1990 vette runs fine after 15-20 min won't start then wait 20 -min runs fine again until i shut it off

Sounds like your ignition module it is common on fords to do this when its cool it will start and run fine but once it heats up the truck dies until you let it cool down again and then your up and running till it heats up again. Some modules are mounted on the fender others are mounted with the distributor have it checked and let me know how it went.

First try and change the fuel filter. this also happened on a van i worked on. I cahnged the filter and it runs fine now.

I would say your starter is faulty. Since the vehicle runs fine, and doesn't start up after being shut off until it has cooled down, it sounds more like a starter that loses contact when it is hot. You could check the starter terminals for 12 volts using a multimeter when the key is turned to Start the engine after warming up the engine and shutting it off. If 12 volts are present, replace the starter. If 12 volts are not present, check the Starter relay.

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Did you replace the fuel filiter there is a big problem with the fuel pumps on these vehicles how long ago did you replace it and you repair the wireing problem to the pump

when it won't start do u hear the hum of the in tank electric fuel pump when the key is tured to run, it will only hum for 1 sencond, so turning it back off and to run repeated times helps to hear the pump.

Sounds like the pgmfi relay is going out. This is a common problem with these cars. They get hot and dont want to work, they cool off and everything is ok.

i have a 1990 vette doing the exact same thing. i have checked; fuel pressure, replaced fuel filter, checked spark, checked engine vacuum and i still cant seem to get the car to run once it's up to temp. any help on this would be great.

check fuel filter and fuel pump. replace if not functioning.

is it fuel injected? you might have a faulty sensor it sounds like a vapor lock, also you fuel injectors might be dirty or bad also check fuel pump pressure or a cloged fuel filter, or it is putting to much gas if your vette has a carb.

sounds like your fuel pump is going out. One thought might be that the fuel filter is causing enough resistance to over heat the pump. I would check the fule pressure to determine a free flow of fuel.