I've had a 1989 L98 IROC for about 7 months. It ran fine for 5
months, then it started misbehaving. After I've been driving for
about 15 minutes, the engine will start to stumble, like it's running
out of fuel. If I let off the throttle and coast, the engine appears
to smooth out, but as I open the throttle, it stumbles, like it isn't
getting enough fuel to match the extra air. If I open the throttle
wide enough, it will die completely, or even backfire (at least,
that's what it sounds like). If I turn it off and wait 10-15 minutes,
it will fire right up and run fine for another 10 minutes, then die.

I don't have a fuel pressure gauge, but I replaced the fuel filter,
just in case. No change.

I backprobed the ECM connector to read the TPS voltage. It's normal.

I suspected that the O2 sensor was giving bogus data and confusing
the ECM when it entered closed loop, so I unplugged the O2 sensor.
The car ran worse, but it did run for a while, then died, just as before.

I bought a serial ALDL cable and tried to log some data, but it didn't
work. WinALDL would only populate the MW2 row and sometimes the
PROMIDA and PROMIDB rows. I saw several posts about this, but none of
them helped. I tried every combination of baud rate and 10k-ohm
jumper, and none worked. I don't know if the ECM has been modified,
but I did verify that it's a '165. Datamaster detected a "GM 160"
ECM, but it couldn't synchronize to start getting data. (Sorry, I
don't remember the exact error message.)

The ECM threw a trouble code when I unplugged the O2 sensor;
otherwise, it hasn't thrown any.

I would sincerely appreciate any advice. At this point, I'm not sure
what to try next, other than testing my ALDL cable on another vehicle.

 

Sounds like a weak ignition component, like the ignition control module or the coil.

Scanner won't show that. Won't set a code, either.

 

 

Thanks for the suggestions, folks. I'll give them a shot.

I had thought it would run for 10-15 minutes and then die after it warmed up. I drove it this morning after it sat for a few days, and it died after only 4 minutes. Apparently, the problem is not related to engine temperature or run time.

When it died, I let it sit for a few minutes, and tried to start it. It fired up, but died immediately. I tried to start it again, but it would only turn over. I'm not sure why letting it "rest" would let it start again. After 10 minutes or so, it started and ran long enough to get home.

 

For the record...

I finally bought a fuel pressure gauge. Pressure is usually in the low-to-mid 30s, but it sometimes drops into the 20s, and that's when it starts to die.

So, I found the problem...or at least part of the problem. I took the air snorkel off, opened the throttle blades, and looked into the plenum. It was filled with a lake of fuel. It turns out, the diaphragm in the fuel pressure regulator was leaking, and fuel was passing through the vacuum line into the plenum.

I bought a Holley adjustable regulator (model 512-501), which came with a diaphragm. I had to grind a flat spot on the mounting flange in order to clear a bracket (for the fuel lines, if I remember correctly). Otherwise, I like it.

After installing it, I disconnected the vacuum line, plugged the vacuum port in the plenum, and started the engine. Initial pressure was in the 20s, so I adjusted the regulator while watching the pressure gauge. It increased until the mid 30s, then stopped increasing, even though I kept adjusting the regulator. So it seems that I also need a fuel pump.

The roads will be covered with salt until Spring, so I haven't taken a test drive yet, but after looking at the old diaphragm, I'm pretty sure at least part of the problem is solved.