This is what it's doing. It runs just fine for maybe 10 min. Then the idle lowers and runs rough for about 5 min and returns to running just fine.

I took the motor out of my 94 Trans am and put it in my 95 formula. Used the pcm from the 94 and had to use the 95 wiring harness. It also got new headers. Went from 1-3/4 pacesetter to 1-7/8 lpp.

The trans am had been sitting for a while so I was thinking maybe bad gas. (Also swaped the gas tank). I put another $20 of fresh gas in and the issue didn't change. Tried several different icms.

Got any ideas?

 

Is this only from a cold start or does it do it when it's warm/ every time you turn it off?

Things to check:

cracked wire/ plug

Fuel pressure drop (when it's running rough). Does it do this while sitting at idle in the driveway? ~Fuel pressure gauge~

Unplug the MAF to see if it changes when doing it

Coolant temp sensor resistance (in water pump) ~digital multimeter~

Oxygen sensors' resistance cold and warm at pigtail connector to engine harness

Age of fuel filter?

Ideally, datalogging would be helpful. These cars should come with an ALDL cable... most good software is free for OBD1 F cars.

EDIT: LPPs are stainless with no coating, right? Breaking in/ heat cycling new, coated headers sometimes results in clogged O2 sensors...

 

I read a similar post and it turned out to be the O2 sensors. I unplugged the passenger side and the problem went away. I ordered new o2 sensors already.

The fuel filter is new, The headers are uncoated. I ordered an OBD2 cable that should be here in a few days. It only does it while warm

 

have you scanned the car to see what your IAC counts are when this issue happens?

 

I haven't been able to connect with the car yet. Waiting on a new cable to show up

 

Got my new o2s and cable. O2s didn't change anything.

Iac counts are pegged at 160 at all times. Runs perfectly fine in open loop. Everything goes to **** as soon as it hits closed loop. O2s seem to be about equal and reacting fine. Maf looks to be reading ok. Had a spare and swapped it out. Nothing changed

 

I'm leaning towards either an open in the IAC circuit or a bad IAC. Ran just fine in the old car but the 95 harness was pretty rough. I had to do some cleaning up on it before using it. Didn't touch the IAC wiring tho.

The IAC is the only parameter I can see not doing what it's supposed to do. Also, I accidentally left it unplugged and it ran the exact same way.

 

Just noticed the LT fuel trims are pegged at 160 also but that may be just because it's running open headers right now

 

Hooked up the Y-pipe. Problem went away. Thanks for the input