i have a 1990 iroc 5.7 the fuel pump will only come on with a toggle switch ive changed the relay an still no help the car will start but only after the oil pressure builds up then it doesnt want to idle until it warms up and if you drive it sometimes the volt gauge will fluxuate it runs fine once it warms up but its something not right and i need all the help i can get all is welcome and appereciated thanks

 

The fuel pump relay needs a ground to operate. It sounds like you may have a loose ground somewhere. Check the frame to negative wires and especially the 3 wires that are bolted to the back of the driver's side cylinder head (nearly impossible to get to). There's only 2 arrows in the pic below, but if you look close, there are 3 terminals.