1988 5.0L Iroc-Z (VIN code F). Factory stock with 88,000 miles.

Ok first the new parts list:

Complete Ignition Tune Up
Air and Fuel Filter
Oil and Filter change
GM vent for the gas tank
All lines to and from the tank have been disconnected and had compressed air blown through them to make sure none are obstructed
All rubber fuel lines connecting hard lines on rear of chassis to fuel tank replaced with new
Gas Cap

Ok now to the problem: It did this before any work was done and is the reason for the new parts being installed. Still does it.

The fuel pump will continue to run after the car is shut off. It does not turn off by itself UNTIL you loosen the gas cap. When you do loosen the gas cap there is a huge relief of pressure from the gas tank. What else should I be looking for?

 

Your fuel pump switch is probably bad and not turning off, it is located on the engine block right above the oil filter. An easy way to test it is to unplug the connector while the fuel pump is still running, if the pump stops, replace the switch.

 

Or the relay which is up above the brake booster.

 

sure its the pump and not the sound of all that pressure pushing its way out of the vent or gas cap?

 

Positive.

That's part of the problem. The pump will run for hours if you don't relieve the pressure by loosening the cap. Vent is new, all lines are clear... why is there such a pressure build up and why isn't it escaping out of the vent?

 

i would say to check your charcoal canister, it may be plugged up, i do not know if the car runs the pump to try to relieve pressure, but that may be part of it, also is your heat shield under there? any recent exhaust work?

 

unplug your oil pressure sending unit right above the oil filter on driverside of engine bay....the sending unit is bad...i know because i just replaced mine...it wouldnt shut off unless i pulled the fuel pump fuse or unplugged the sending unit

 

Heat shield is there. Exhaust is new, but changed before my BIL bought the car.

I will pick up new sending unit and relay since they are cheap enough. Still stumped on why it builds so much pressure in the tank....