Quick description of car: 1985 automatic, 120,000 miles, stock. Little project for me and my dad

Long story short, I have a 1985 C4 that ran and drove really well, up until 2 months ago where I did a close to WOT pull and after letting off, the car stumbled and died.
Started back up, drove a few miles, died again. Repeat for how ever many tries until I got it home.

Replaced the fuel pump, and drove the car 100 miles or so and it ran great.
Had a guy come take a look at the car since its for sale, and he drove it and it was great. But, when he left, we turned the car on to park it, and it stalled right away. We started it up and it was idling real bad, limped it into the garage. Its had a little difficulty at times after being driven to get started again, usually a quick tap on the throttle set it back to normal idle.
Changed the IAC, took all the steps to reset it, car seemed to be great, drove for 15 miles or so, and it died about a mile away from my house.
Now it won't start, and when it does it either dies right away or idles extremely low or at 1500rpms.

We did all the steps to reset the IAC again and again, no engine codes come up, just looking to see where to go from here.
Vacuum lines are all good, just rechecked the fuel pump, any help is greatly appreciated

Tried doing the minimum idle adjustment with setting the TBS to .054v and the car won't allow me to change the idle. As soon as we try coaxing the engine with a little gas, it dies right away.

 

i would start by removing the dizzy and checking for cracked parts or a wire hanging off the ICM.The icm is just below the rotor with 5 wires that must be secure.

 

Would that cause the car to start sometimes and have a poor idle, then not start at all other times?

 

It sure could also have the coil tested while you are at it. If you have fuel pressure then something electrical is glitching.

 

Thanks I'll take a look at that. Currently the fuel pump isn't turning on now unless making it turn on with the "G" pin, so we replaced the fuel pump relay with no luck, and the fuse isn't blown either.

 

When you turn the key to "RUN", it should prime for 2 seconds and then shut off. After it gets ignition pulses, it would start up and keep going.

 

you haven't mentioned fuel pressure...do you have a gauge on it?....did you eliminate your pulsator with the new pump?...sure all the fuel lines in the tank are secure?....the connector on both sides of the bulkhead where the wires go in to the tank should be checked......good luck...

 

We weren't able to get a gauge, we pressed on the Schrader valve with pressure at first, and now its not holding pressure after manually running the pump.
I don't believe we eliminated the pulsator. Yesterday it ran great for 20 minutes, and now its back to square one

 

We're not getting that initial prime when turning the key to "run"

 

Check all your fuses and wiring and GROUNDS. I f you are still not hearing the pump, then you are going to need a dvom or better yet a Power Probe. This tool will be needed to test your circuitry and relays. First check for B+power and ground at pump. Can you get this far?

 

It has a timer. So if you keep flipping the key on and off, it won't. So much time has to elapse between initial primes.. I'll check my FSM later today and see if I can find a time.

 

Crimp off the return line and see if you can hold pressure when you manually run the pump. If you can, it may be a bad regulator. If you can't, injector is leaking or the pulzator

 

they usually give you a section of hose to replace the pulsator.....it happened to me, changed the fuel pump and not the pulsator....the line would blow out of the pulsator and then stick back in when you stopped the engine....couldn't figure it out for awhile....really need to have a fuel pressure gauge on it....it would sure help you.....

 

I replaced the ICM, FP relay, IAC, oil pressure switch, and a few frayed looking cables with no luck, however most of these parts didn't look too good and they were cheap.
My '90 K1500 had this problem and it was the oil pressure switch, thought I could get lucky and have it be this but nope.

I just picked up a fuel pressure gauge and an ECM today just in case (70 bucks, couldn't pass it up) so hopefully tomorrow it finally gets fixed.
Thank you to everyone that's given suggestions so far, I know its hard to take a stab in the dark at a 34 year old car without looking at it, I appreciate it.

 

Check the grounds for the fuel pump. I would start by testing the pump at it's harness connection under the gas fill door. BE VERY CAREFUL in this area, I use a Power Probe and test the ground and the battery voltage. There are three wires going into the fuel pump assembly, One is 12 Vdc, one is the signal for the fuel level sending unit and the last one is the ground for both the pump and the sending unit. If there is much of a voltage drop then your pump might not be able to energize and pump gasoline. Test the ground and verify the Fuel Pump Relay output voltage is making the trip to the fuel pump without any losses.

If the power is not making it to the fuse block then you have to go to the post that supplies power to the fusible links which is below and behind the battery on my 1988 C4. My car had corrosion on the post and low voltages everywhere, it was not able to prime the fuel pump either until I cleaned the post and all the connectors that are there. Start checking voltages at the battery and then go to the Fuse Panel, the voltages should all be the same or very close. Test the output of all the fuses just to be sure they are all getting power. I had aluminum fuses that corroded in the fuse panel and they would not conduct electricity to the circuits either. It seems to be placed in a bad spot where water from the door seal gets on and into the fuse panel. I ended up cleaning everything and replacing all the fuses with better quality parts.

I hope that you can get Your Corvette back on the road, Reliably!

Best Regards,
Chris

 

Replaced the ECM, and now the car runs after a few cranks to build the pressure for the oil pressure switch, but the fuel pump still won't run off the relay.
The relay works, tested it and swapped it with another relay to confirm, getting power to the relay, and there's voltage in the fuel pump connector.

So tomorrow will be all grounds and fusible links, I think we're finally getting close.

 

On the 85 you need to replace the other oil pressure switch next to the oil pressure switch if it is old and rusty also. it gets pressure to get the fuel pump going we believe.

 

The fusible links are not behind the battery like the 1988/89. I own both the 89 and an 85. The 89 has the post with all the links the 85 does not. I believe that the links go through the firewall connector C100. And yes the oil pressure sender has an effect on fuel pressure. If you loose oil pressure the car shuts off. I found that out when my hose to my external oil cooler came loose and the oil came out of the car. No damage to the motor. So if the sender is bad it can cause you problems.
Aside from changing parts you may want to look at the FSM for ways to check for broken wires. Although if it was running well I find it hard to believe one would crap out while te car was running. Unless it touched the exhaust manifold and got burned and shorted.
Good luck

 

Will this myth ever die?

The Fuel Pump Relay is the primary power source for the Fuel Pump

ECM Primes for two seconds, FP relay sends power once ignition signals are sent, the Oil Pressure switch is a BACK UP, or a second leg of power. It's failure or its detection of low oil pressure will not stop the fuel pump from working unless the relay is also bad.

Look at the wiring diagram:

 

But everybody says......