@ihatebarkingdogs Thanks!
You may be on to something. Going back years and 2 battery replacement cycles, when the battery is weak, this same problem happened. It would crank but not start.
Replaced the battery and all was fine. Fast forward 10 years and now the pump is weaker and even on a full charge it doesn't run fully during cranking.

In response to other suggestions, over the weekend I tried cranking long enough to get the oil pressure up, but to no avail.

Since then I have tried this twice and it worked...before cranking, I turn the ignition to "on", let everything energize (and the fuel pump run its prime cycle for 2-sec), then off. Repeat 4 more times. Then crank it and it starts. Theory: If the pump is weak, one 2-sec prime cycle may not be enough, so doing more than one gets the pressure up. (5 probably more than needed) (will need to check with fuel pressure gauge as suggested). Does this make sense?

 

Have you tried testing the pump with a gauge? I think it would be the same on an LT1 motor where there is a test port? Turn the key on and see if the pressure jumps up. If it does, what does it settle at? Does it bleed down fast.

 

Fuel bleed port on underside of right fuel rail on this one.
Just ordered the Fuel Pressure test gauge set on Amazon. $32. Delivery tomorrow.
Stay tuned...

 

Great suggestions to investigate the fuel pressure.
I just tested the pressure with interesting results.
Prime operation with key in "on" position: 12 psi.
After starting, pressure builds to 38 psi.

Looks like the problem involves the prime relay circuit not delivering enough juice to the pump.

Any ideas on the root cause?

 

Good test. Now prove the theory. Splice in a wire to the harness at the fuel pump, measure the voltage while cranking and while running. Compare that with battery voltage while cranking/running. If it's meaningfully lower, then you know that you've got voltage drop between the battery and the pump. Is through the relay? Harness? You have to narrow it down by testing w/a volt meter.

 

@Tom400CFI ...thanks for the plan; I'll do that next!
I wonder if anyone has had similar issues.

 

Do the test as Tom suggested but I wonder if the pump is weak another thing you can do is jump the wire to the pump at the ALDL doe 2 seconds and see if you can get the same results

 

Thanks aklim. I don't think the 93 has the ALDL input for the pump. Not sure where the Packard one-terminal is located.

Per ihatebarking dogs: Last year for the ALDL Pin G test is 1989. 90-up have a loose Packard one-terminal connector hanging out of the engine harness. Adjacent to the ECM. I think it is still a red wire, but may be grey. The LT-5 does not have this test terminal.

 

IT'S THE FUEL PUMP!

Today:
Battery: 11.9V
Prime: 0 psi with 11.1V measured at the pump for 2 sec. (Intermittent, sometimes prime=10 psi)
Engine Running: 13.9V at the pump, producing 20 psi, sometimes 30 psi

At this point: Could be the Prime circuit not providing enough voltage to the pump or a bad pump.
Next: disconnected the alternator so that the Prime Circuit and Oil Pressure Circuit both use battery power

Prime: 0 psi, 11.1V
Running: 0 psi at 11.1V

Conclusion:
The two circuits produce the same voltage at the pump when using the same voltage source (Battery) The prime circuit is not deficient.
When the pump gets 13.9V it runs, but at diminished output (Once started the alternator provides 13.9V as it does right now)
When the pump gets 11.1V during prime it often does not produce output.
The pump should run on 11.1V
The pump is bad.

I'll replace the pump and let you know what happens.

As an aside, I recently flunked my California emissions test. I'm hoping that low fuel pressure might be a small factor.
Measured HC=36 ppm, Allowable=34ppm, so not failing by much.

Thanks everyone for your help!!!

 

Hot dog! Nice work. That is some proper diagnosing you did there.

 

Had a little coaching to help! Thanks again!

BTW: Found the pump assembly, sending unit, filler neck bracket by Delphi for 72.99 at Autozone. Next day delivery! (might buy two...haha)
Crock Auto wants $283 or same.

 

Maybe this will help:
https://www.smogtips.com/failed-high...carbons-HC.cfm

 

I'm still baffled as to what the pump has to do with the low voltage for priming unless that circuit is bad somehow.

 

Ah. I see. Good point

 

it does. Its not an interlock, it's a backup. And I dont know where people get the idea that there is a priming circuit and a run circuit.

 

I had a very similar problem and solved it.
The battery voltage of 11.1 IS NOT OKAY! Your battery voltage should be 12.5-12.7 open circuit. Your pump on and key in run position should not push the voltage to 11.1 that fast. Either Charge the poop out of that battery and see if it comes back or replace it

On my 1988 Corvette I had low voltages everywhere including the fuel pump and the fuel pump relay. The problem was the little post under the battery that has 7 or 8 Fusible Links that supply most of the car with power. My post had so much corrosion that it would get warm when the car was running. I removed the terminals and cleaned each and every one which brought full voltage to the interior of the car. I just cleaned the post again as I saw it had corroded up again. This time I greased the connectors and the post and then I put a vacuum cap over the whole post. This post may be in a different location on your Corvette but on the 1988 it was just below and rear of the battery. The 7 or 8 Fusible Links supply most of the car with power. If you have a corroded connection that would explain the voltage drops that you are seeing.

Ground Straps are Critical for our Corvettes. I had a broken one on my exhaust and after fixing it my radio antenna started working again. Start at the battery and measure the voltage, then measure the voltage at the fuse block. On my car the fuse block had low voltage and was corroded. The corrosion on the aluminum tabs in the fuse box made me discover another problem. None of my fuses were letting power pass through them, the aluminum blades had a layer of corrosion that prevented contact. I replaced the fuses and cleaned the fuse holders with a points file. Then the inside of the car would work and I could finally crank my car. After finding a few other bad grounds I finally have a functioning Corvette.

Corrosion is a killer, and just because you can't see it does't mean it is not there. On a a car with Electronic Fuel Injection having good connections is critical for the proper functioning of your Corvette.

Good Luck and may your Corvette be putting-smiles on your face instead of making you work on it.

Best Regards,
Chris

 

Oil pressure switch...I presume the intent is to cut off the fuel if the oil pressure gets too low so the engine is not destroyed.

 

AFAIK it isn't more than a backup based on the diagram.

 

No. You'd think that might be a good idea...but it is only a back up to the fuel pump relay failing.

 

@64Scout You are correct. Per the FSM, ECM closes the fuel pump relay if pulses from the ignition are detected. In parallel, the oil pressure switch also provides power when pressure is reached. Either will run the pump. ECM turns off the pump after 2 sec. if no pulses detected. Turning the key to "on" will run the pump for 2 sec. and charge the fuel system. Calling this "priming" is not a bad description.