1998 GMC 3500 No Start--Help!

[Mikes91]

My wife's '98 GMC 3500 won't start. We tested the fuel pump and came back with about 12 PSI. Dropped tank and replaced fuel pump. Turns out the pump should be for a '97 truck because the truck is a pre-June build and uses the '97 pump.

Now have 15 PSI with the new pump, but the truck still will not start. It will start and run if gasoline is put in the intake. She has a Vortec 350 in the truck.

I think, but am NOT sure, that the '97 trucks with this fuel pump needed 51-55 PSI to run. We are getting 11 volts at the pump, which is full battery power with the key "on" and daytime running lights on. I think that pretty much rules out an electrical issue... 1 volt won't make that big a difference in pump pressure.


Any ideas? We have spark. Timing chain shouldn't be an issue... I think the issue is why we're not getting 51+ PSI...


Thanks,

Mike

 

[tonkadoctor]

1. dirty fuel filter (biggest cause of fuel pump failure)
2. dirty strainer in tank
3. wrong pump
4. bad pump.... Yes new ones can be bad.
5. kinked fuel line.
6. debris in fuel lines
7. debris in tank plugging the suction strainer

EFI gasser cars and trucks that call for a specific fuel pressure need that pressure as a minimum or the computer will not tell the injectors to fire.... If it calls for 50 PSI and you only have 49 psi it ain't gonna start. You really need to find out the correct spec and go from there.

On a positive note: Be happy it died which makes it easier to diagnose, worst ones are the ones that start, let you drive to walmart and then won't start again until hours later after it's towed in or the next morning when the pump cools off which then it gives a good reading until it dies again. Diagnosing one that is dying a slow death is a PITA.

 

[Mikes91]

I can blow through the fuel filter... can't see any restriction there but will replace anyway. The strainer on the pump is known good... Lines have no kinks... Not sure about the suction strainer.

This was a sudden failure; i.e., she drove a friend's house and the truck wouldn't start after being shut off. From all I can tell, it ran fine up till then.

Only things I can think of are: debris in fuel lines (we'll hook a gauge up to them once we get the right fittings) or wrong/weak new pump.

Maybe an intake problem? I just can't see why this thing would run fine, then fail suddenly when trying to start. I drove the truck the day before and it seemed OK.


Thank you,

Mike