engine fallin on it's face whenever engine warms up

[vinnye89]

Hey guys, I have this problem with my Firebird and I figured you guys would be able to help me out.

I just got my LT1 completely rebuilt. Now we have the motor back in the engine bay and everything hooked up. Whenever I first start the car, I can rev the engine and it'll have power, but whenver the engine is warmed up, it falls flat on it's face. I try to rev it and it wants to die. I've changed the computer so far but that didn't do anything. Does anyone else have any idea what it could be?
thanks in advance.

[Injuneer]

A couple possibilities.... first would be some component that breaks down when it heats up. Could be something like the IC Module, optical module in the Opti, plug wire insulation, etc. Or, it might be something related to "closed loop" operation, which only occurs after the coolant has warmed up and the O2 sensors are hot enough to work.

What are the mods? How long after cold start does it run well? When does it start to lose power. Does it start to misfire? Is it just some roughness or surging?

[fastTa]

I agree that it is likely ignition or closed loop related. It is fairly common for the ignition control module (ICM) that is located on the front of the driver's side cylinder head to get heat soaked and create problems. Could also be the Optispark distributor.

Best thing to do is to get it on a scanner and see what is going on. You can also retrieve the codes to give you a place to start.

What year is your car? It would help to know whether it is OBD-I or OBD-II.

What led you to replace the PCM?

[vinnye89]

this is what i got. 95 Formula firebird
boared 30 over with flat top pistons and oversized piston rings. gasket matched intake and heads. and the heads have been reworked. I replace the optispark cover. and of course new sparkplugs and wires. My mechanic said he hooked up the scanner and nothing came up. Thats why i changed the computer in the first place. He said last week that he was going to check for fuel pressure because he thought maybe it was the fuel pump goin out, but that doesnt make sense to me, cuz why would it be fine whenever it first starts? He still hasn't touched it and he's had my car for over 6months and i'm tired of waitin on him. So a friend of mine and i are goin to go ahead and finish the job.

Don't know really if its a OBD I or OBD II, but i'm thinking its OBD I.
I'll get back to you on exactly what it does, but i believe it idles fine but then you give it some juice and it rough. But I'm goin by there later on tday so i'll check. Thanks again guys

[Injuneer]

The 95's are an OBD-I PCM with a 16-pin OBD-II ALDL connector..... sort of makes it difficult to scan unless you have the correct adapter for the ALDL.

Did you make any changes to the injectors during the rebuild? Different cam? Any PCM programming?

[vinnye89]

I haven't done any reprograming or changes to the injectors...haven't been able to get a hold of my guy so i haven't been able to check out more details on the car, or let him know of ya'll thoughts. hopfully i can talk to him today

[DeWynter]

Well without a tune it could be starved for fuel?

[vinnye89]

I mean i did get the PCM flashed if that's what your talkin about.

[Injuneer]

You only mentioned the heads are reworked.... is there a different cam as well? You're saying the PCM was "tuned" for the specific mods in the car???

It could be something as simple as a faulty coolant temp sensor.... if it showing a very low coolant temp, it will start and run fine cold, and as soon as the engine warms up it will be running too rich.

Your description of the problem is very vague... its hard to do this over the 'net, without a chance to see the engine run. A suggestion:

Describe in detail all the work that was done to the engine, and all the parts that are different than stock.

Describe how the engine runs, from cold start through warmup. Appears this is just sitting in a shop? Or has it been driven on the street?

What diagnostic work has been done? Has it been scanned for codes? Have all the vital sensors been checked to make sure they are working and putting out the correct data? Have you pulled plugs to see their condition?

Without a lot more input on the above points, I doubt anyone will be able to help you. Too much detail is way better than too little.