I have a 98 z71 pickup, 5.7 vortec. can someone tell me what the fuel pressure should be? with key on engine off it is 56, with engine running it is at 52 for a few min then drops to 42 at which point it quits. fuel filter is new. Thanks, Russell

 

Could be a bad pressure regulator or pump. That 55psi pressure reading sounds about right when the motor is off then it cant keep up when running.

Just my 2 cents, lets see what the pros have to say.

 

When I first got my truck the pump and regulator were bad. If it takes a while to crank, but kinda starts faster if you give it some gas, it's the regulator. If it starts fast, but the fuel pressure problem is still there, it is the pump. What problems are you experiencing that made you check the fuel pressure? The drop you are experiencing was the fuel pump for me. Is the dying part the only problem you've experienced so far? I say the pump is the problem. The Fuel pressure regulator (fpr) is cheaper though. About 50 at autozone or oreilly compared to a 300 dollar fuel pump.

 

I have the same issue with my 2000 escalade. You put to your foot to the floor and it acts like it is has no fuel but as soon as you let off the gas it will pick back up till you touch the gas again,only you could bump it and it acts out. I also noticed it jerking and I get an occasional backfire but only on wot. I've been fighting this thing for months with no avail.
I've tried fpr,injectors. I'm doing a fp test tonight. So far at idle I get 50 psi and accelerating in park it goes to about 55-62. Any suggestions I'm really getting sick of this. I'm embarrassed tho drive it.

 

I was having some backfiring at start up, and some hesitating when I gave it gas. It seemed like it would die if I tried to accelerate when it was cold. It does better today, now that is it warm out.

It starts up and seems to be idling ok now, but still has the hesitation. I tested the fuel pressure and when I switched the key to the on position there was no pre-start pressure. when I cranked it over it starts and then the pressure slowly climbs to 50 psi. It takes about 30 seconds to get to 50. I revved it a little and it went up to 53psi.

I have a 1997 Sierra 1500 with Vortec fuel injection.

 

Need a few more details about your truck because it matters for how the fuel pump is powered-

1) What is the engine displacement (4.3, 5.0, etc)
2) Does it have dual tanks, even if it’s only functional on one right now?

 

engine size is 5.7 and it has a single tank.

 

Ok, I’m thinking you may have a faulty FP relay (or there may be a problem with one of the related circuits), so here’s a few things to try:

1) With the engine running, disconnect the oil pressure switch at the back of the engine by the distributor. The oil pressure switch provides a bypass of the FP supply circuit in the event of FP relay failure. If the engine dies when you disconnect the oil pressure switch, the FP relay may be faulty, but you want to verify the circuits first.

2) Make the following checks with a test light at the FP relay with the connector unhooked from the relay (use diagram below for clarity).
-With test light connected to battery POSITIVE, probe the black/white wire to verify the ground
-With test light connected to battery NEGATIVE for the remaining checks, probe the orange wire (hot at all times), probe the dark green/white wire and check for the light to turn on for about 2-3 seconds when the key is turned on without cranking.
-Jumper the orange wire to the gray wire - the fuel pump should turn on

Regarding the red wire labeled as “Fuel pump prime connector” - I can’t say that I’ve seen this on similar Chevy trucks, so I’m not sure if there’s a terminated lead there or if it joins the wiring harness or what. If it’s a terminated lead, jumper it to battery positive with the relay installed and the fuel pump should turn on. If it joins the wiring harness, that may actually be where you want to test for a 2-3 second prime when the key is turned on (unfortunately the wiring diagram provides no other details on that circuit and I’ve been unable to find a reference to it in the service manual).

If the circuitry for the relay checks out, replace the FP relay.

 

instead of pulling the oil pressure switch pull the relay. It will tell you the same thing and you don’t have to climb over the engine to get to it. Also if the switch is bad your gauge will read zero or pegged out. Pressure for these fi engines spec is 60-66 psi. 50 is ok to start up but it will be lean throughout throttle. Min is 60 psi or your going to have nothing but issues

 

so giving it throttle it should stay atleast 60psi my 5.7 idle is at 52ish give throttle it jumps to 60 and steadily drops back to 50s while full throttle could be a bad pump?