Rb20 miss at idle and in first gear

[j-min]

so my RB20det in my '91 GTS-T idles like it has a gnarly cam in it.

It's not ALWAYS been this bad, its been slowly getting worse. It only idles terribly when its cold. It is also quite hard to start, I have to pedal the gas or try to start it 2-3 times to get going. Under full throttle in first gear, I get a big missfire around 3-4500 rpm, wideband goes full rich. I assume some kind of spark blow out.

Steps taken:
I first assumed the IACV was dead/dirty. Removed that, cleaned it. Re-installed, no change. It does seem to bias idle when the car is nice and hot, however. If i unplug it while running the car tries to die.

TPS is in check.

Put in new spark plugs, gapped to .8mm

Have tried 2 different MAFs, no chance. Timing is set to standard.

My wideband gauge also says it runs super lean at idle (like 18:1 or higher, my AEM gauge only goes to 18)


Ideas? Hidden vacuum leak under intake manifold? Coilpaks gone?



For reference, the car has a stock rb20 engine with:
GTR injectors/resistor
300zx maf
rom tune
fmic/exhaust
rb25 turbo running 13 psi ish

[Skym]

Points towards intake manifold or exhaust manifold leak if going to 18 AFR. If running richer at higher rpm, that's the knock / low octane maps and can be due to engine running lean, knocking.

The paper stock intake manifold gaskets are known to blow on GTS, GTR engines and there's upgraded gaskets from Cometic for stock intake manifold for GTS engine.

The stock exhaust manifold studs are known to break and should be replaced every I think was 10 years. It's usually because the hanger bushes for the exhaust need replacing. Part of routine maintenance.

[j-min]

Exhaust side doesnt leak- intake side is a distinct possibility because this problem only seems to be getting worse. I'm removing the manifold next month to fo a FFIM setup, so I'm sure I'll find something.


It only runs excessively lean at idle. WOT is fine. Transient load through in town driving is super rich (11s and 12:1) but that could also just be attributed to a presumably rough JDM tune. thanks for the insight.

[Skym]

You're welcome.

[Underoath.]

you could have a bad igniter. ive seen that many times.

[ToroGTS]

The "lean" reading could be caused by a non ignited intake charge as well. If it "blips" lean while it stumbles, its an ignition issue. If it's a consistent lean reading while idling, it's a post maf vacuum leak. Could be as simple as a loose pipe coupler.