Strange misfire, 1JZ 4th/5th gear only

[joeshaw123]

I seem to be having a weird misfire at high load pretty much WOT, low to mid rpm (2500ish), 4/5th gear only. 1-3rd pull fine from idle to the top end no issues. 4th OK when not loaded up but on an incline in 4th or any sort of road in 5th it seems to do this misfire/backfire almost full cut feeling thing.

I'm trying to rule out any hardware issues before I take the car to be properly tuned, can anyone see anything untoward on my map? (only rough map I've been cruising around on to fault-find before mapping). If the map looks OKish, I'll have to look for issues with coils and wiring etc.

I've changed plugs from BKR7E (0.9mm gap) to BKR8IEX 0.7mm gap, and the coils (VAG) are relatively new. No change at all.

I've upped the dwell on vag coils from 1.8ms to 2.2-2.5ms with no change.

I've attached the map i've been using while fault finding and most recent log with misfire like incidents at 1:04 and 1:16. Note i've not set knock gain levels at all so they read prettttty high across the board!

Any thoughts at all appreciated, thanks.

MARK II 2023-07-30.pclx PC Datalog - 2023-07-30 8;28;56 am.llgx

Forgot to say it never did this before on the old hybrid setup but since then gone for 264 cams and G30-770 turbo.

The turbo is on the smallest rear housing and I was worrying about compressor surge but don't think it would feel like this.

[Adamw]

There are no obvious control issues in the log. The first example at 1:04 appears to be a misfire from the lambda and engine speed ROC trends, but the one at 1:16 doesnt have a lot of clues.  

Since you have an analog wideband, the first thing I would try is adding and removing 10% fuel from area I've highlighted below to see if that gives us any clues.  

You could try taking 10deg cam advance out in the same area as a test too.  

[joeshaw123]

Thanks for the info Adam.

I tried adding and subtracting 5% then 10% of fuel and it made very little difference to running apart from the AFRs looking a bit off. It seemed to bog a little with +10% and felt a bit weak with -10%. I do mean to get a CAN lambda setup at some point but its analogue at the minute and staying that way, I'll probably go CAN lambda next year when I maybe look at e-throttle too.

However - adusting the VVT cured it. I'm not sure I understand the mechanics of why cam advance improves torque and how you find the optimum values for a given rpm/tps%, but I went from 42 (do these max at 40 anyway?) down to 36 which massively improved it, then to 34 which stopped the issue altogether. I've not had the vvt tuned properly since fitting the 264s.

Why would this happen, and why only in higher gear/higher load and not the lower gears? Is it just that you're spending longer in each load cell for the issue to show itself in the higher gears?

Thanks again.