Vortec Fuel pressure bleeding off

[kennythewelder]

97 Vortec 5.7L 240,000 miles. Truck runs grate, with plenty of power, but is getting a little hard to start but only sometimes. I needed to do an oil change today, so I go to auto zone yesterday to get my oil, and filter. I figured while I was there, I would do a fuel pressure test. I'm getting just over 60 PSI prestart, and in the mid 50s after start. Rev the engine and fuel pressure comes up. OK so all looks good there. There are no leaks anywhere that I can see, including inside of the intake. So what stops the fuel pressure from bleeding off back into the tank.

 

[Oldblue98]

97 Vortec 5.7L 240,000 miles. Truck runs grate, with plenty of power, but is getting a little hard to start but only sometimes. I needed to do an oil change today, so I go to auto zone yesterday to get my oil, and filter. I figured while I was there, I would do a fuel pressure test. I'm getting just over 60 PSI prestart, and in the mid 50s after start. Rev the engine and fuel pressure comes up. OK so all looks good there. There are no leaks anywhere that I can see, including inside of the intake. So what stops the fuel pressure from bleeding off back into the tank.

Ken K posted this awhile back, maybe this will help

The check valve is located inside the fuel pump pressure outlet. It is not spring load, only gravity allows it to seat. Over time, fine dust & dirt finds it way into the fuel tank via the gas station. I had over a hundred shops that stop fuel pump failures & returns when inspecting the strainer. Dark strainer indicate clean the tank with hot water and agitation is the fix.
Your fuel pressure regulator is the stop for fuel returning to the tank when off. Otherwise, leakage at the injectors will cause this issue. As said, the eco will power the pump for two seconds on GM's during key on, but on a return type system, that is about 18 feet of line to fill and two seconds is not enough. Try some "Techron" in the tank to clean the injectors to help them seat. Run on low tank and repeat.
In the dealer, we isolate the system and watch for leak-down thru the pump, regulator or injectors. Adaptor hoses with shut-off valve in line help us find where if any, leakage is occurring. If it injectors, then ACDelco X66-P or GM's Top Engine Cleaner is used. Caution; Do not let this 10% mixture of X66-P and fuel return to the tank, only the injectors and fuel rail.

Chevron makes "Techron" and is the main ingredient in "Top Tier" fuel but at a lower %. Warranty allows us to clean injectors once, then directs the customer to use only "Top Tier" fuels. They sell "Techron" fuel injector cleaner in almost all local parts stores. It can only help, where ever you leak down is happening. Cheapest fix also, unless the hose on your pressure regulator has gas in it.

 

[east302]

The injectors need at least 40-psi to open, but GM literature says that 44-psi (measured at the upstream schrader valve) is the margin for being inoperable. Factory spec at the schrader valve is 60-66 psi. The pump is a constant speed pump, so the regulator will modulate to send excess fuel back to the tank and maintain an upstream pressure at the injectors.

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Not a specific GM, but here’s a return type injection system...

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You would install a shutoff valve in the return line (near the filter) to check for a failed regulator or leaking injector.


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[east302]

Here’s the test procedure if you need it. I don’t know if a parts store rents the shutoff valve kits, but you could probably rig something together with a ball valve or maybe find a test kit online.

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[kennythewelder]

Thanks

 

[Supercharged111]

Kenny have you considered the possibility of a faulty FPR? They're known to suck and when they do leak you don't see it because they live inside the intake.

 

[kennythewelder]

Kenny have you considered the possibility of a faulty FPR? They're known to suck and when they do leak you don't see it because they live inside the intake.

yea, it may be time to up grade to the new style spider, but I will put that off as long as I can. Money is always tight these days.