I have an 1989 4.3 it wont start unless i press the gas pedal down. then once it is started it idles fine but as soon as you put it into gear and try to give it throttle it dies, but if i am lucky enough to get it to go it will run ok for a short period then it will begin to miss and then run smooth for a few miles then miss again but it misses more times than not. i have replaced all the spark components cap,button,wires and plugs but it didnt help.

Had this problem too, and you need to check that your TPS sensor plug.harness is tight, the best way to do that is to actually unplug it, blow it out and then plug it back in. But the same problem happened again and it turned out to be the CPS plug had come loose, again check it and make sure it's seated well, and everyone will tell you to replace them, but if take a wooden dole, or stick and can tap yours gently down towards the transmission I've found that it works well after that.

http://www.fototime.com/554D75F2A0D08B0/standard.jpg

Clutch plate and pressure plate seem to be too weak to pull the car. Or one of the CV joints is detached from the gear box. To test the clutch plate, park the car on flat surface. Pull the hand brake up, start the car, depress the foot brake pedal, depress the clutch pedal, put the car in 1st gear, press the gas pedal and leave the clutch pedal at once, while keeping the brake pedal depressed.
Follow one of the two following statements:
1. Engine does not shut down and remain start. (Clutch plate, pressure plate and clutch bearing defective - Replace all as a set)
2. Engine shuts down. (Clutch system is OK and does not require any repair).

my truck starts right up if I hold the petal to the floor but wont start if I don't hold petal to floor

Chances are it could be over fueling with an injector stuck open or a vacuum leak from the intake manifold gaskets more than likely

A variance of 20 PSI indicates a problem.

i think you might be right it sounds like your engine is getting over fed fuel you are probably not getting very good fuel mileage either the 3.8 and 5.0 would defintely require the same volume of fuel it is probably playing havoc on the pressure regulators i would get another fuel pump and start there

Have you scanned for fault codes? How about the Throttle position sensor, have you tested it?

Try depressing slightly the gas pedal to see if the engine continues to run when you put it in gear. Might have to depredd gas pedal and brake simultaniusly. if the engine continues to run, then the problem is in the idle circuit. Try decarboning the throttle body and the idle speed control.

replace the gas filter just follow the gas line of the carb it should be easy to find

First, try a new accelerator pump. They're less than $10 and easy to install. On the side of the carburator, near the linkage. It's simple a small rubber diaphragm.