Fuel pressure question

[lukeapre]

Gentlemen, I have a question. 99 7.4 2500 suburban. No CEL. All original engine parts except 5.0 injectors and all MSD ignition(distributor, coil, plugs, wires). I did replace fpr years back. So I dropped my fuel tank to rubberize frame and I snapped a nipple off my fuel pump. Had to replace. Finished install and just to check, I hooked my gauge to check fuel pressure once done and it reads 49-52. No rise when I hit the throttle, no rise when I pinch the return. Rigged my broke pump to work for a minute to test it and find the same results. Why no rise in fuel pressure? Doesn’t make sense to me.

 

[Schurkey]

Faulty pressure gauge?

 

[Scooterwrench]

49-52 should be max.
Don't the later EFI's have a VRFPR? If so pressure should be lower at idle and part throttle and increase with the drop in manifold vacuum.

 

[lukeapre]

Vrfpr? Variable rate FPR? I’ve never heard mention of that. Faulty gauge. Possibly. I can say it reads air accurately. Meaning I thought the same thing. Fit it to my air compressor hose and changed the psi up and down and the gauge responded correctly. I thought to at least be close. Odd thing is the gauge fluctuates maybe a pound or 2 at most when hooked to my truck. Also I thought fuel pressure for this engine was closer to 60.

 

[toolmakerson]

Gentlemen, I have a question. 99 7.4 2500 suburban. No CEL. All original engine parts except 5.0 injectors and all MSD ignition(distributor, coil, plugs, wires). I did replace fpr years back. So I dropped my fuel tank to rubberize frame and I snapped a nipple off my fuel pump. Had to replace. Finished install and just to check, I hooked my gauge to check fuel pressure once done and it reads 49-52. No rise when I hit the throttle, no rise when I pinch the return. Rigged my broke pump to work for a minute to test it and find the same results. Why no rise in fuel pressure? Doesn’t make sense to me.

replace fuel filter ? confirmed lines not plugged ? manual has a whole procedure to check whole system step by step.

 

[Carlaisle]

Factory spec on that engine is 56-62 key on engine off. There is no running spec. If the pressure doesn't go higher when you pinch the return something is amiss. Either the pump is incapable of producing higher pressure or the fuel is going somewhere.

 

[Schurkey]

Vrfpr? Variable rate FPR?

Vacuum-referenced fuel pressure regulator. Pressure varies with manifold vacuum--common as dirt on OEM fuel pressure regulators that aren't for TBI.

Factory spec on that engine is 56-62 key on engine off. There is no running spec.

Sounds right.

If the pressure doesn't go higher when you pinch the return something is amiss. Either the pump is incapable or producing higher pressure or the fuel is going somewhere.

If the return plumbing is blocked, I'd expect > 80 psi.

Faulty pump, punctured hose inside the tank bleeding fuel, lack of voltage/amperage to the pump, etc.

 

[lukeapre]

Vrfpr yes. There’s a little vacuum line on the top I can see, I assume that’s what that is. Voltage was good taken at the pump. I don’t know if the return is blocked, but you’d think the pressure at the gauge would be high if that were the case. Fuel filter is a year old, I’ll check pressure before it see what happens. Fuel pressure bounces around 1-2 pounds koeo, when started, also when given some gas by hand at the throttle. Also when I turn the engine off with gauge attached it drops a couple pounds and will not move. Does not bleed at all. Had it on the gauge on there for hours. I’m going to drop the tank again tomorrow, disconnect lines at engine and check for clogs. Where is this fuel going? Almost certain it’s something stupid, and it’s something I’m responsible for. Lol. Thanks for all the help. I will return with my findings.

 

[Supercharged111]

Gentlemen, I have a question. 99 7.4 2500 suburban. No CEL. All original engine parts except 5.0 injectors and all MSD ignition(distributor, coil, plugs, wires). I did replace fpr years back. So I dropped my fuel tank to rubberize frame and I snapped a nipple off my fuel pump. Had to replace. Finished install and just to check, I hooked my gauge to check fuel pressure once done and it reads 49-52. No rise when I hit the throttle, no rise when I pinch the return. Rigged my broke pump to work for a minute to test it and find the same results. Why no rise in fuel pressure? Doesn’t make sense to me.

What kind of fuel pump did you install?

 

[toolmakerson]

Vrfpr yes. There’s a little vacuum line on the top I can see, I assume that’s what that is. Voltage was good taken at the pump. I don’t know if the return is blocked, but you’d think the pressure at the gauge would be high if that were the case. Fuel filter is a year old, I’ll check pressure before it see what happens. Fuel pressure bounces around 1-2 pounds koeo, when started, also when given some gas by hand at the throttle. Also when I turn the engine off with gauge attached it drops a couple pounds and will not move. Does not bleed at all. Had it on the gauge on there for hours. I’m going to drop the tank again tomorrow, disconnect lines at engine and check for clogs. Where is this fuel going? Almost certain it’s something stupid, and it’s something I’m responsible for. Lol. Thanks for all the help. I will return with my findings.

Rig up your fuel guage right after the pump and run the pump, see what psi you get. this takes the filter and line out of the equation. I think that's one of the steps in the manual. I skipped all the steps cuz I just replaced my whole system, tank, lines, everything. But if the "new" pump was crap the problem would of course persist. After installation I got 60 so whew i was ok. (I say whew in this age of "new junk parts"). The pump hot wire and ground wire/connection not being 100% means potential for weak/intermittant pumping. As stated by others any problems/leaks in the whole sending unit inside the tank creates problem though everything else is fine, even pump itself.