have engine from a 94 corvette installed in custom 51 chevy truck. used painless wiring harness and had ecu flashed to remove vats, still starts and dies after 3.5 seconds. Any thoughts? new Optispark unit installed. new plus wires and has dual o2 sensors installed

 

have you confirmed fuel pressure is still good when motor shuts off?

thought is ECU is not seeing oil pressure and shutting off engine

you have solid FP & OP when shut off happens??

 

I have FP, I don't know on OP,. If oil is low it can shut engine down at 3.5 seconds? That would be something I wouldn't think of but would be something I'll check in am

 

did you open up the donor motor or just install it as is in the truck?

if you did open up the motor did you reuse the stock oil pump drive shaft?

Confirm the oil pressure is there...even if the oil level is full there could be something preventing "oil pressure" (why I asked if you opened up motor)

You have a custom install using a aftermarket wiring so there may be something not right in the wiring, IDK.

was just asking to check fuel pressure and oil pressure while the engine shut off occurs. You would need fuel and oil pressure gauges installed to do this. I believe the ECU will shut off engine if oil pressure is to low

 

How much fuel pressure do you have? If you are checking just by hitting the Schrader valve and see fuel coming out that doesn't mean it's enough to keep the engine running. Check with a gauge if you haven't already.
After this 3.5 seconds can you immediately start the engine again? If VATS was the problem I thought you had to wait something like 10 or 15min before attempting to start again?

 

I "think" the oil pressure switch cuts of fuel to injectors if it does not see oil pressure. I don't think it does on 94-95 (OBD1). Not sure if will start first...but since oil pressure is "zero" before you turn the key maybe it will start for a few seconds then when oil pressure switch see's no oil pressure it cuts fuel to injectors.

with a fuel pressure gauge it should immediately jump to 43psi then drop a few lbs once fuel pump primes. If oil pressure switch is cutting of fuel injectors and kiling motor the FP obviously would drop but since the engine cut off would happen at the same time would be hard to determine if it was a fuel pressure issue

so the ? is...do you have oil pressure "when" the engine does actually start or does it stay a zero?

OP, what year is the donor motor/ECU?

 

If the oil pressure sending unit on the rear of the motor is not functioning properly it can cause this issue. I had this issue on my Buick, I didn't notice that I bumped it and cracked it at the base. Bought new sending unit and it started up and stayed running. Check fuel pressure with a gauge also.