Unsolvable RB25DET Misfire

[BigBoi]

So, this is basically me, giving up and asking for help. I'm truly stumped what's going on with this car. I've had this car for almost a year (R33 GTST, S2 Engine) and it's always had a mild misfire on idle. Always drove fine so was never a huge bother, but I did try a few things to no avail. Again, only mild and on idle, so not a real worry. I have replaced basically everything on it:

Injectors,turbo,manifold gaskets,sump,fuel rail,plenum etc. Basically all bolt ons are new. Engine was out of the car but not torn down. Should have but ran out of time. Anyway, basically every and all gaskets are new, same with belts etc. 

Running an APEXI PFC tuned on a MAF sensor. Making ~300RWKW.

Recently, the misfire has now moved up to being rather suddenly worse (within the space of a few days/weeks) and now the car runs oddly rich at 9:1 under boost and misfires everywhere. Used to be around 11:1 or so. The idle is very lean at 16-17:1 and bounces between 850 and 1000 RPM. Runs awful, backfires all through the rev range and is not safe to drive.

When I unplug the cylinder 1 coil or injector, the idle barely changes compared to the other 5 cylinders. IT DOES THIS RANDOMLY- sometimes it will make a decent idle change then most of the time it won't change idle at all. Since then, i've checked or replaced all of these items with very little improvement.

- Swapped injector and coil to another cylinder seperately- STAYED ON CYLINDER 1? All other 5 cylinders make changes to idle when either coil or injector are unplugged. Cylinder 1 barely makes any at all, most of the time none. 

- Compression tested: All cylinders, WOT, warm engine, plugs out..... = 125 PSI across all 6 give or take 3-4PSI. Slightly lower than expected but very even. Done on 2 of the same compression testers for reference. No smoke, no blowby in catch can, seems fine.

- Known working coils put in

- Known good coil loom in

- Voltage tested injector loom and resistance tested injectors themselves, 12v on both sides when placed multimeter grounded with chassis. Was saying 6Volts for a bit but then it went to 12, and i'm not sure why.

- CAS swapped out for a test

- MAF cleaned, car dies when unplugged. Had no issues with MAF tuning the car either.

- ECU wiring looks a bit piss poor but nothing seems loose or damaged.

- Regrounded engine block to chassis

- Changed plugs, lowered gap to 0.7mm (BCP7ES I believe?) but these were brand new plugs swapped for brand new plugs anyway.

Sorry for the long post, but i'm totally stumped where to go from here. This small misfire has suddenly gone big and i've done heaps to figure it out. Now hopefully someone can help.

Rant over- all help appreciated

Edited December 15, 2021 by BigBoi

[Duncan]

Perhaps an intermittent short/break in the wiring loom.  When the issue is occurring turn off car and check continuity from coil1 to the ECU.

Also, just to confirm my memory, S2 R33 didn't have a separate ignitor on the engine, it is built into each coil, right? I had a nasty but intermittent misfire when my ignitor failed

[Dose Pipe Sutututu]

What's your battery voltage like? Also did you connect both Z32 GNDs back to sensor GND and not into the chassis like most?

If you ground off any sensor to chassis ground you'll run into issues too.

[BigBoi]

On 12/16/2021 at 6:57 AM, Dose Pipe Sutututu said:

What's your battery voltage like? Also did you connect both Z32 GNDs back to sensor GND and not into the chassis like most?

If you ground off any sensor to chassis ground you'll run into issues too.

I believe it's always been to ECU as ran good on boost before, just randomly went bad a few weeks back.

Will double check tonight but nearly certain its pinned to ECU

[BigBoi]

On 12/16/2021 at 3:57 AM, Duncan said:

Perhaps an intermittent short/break in the wiring loom.  When the issue is occurring turn off car and check continuity from coil1 to the ECU.

Also, just to confirm my memory, S2 R33 didn't have a separate ignitor on the engine, it is built into each coil, right? I had a nasty but intermittent misfire when my ignitor failed

I'll try that, it isn't really intermittent anymore, misfires terribly just can't seem to get cylinder 1 narrowed down as to why that cylinder makes no difference sometimes and a bit others

[Dose Pipe Sutututu]

There's also a bunch of grounds from the loom that's grounded next to the water outlet on the intake manifold.

Check if any of those have snapped.

[BigBoi]

 

On 12/17/2021 at 6:27 AM, Dose Pipe Sutututu said:

There's also a bunch of grounds from the loom that's grounded next to the water outlet on the intake manifold.

Check if any of those have snapped.

Problem solved - wasn't this either!

[BigBoi]

So for anyone interested, after tearing apart my loom, accidentally and scarily zapping myself by shorting out my injector harness on my manifold (touched it when it had a stripped spot woops!), and spending countless hours replacing and swapping just about every electrical or fuel related part- it was the spark plugs.

I don't quite know how, or why. Brand new NGK BCPR7ES gapped at 0.7mm with around 2500km on them, swapped two of them out a few weeks ago with no change (all supercheap had in stock, so the 'dirtiest' plugs got changed) then eventually just swapped them to NGK BKP7E at 0.7mm gap. This seems to have fixed it.

It misfired on two sets of plugs, both good condition, a heat range 6, then a heat range 7- now seems a lot better on this set. I have no idea why this made such a difference but i'll keep posted if this changes. Third time is the charm?

 

Anyway, I appreciate the helpful replies, thanks fellas.

 

Edited December 17, 2021 by BigBoi