Hi all,

The car is a mildly-modified EF Falcon XR6 automatic.

Have had an occasional stumble at WOT for a while but has been getting progressively worse over the past few days and is now a full-on misfire. It occurs only at WOT from 2800 RPM (when it comes up on the torque converter) to 4000 RPM, so only at maximum torque/load. Once it is past 4000 RPM is pulls just fine to redline. I stumbles so hard that the revs are bouncing around a bit and even drop once or twice on the way up.

Drags are on tomorrow night, so I need to solve the issue quickly. I am working, so I basically have only tonight to sort it out.

It can't be fuel or injectors, as it pulls hard right out to 6850 RPM at WOT - at maximum fuel flow. I checked the AFR and it's 12.2 to 12.6 through this rev range, so definitely not leaning out.

I thought it might be the knock sensor detecting knock and pulling timing out. But I pulled out 3-4 degrees of advance and richened up 10% in the affected areas and it made no difference.

I checked the spark plugs and they look OK (white one side, tan/brown the other). Though a couple of them have bright white side electrodes which I haven't seen before. I have just picked up a new set of plugs to install tonight and hope that solves it, but I'm not confident, so need a plan B.

I did quick-and-dirty lead/coilpack test last night - testing two leads and the secondary circuit as one unit - and it seems fine. Cyl 1-6 was a total of 26k ohms, 2-5 was 20k, and 3-4 was 19k - all of which are well within spec in total: 9.2k for the coil leaves at most 17k total for the two longest leads. Could the coilpack be faulty anyway? I don't want to go through the cost and hassle of changing it unnecessarily.

It can't be a vacuum leak, as there is almost no vacuum at WOT, and a leak would make a poofteenth difference to total airflow at WOT anyway.

One earlier post mentioned unplugging the Cam Angle Sensor. Is it safe to try that, in case the CAS is dodgy? It shouldn't affect fuel in the batch-injection EF, but does the ECU lose it's ignition timing reference?

Anything else I should be checking? I will pull error codes as well to see if they tell me anything. Thanks for the help.

I would have thought leads with the breakdown under load.

Could be the coil pack breaking down but unlikely.

With you AFR's, doubt a fuel telated issue. Sounds more electrical, plugs or leads.

Thanks for that advice. I do have a brand new set of leads on the shelf for when these ones expire, so I might just replace them anyway.

Much obliged.

Update:

SWC, you were spot on! It was the plugs, even though they looked fine. Replaced them and it's all sweet again. So it's Night Speed Drag Wars at Meremere Dragway tomorrow!

Thanks heaps for the help.

It turns out that the ignition leads were also on their last legs and causing an intermittent misfire. Have replaced them and it runs like a dream now.