Mis-fire

[A80Avant]

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

Ok
So Andy has changed spark plugs 3 times, injectors once, compression is tested, and everything from the above still produces a problem.

If its too rich or lean he can tell us from the gauge.

I'm just wondering if its a wiring thing...

[UrS4boy]

One missing and hesitation diagnostic procedure

...I'm just wondering if its a wiring thing...

We had a situation on my home forum with a "noob" (to UrS4s) who had a miss. We all pointed him to check his plugs, boots, coils, POSs, etc. etc. Turns out he had a cut in one of the no. 5 injector wires that, when fixed, cured the problem.

I have added the injector wiring as a potential problem in this missing and hesitation diagnostic:

http://forums.quattroworld.com/s4s6/msgs/22611.phtml

[A80Avant]

Now that you mention that a friend of mine with an rs2 had similar issues but with one injector plug. Not a misfire exactly but the car would feel lean and kind of hesitate on a low rev range. Once he fiddled around with the injector plug on one of the cylinders it never came back

[prj]

I think it is quite unlikely to be the wiring issue, and I also think it could be something spark related.

The reason I say this, is because the car is fine at 1 bar and the issue starts manifesting itself from a higher boost pressure.
So it's either way too rich/lean (we still don't have numbers from the wideband) or it is ignition related.

The reason I think it's ignition related, is because the higher the pressure in the cylinder, the more difficult it gets to ignite the mixture, as the resistance between the plug electrodes rises. So usually, when you get such misses and it is in direct connection with increasing boost pressure, it is something to do with ignition.
Wiring faults usually are more intermittent - if it can be reproduced at will by winding the boost up, and then disposed of by winding it back down, then it's unlikely to be dodgy wiring, although checking that does not hurt.

[kenax]

Which spark plug leads do you use? Which ECU?

Edit: and what range of power are we speaking here?

[prj]

About 350-380hp, 3B ecu...
Should not be any problems at that level, something must be wrong with hardware.

[kenax]

Which ignition? AAN? distributor with wires?

I'm with prj here, it's most likely ignition caused.

Check for white spots and cracks on plug wires or the insulation of the cop-coils. When misfire symptoms suddenly appear, and they are manifesting themselves at spool or near max torque, the spark just doesn't find it's way between the plug electrodes. If it's higher up in rpm, it may be that there's something with the coil(s) and the charge time reserved for them is not engouh anymore, for some reason, and the spark energy is not enough to cause a breakthrough at high cylinder pressure.

[prj]

The 3B is distributor based. Single coil, distributor and then leads to each plug.

[Dybro]

Does the plugs have the right spark gap?
If it's too big it could cause the problem.
As I remember, the OE f5dpor's only have ~ 0.5mm gap.....

[GazzaG60]

id go ignition to be honest. I had similar misfires when I ran over 1.5 bar or so. coil dwell was my problem. needed more charge throwing in.
this was vems but I never had any missfires before boosting over 1.5bar.
stock plugs were the best after trying a few types.

[AndyP]

I'll answer all the questions when I get home tonight.

Keep the suggestions coming though

[kamuto]

I think that will be spark gap

[kenax]

I dont think several set of plugs would have wrong gaps that would cause misfires at that power level. Strike through or coil failure. Have cams been adjusted? Can the distributor be misaligned? Is the distributor in good shape? Worn conductors in the distributor eat spark energy

[stocious]

bad engine earth , can that effect 3B ignition ?

[Error404]

Maybe the old 3B coil is simply not man enough for the job in this instance?