1998 Pontiac Grand Prix GTP won't start at times unless gas pedal is pressed half...

[thebax2006]

...way down.? It starts OK for several days and then won't start unless the gas pedal is depressed. When acting up the motor won't idle for up to 3 miles of driving. This has been happening on and off at random for 4 months. The fuel pressure is in specs. at idle and at 3000 rpm. No fuel leaks in pressure regulator vacuum port or hose. The throttle body is clean. No SES light or codes. I'm an Import Master Tech and this ones got me. Thanks for any clues.
It does not matter if the motor is cold or at operating temperature. It can be running fine, I turn the motor off and at start up a few minutes later it will either be OK or act up again. You never know when it will act up or run normal.

 

[Goatman]

I would start with any grounds that are in the ECM area sometimes a corroded ground will cause an intermittant open ..
you may also have any number of wireing issues elsewhere in the system ..
If the fuel pressure is correct have you checked it while the problem is occuring, has the fuel filter been changed lately?
are there any vacuum leaks or bad hoses /gaskets
some 3800 series engines had intake gasket leak issues caused by the composite intake sealing surface distorting , they would need the intake replaced .. this was usually accompanied by a coolant leak over time but not allways .. some stuff to look for anyway ..