.random Power Fc Leaness Under Vacuum, But Wot Is Ok

[mutex]

Hoping someone has stumbled across this before.

I have an R34 skyline gtt with the previous generation power fc and an aftermarket wideband sensor.
Sometimes on warm starts (say water > 50 deg) the afr is 0.2 to 0.5 (or more) leaner under vaccum.
It doesn't happen all the time, most of the time it follows what's in the fuel map.

Sometimes it can be replicated at idle by letting car warm up, turn off, wait, start up again.
When left running for a while you can see it randomly lean, then return to fuel map value, or stay in a leaner mode of operation.

It's like a random vacuum leak, frustrating to tune.

What I know:

wot unaffected

o2 closed loop disabled, but oem o2 sensor connected

good battery 14.4v (through datalogit)
sensor check ok
stock bov
no air temp sensor on this model power fc
swapped z32 maf with another and no different
water temp correction is 1.000 at 80 deg, so afr should be the fuel map value

ignition timing unchanged at idle, even when running lean
injector duty unchanged at idle, even when running lean
fuel pump voltage unchanged at idle, even when running lean
can't be a dodgy wideband o2 sensor as engine note is audibly different when lean at idle
disconnected fuel purge canister loom (since its the only solenoid I can see), no difference
tightened all the intake clamps and turbo nuts, no difference

Going to check:

source another wideband o2 sensor (check just in case)
disconnect oem o2 sensor (shouldn't be doing anything but you never know)
measure fuel pressure
check vacuum leaks
disconnect idle stepper
check spark plugs

I'm thinking its a bug in the power fc logic.

I have had this issue for a few years and blamed hot fuel at track days, but in garage at home I can replicate and the fuel ain't hot.

Ideas welcomed.

[SargeRX8]

My car does this. Also a power fc. Most of the times idle is fine then randomly it will idle leaner to the point it pops. I cant understand wtf is going in. I cant recreate the scenario. Ive tested for leaks etc and come clean.

When it happens the only other thing I noticed is there is less vacuum. According to my profc b when its running fine its at -96 but when it decides to play around its at -87. Ive also noticed the timing starts jumping to 16 then back to 15. Idle does not change but the sound of the car will and it pops.

[Guilt-Toy]

id be checking the fuel pressure mate. see if its changing. do you have a intank walbro or something ?

[mutex]

Just spent a couple of hours trying to reproduce with extra gauges and can't make it happen atm.

I target 14.8 afr at idle, cell 2,2, 15 deg timing.

From logs (going back to 2008) at idle there are differences when it decides to go lean:

normal 14.9 afr, ends up being around 15.5 afr, sometimes > 16 afr

injector duty is up from 0.9 to 1.1 %

air flow voltage is up from 1100 to 1200 mv

vacuum is down from -520 to -500 mmhg

rpm is up from 750 to 800 rpm

timing remains at 15 deg, water temp > 60 deg

Seems to be directly heat related, car left a while, then warm start.

Two things from yesterday's diagnostics which I need more data on to rule out.

I unplugged the idle stepper connector and afrs appeared to be stable.

When I reconnected the connector and power fc went into some recalibration mode.

I found a small nick on one of the fuel pressure reg hoses and replaced the hose (I'd say from dyno operators).

However, I could maintain a vacuum on this pipe so don't believe the nick penetrated.

Anyhow putting car back together as I have a hill climb this weekend.

Will update this topic if I discover anything.