Engine randomly shuts off

[87chevyk10]

Hi, i am working on a 89 305 tpi for a guy. he said that when he is driving it randomly shuts off and is very inconsistent when it does shut off. he can not guess when its about to shut off. after it shuts off it wont start up until 5 minutes later. here is a list of things he changed or did:
tune up
fuel filter
distributor
Fuel injectors
Fuel pressure regulator
strainer/ float sending
Ecm (twice)
ignition switch
02 sensor
tps
egr valve
maf sensor
lac valve
ignition module
coil
2 relays
so can you guys help me out here and tell me what possibly it could be?
After it shuts off it starts up again but the idle goes up and down. when u hit the gas it shuts off. and when you start it up again the idle still will go up and down but when u hit the gas it would stay running. And the guy i am working on this for replaced all the things listed above by a garage and by his self. And now i am trying to figure this out for him. he took it to a couple of garages and they dont know.

[LONGHAIR]

Randomly shutting off pretty much has to be an electrical issue, but "unpredictable" makes it far worse to figure out.
Could be as simple as a loose ground.
Most of the things that have been replaced could make it run badly (or not at all) if they were bad, but the random shut off that runs fine the rest of the time is not easily connected to most of them.
Were these new ECMs?

[cableguy0]

Was the fuel pump replaced? How much fuel pressure does it have? I have seen a few Gm vehicles recently that show 50lbs+ of fuel pressure but are actually not moving any fuel just building air pressure. No clue how it manages to do it but I have seen it 4 or 5 times in the last year. Open the valve on my gauge set and fuel barely dribbles out showing 50psi or more.

[cableguy0]

Also is it throwing any codes at all? The computers in the obd1 gms are notoriously horrible and the replacements from the parts store aren't much better.

[87chevyk10]

Originally Posted by LONGHAIR

Randomly shutting off pretty much has to be an electrical issue, but "unpredictable" makes it far worse to figure out.
Could be as simple as a loose ground.
Most of the things that have been replaced could make it run badly (or not at all) if they were bad, but the random shut off that runs fine the rest of the time is not easily connected to most of them.
Were these new ECMs?

Tried 2 different brand new Ecms. I will go through and check the grounds tomorrow thanks.

[87chevyk10]

Originally Posted by cableguy0

Also is it throwing any codes at all? The computers in the obd1 gms are notoriously horrible and the replacements from the parts store aren't much better.

New fuel pump. I dont think it's throwing any codes but I will double check tomorrow. And also will check fuel pressure and stuff tomorrow thanks.

[Special-Ed]

did you check the idle circuit(air) and clean the throttle butterfly... carbon build up can cause this exact issue... it was very common with the quad4 and other motors from pontiac/gm and other motors from back in the day.

[87chevyk10]

Originally Posted by Special-Ed

did you check the idle circuit(air) and clean the throttle butterfly... carbon build up can cause this exact issue... it was very common with the quad4 and other motors from pontiac/gm and other motors from back in the day.

No, I will tomorrow thanks! Keep throwing stuff at me that I can do.

[68gmsee]

I had the throttle position sensor act up on one of my vehicle and do strange things with the idle and surging. Since you show it's been replaced, check the connector very carefully for loose, broken or corroded wires. Maybe try to follow it back.

[Gumby]

Did ya open the ECM's and inspect the board with eyes ands nose for burnt section?

won't always toss a code.

had a CCP valve mess with me for a few years.

canister Control Purge