or swapped for trial purposes, the following items:-

Ignition coil
Distributor
Distributor cap
Rotor arm
HT leads – 3 sets
Spark plugs – 3 sets ( NGK & Bosch )
MAF
Lambda sensor
TPS switch
Fuel pump relay
Injectors
Replaced all fuel injector plugs
Fuel pressure regulator
Compression tested and all OK
Boost leak tested and all OK

The car is mapped to 1.5bar and as I have an EBC I can set the boost to whatever I want. If I leave it at 1bar it runs fine 99.9% of the time, try and run it at the full 1.5bar and it gets to around 1.2-1.3bar and starts mis-firing. I don’t think it’s missing on all cylinders as it does not slow down as much as it does with an over boost cut. Also, when it does mis-fire the AFR gauge goes lean, does this mean it’s losing fuel rather than spark?

My next move it to try another ECU, any other suggestions are welcome as this is driving me nuts!

[prj]

Misfire is always lean.
Because the O2 sensor measures oxygen. No combustion -> lots of oxygen going into the exhaust because it was not used for burning the mixture -> lambda sensor shows lean.

Measure fuel pressure at 1.5 bar boost, should be 3+1.5 -> 4.5 bar.
If that's alright, then you have got a spark problem. ECU got nothing to do with this, leave it alone.

What mixture are you seeing at WOT at 1 bar?

[A80Avant]

Having changed 3 sets of spark plugs I'd say they are ruled out.

Are you sure the replacement coils are ok?

[AndyP]

I guess the coil could still be faulty, but the fact that the symptoms did not change at all when I fitted it makes me think it was OK.

[A80Avant]

That makes sense. Hmmm... it is intriguing that.

[stocious]

Just for reference is it the small rotor arm fitted ?

[AndyP]

Just for reference is it the small rotor arm fitted ?

Yes, it's the correct narrow one.

[QuattroFever]

has it run fine at those levels before or an ongoing issue since the build?

[Tonks4x4]

What's the condition of your knock sensors?
Is your MTFS acting up?
Have you taken the wastegate off and checked for play in the valve?

[QuattroFever]

hows your oil consumption? what the plugs look like when you plucked them?

[prj]

Sorry, but how are going knock sensors MTFS or wastegate cause misfiring on 1 or 2 cylinders?
Clearly it's misfiring, as the misfires show on the wideband...

Unless it's going way too rich so that the mixture can not be ignited anymore, it is most likely going to be a spark problem.

[Tonks4x4]

Sorry, but how are going knock sensors MTFS or wastegate cause misfiring on 1 or 2 cylinders?
Clearly it's misfiring, as the misfires show on the wideband...

Unless it's going way too rich so that the mixture can not be ignited anymore, it is most likely going to be a spark problem.

These items are not on his list!

Would it hurt to check them?

When you have eliminated the impossible, whatever remains, however improbable, must be the problem.

[prj]

IMO it's better to diagnose the problem and replace the thing that needs replacing, as to randomly swapping bits not related to the issue.

We have so far confirmation that it's misfiring on a few cylinders and the misfires show up as lean spikes.

The car can misfire because it's way too lean, way too rich, has issues with an injector, has issues with the spark or lacks compression in a cylinder.
Checking other things seems like a waste of time to me, but what do I know ... I'll just shut up and see how it goes

[Tonks4x4]

Checking other things seems like a waste of time to me.

Yes we all know what happens when things are not checked... For example if all the debris of an injector seat is removed from an inlet tract. That does not seem like a waste of time to me at all.

but what do I know ...

You know more than anyone else on the planet about Motronic and I have a huge respect for that. But maybe you should stick to what you know.

I'll just shut up and see how it goes

Perhaps this way you will manage not to offend anyone....Whilst brushing up on your public relation skills

[prj]

These forum threads just always end up with a million people saying million things unrelated to the issue, and in the end what it boils down to is someone could just say "check every nut and bolt in the car" and be done with it - and it sums up the thread.

I like approaching things systematically, it has worked for me on hundreds of occasions and I simply asked what does wastegate or other things have to do with this when boost is nice and steady and there are misfires showing up on the wideband.

Instead I got attitude - because there is no connection between a cylinder misfiring and play in the wastegate or MFTS, as the boost is nice and steady.
As to offending people, well I'm sorry you got offended, wasn't my intention. But frankly, people get offended all the time for all the wrong reasons, and I can't please everyone.

Oh and, as for finding faults in an I5 engine - I have experience fixing about a hundred cars which others could not fix, and people saying "it's the ECU", when it's got nothing to do with it.