Greetings from the land down under :)

I am working with my father in law's on his 94 "vette", we do not know the history of this vehicle, sometimes it starts, sometimes it doesn't. 

This is what we have discovered so far.

Spark ✔️ Fuel Pressure ✔️  Turns Over ✔️

Things it doesn't do

Fire up every time 😞

Idle smoothly 😞

We haven't checked the fuel pump relay as the pump does work and stops when pressure is achieved at the rail.

When it does start with the fuel pump connected, it runs very rich and rough, doesn't idle well.  When you disconnect the fuel pump, it runs smooth and nowhere near as rich, but obviously stops when the fuel in the rail runs out. 

Has anyone heard of this problem, could it be an earthing issue? or something else?

Thanks 

The description you give sounds like either the fuel pump has been changed to one with too high pressure('94  LT1 should have 40-42lbs running) and it is over pressurizing the injectors, OR, the fuel pressure regulator system is malfunctioning(vacuum line off, perhaps?). I would suggest installing a fuel pressure gauge to see what the actual pressure is while running, and go from there.  Depending on what the fuel pressure test reveals, it is also possible the injectors are leaking. Start with finding the actual, running fuel pressure. 👍

Joel Adams  
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Had the fuel pressure checked and mechanic was happy with it, checked the fuel pressure at the pump, and after the regulator and again at the fuel rail. all checks out ok.  leaning towards maybe faulty injectors?  Planning on doing some electrical testing next week to see if anything presents itself there or not.

Have you had a scanner on it to see if there are any codes? A faulty temp sensor could cause rough running if the PCM thinks it is -45* ambient temp. Other skewed sensor readings could cause it too. A leaking injector(s) can be determined by powering up the fuel pump with a pressure gauge on the rail, engine not running. 

Joel Adams  
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Tried connecting to the ECU via the OBD, scanning tool couldn't connect, possibly broken wire or something.

Checked all fuses with multimeter and they seem fine.  

There is evidence of mice being in the car, however unable to see if they have been nibbling at stuff, will definitely investigate this further.

Vehicle was imported into Australia late last year.

I would definitely try to find the problem with this issue first. The PCM not having comm with to ALDL connector is a big red flag for me. Sometimes it is just a fuse, but you say all is well there. If the comm is down, then chances are other circuits are also compromised. 

Joel Adams  
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Try idle air control valve it will all the systems you are having

Thanks TLAVAC,

Would that be the valve near the rear of the motor (firewall end) that sits just below the fuel rail?

Cheers