I'm actually helping a friend out with is GMC 4.3 V6 its a 1998.
His symptom is when the truck is cold it will start fine and run 15,30 min's. then it will stall out and not start back up.
Temp is running at 210
Its not throwing any codes

I put a fuel pressure gauge on the truck along with a spark tester. Started the truck up when cold. FP was 51 and spark showed a nice strong light.

After a few min's the the FP dipped down to 40psi and the truck stalled. On restart FP was 40, spark was fine but didn't start up.
Sat for a few min's, turn the key FP up to 60 and starts up.

The whole time spark is fine so this is defiently a fueling issue.

Turn key on... pressure jumps to then drops to 0
Turn key off.... pressure again jumps to 60 then 0

So at this point I do believe the system is losing pressure... I did replace the FPR and that didn't solve the problem except that now when its running the pressure stays constant and it will run smoother until the psi drops off again.

The fuel pump relay is working.

I plan on removing the tank today and taking a look at the fuel pump.

Do you think I'm going in the right direction? anything else I should be looking at?
thanks

 

Put in a new fuel pump, pressure came right up and stayed.
Ran great.

Thanks for your help!