7M Supra bucks at light load

[IRONMAN]

My 88 Turbo is jumping a little under light loads. Is 100% replicable with quick drive and is consistent. It happens at less than about 5-10 % throttle.
Cars idles fine. Car runs fine at more than 5-10% throttle. Boosts fine and reliably at 10psi. Gets expected gas mileage. Has no codes. IAC is perfect. No intake/boost or vacuum leaks on any hoses or accessories. Coil packs, plugs, wires, cps, tps, afm, map, injectors, fuel pressure, regulator, pump, wiring, water temp, knock, and oxygen sensors/parts are all perfect and in spec.
It is moderate kick and jump as if it is dieing then firing then dieing then firing, but does so at a rate of about 4-5 times a second.

Mods:
full 3 inch exhaust turbo back. (no cats. Just one open muffler)
3 inch intercooler with 2.5 piping.
upgraded ct26
boost gauge
GE Heads and pistons
complete engine rebuild with 1200 miles on it.

This problem has been very consistent and is just enough to be annoying and a major pain without being dangerous or undrivable.

Does anyone have any idea?

 

[super51fan]

It sounds like secondary igntion breaking down based on info. Light load = max spark advance and lean. I would spray the plug wires with some glass cleaner to see if it gets worse. Correct OHM's on plug wires have noting to do with insulation of wires. I would start with new plugs and OEM wires. Good luck.

 

[Sparky33]

i had a similair issue idled fine bucked under light load if you floored it, it would boost fine. i sent my injectors out to be flowed, and it turned out they had a poor flow pattern and sure enough fixed my problem.

 

[7M4EVR]

Well since you ruled everything else out....does this happen to be a manual?...sounds to me like you might not be giving it enough gas as you let the clutch out....baahahaha.

 

[7mLove]

i second sparkys idea i had a similar problem but not as bad as yours seems to be, send out your inj to get tested..

 

[IRONMAN]

I have driving manuals for years. I can't stand autos. This is also happens when I've been in gear and not just when shifting. However that is good advice. I will check my wires first. Then I'll get money for new injectors if not that. I probably need them anyway.

 

[IndigoMKII]

sounds like your tps is out of calibration. the afm is seeing more air but not seeing a throttle position increase so adjustments are not being made. I would be willing to bet that your tps isn't seeing off idl til 5% throttle. this can be checked by probing your tps and slowly opening the throttle. when the throttle is opened idl should open or read infinite resistance.

 

[Backlash2032]

Either probe the idl contacts on the tps or disconnect it and see if it runs right.

Sent from my HTC Droid DNA

 

[super51fan]

Why don't you make a video of the problem. I am guessing that no check engine light and no stored codes.

 

[IRONMAN]

I just rechecked my codes and I did come up with a AFM code. However it runs fine and the afm works well. I don't have any lean spots or over rich spots. I also have tested the AFM for spec resistances. Everything checks out. Maybe the afm just can't read that low or is somehow malfunctioning?

 

[super51fan]

I think the code is for the airflow meter circuit. Not just the airflow meter itself. Might have a wiring issue. !st try a known good airflow meter. If still having issues you will need to test and inspect your wiring. BTW. Afm is probably one of the biggest input to ecu for load and driveability. Runs fine does not include bucking IMO.

I also see you have a thread for boost issues. I would get to the bottom of why you are setting codes for afm. Good luck.

 

[IRONMAN]

New AFM with new wires. That is what is confusing about my car. I half expect that my ECU is just bad. I have rewired so much in the car to still get issues with brand new sensors.

 

[Dan_Gyoba]

IndigoMk3 already mentioned, but this does sound exactly like what was happening with my ex-boss' '89T, which was a bad TPS. TPS was correctly adjusted, but would occasionally short the IDL to E2, causing a momentary fuel cut, which was perceivable as a quick buck. (And if I hadn't known what the problem was and showed him how to fix it, I could have bought the car cheap. D'oh!)

 

[IRONMAN]

Haha. When I adjusted the TPS is got better but it would only get so much better before I was not ever reaching the idle switch and getting the TPS code.

 

[Dan_Gyoba]

So unplug it. You'll get the TPS code, but if the problem goes away, then it's a bad TPS.