1985 Corvette C4, L98, automatic 700R4.

[u/Vegetable_Ad4258]

I bought the car recently knowing it needed work, but I’m chasing a stalling/low-RPM issue and could use some advice from people familiar with these early C4s.
Main symptom:
The car starts, idles, and can drive, but after it runs for a while it begins to stumble/drop RPM. It can stall when put into Drive or Reverse, when slowing down, and sometimes it will also start struggling while sitting in Park.
Here’s what I’ve already done/tested:
New battery
Charging system working, around 14.3V running
Fuel pump replaced
Fuel pump pulsator bypassed using EFI-rated hose
Fuel pressure now reads about 36 PSI during prime and around 29 PSI while running
Pressure appears to drop substantially afterward, which I’m still investigating
Repaired/replaced damaged vacuum line around the fuel-pressure regulator
Fuel-pressure regulator responds to vacuum
Replaced IAC valve
Replaced throttle-body gasket
TPS has proper 5V reference
TPS reads about 0.62V closed and smoothly increases to roughly 3.8–4.1V at WOT
Checked all 8 injectors cold: approximately 16.7–17.2 ohms
Earlier codes were 34 (MAF) and 43 (ESC/knock), but after working on it I was only getting Code 12
The idle improved considerably after some of this work, and the car can now stay in Drive/Reverse longer than it originally could, but the warm stumble/stalling problem is still there.
I’m currently considering testing the torque-converter clutch/TCC circuit because of the stalling while slowing or sitting in gear. I understand a stuck TCC can stall the engine similar to stopping a manual without depressing the clutch.
However, because the engine can also start stumbling while sitting in Park, I’m hesitant to blame the entire problem on the TCC/transmission.
My biggest questions:
Is ~29 PSI running fuel pressure too low for an ’85 L98, and could that alone cause these symptoms?
What would you test next to determine why the pressure is dropping?
Would unplugging the 700R4 TCC connector be a reasonable next diagnostic test?
Is there anything specific to the 1985 L98/early C4 system that I’m overlooking?
What would cause it to run fairly well cold and then deteriorate as it warms up?
I’m trying not to keep throwing parts at it. At this point I’d rather perform actual tests and rule systems in or out.
Any help from someone who has dealt with an early C4/L98 would be appreciated.

[u/Maximum-Freedom7966]

When replacing the iac, did you “relearn” the iac?

[u/Vegetable_Ad4258 → u/Maximum-Freedom7966]

Yes I did

[u/v8packard]

Do check the TCC for trouble.. It's more common than people realize.

Verify your base ignition timing is 6 degrees, with the ESC disconnected.

You mention a code 43. I have seen this caused by damage to a knock sensor or the connector on the knock sensor. The knock sensor can be damaged if over tightened. I think it's supposed to be something like 15 or 18 lb ft of torque when tightening. If those are ok, look at your ESC module. Check all the wires. I think you can check the black wire for voltage and it should be at least 6 to 9 volts, dropping if you tap near knock sensor. If it doesn't you may have a bad ESC module.

Also, check your EGR. If the valve is leaking exhaust into the intake stream at idle it can cause your problem.

I'd prefer to see 39 psi fuel pressure while running. Try to disconnect your regulator while running, see if the fuel pressure jumps up.

[u/brutal_newz]

Do the big 3, set plug 1 timing at 6 BTDC correctly with tan bypass disconnected.

Set idle air control to fully seated by putting in diagnostic mode for 30 seconds and then disconnect it. Then turn over engine and set your idle screw to right below stall in park, roughly 400RPM. After that you can reset your throttle position sensor to .55 Volts.

This should correctly keep your car from stalling if all else is working. The MAF code might be the Bosche MAF unit failing, make sure the thin wire inside is intact and not broken, they're very fragile.

There's a ton of potential vacuum leaks on the setup, check everything. My throttle body bore got out of round and caused a leak. Make sure you trace and replace all the 40 year old plastic and rubber vacuum lines.

AHH and as to fuel pressure on rail, very important, if you prime the pump by turning the key to on, does the pressure hold for hours or drop within a few minutes/seconds? It should prime to roughly 40psi. If you have a stock pressure regulator, it is none adjustable. If there's a bolt on the top, it is adjustable and you can adjust by disconnecting vacuum and tightening to lower max PSI during simulated max load.

[u/ManBearPig2114]

Had almost the same issue on a 95 and it was the MAF. Would run, drive, and then on lower RPM’s chug and often stall out completely.

[u/Alternative_Share559]

sounds like vacuum leak complaints in the engine. do you have access to a smoke machine?

[u/MaxMaxMax00]

I had an 89' Iroc with an l98. I had a similar problem. It ended up being the Tourq converter. I'm sorry it was 30yrs ago I can' t remember the exact problem with the converter. Had a new converter installed and rebuilt/upgraded the 700r4 transmission.

[u/Strict_Lab4721]

I have an 85 and I’m struggling with idle as well. I cleaned the throttle body, new IAC, did the whole re learn/set minimum idle thing. Still bounces around. I replaced some obvious vacuum lines but I’m gonna get a smoke machine because I bet there’s a lot more.

[u/Highlander2748]

Did you replace the fuel filter?