Fuel PSI drop??
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Joined: Nov 2004
Posts: 100
From: Waynesville I.L.
Fuel PSI drop??
I've been out of the game for quite a while cars been setting for about 7 years. So my question/issue is when I turn the key to run my fuel psi comes up to roughly 43 psi the after pump stops running it starts to drop slowly I found a couple fuel leaks on my aeromotive fuel psi regulator fixed them no more leaks but still drops. I don't remember the fuel psi doing this before is this normal?? Also car starts seems to run good but noticed when I rev the engine fuel psi drop below 40 say roughly 35 I also don't remember this before. Is that normal thanks for the help.
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Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
Re: Fuel PSI drop??
How fast does it drop? 5-10 PSI in 10 minutes or so isn’t a problem. If the pressure drops rapidly, could be:
- faulty check valve in the fuel pump
- faulty fuel pressure regulator
- leaks in fuel line, including the flex-line in the tank
That said, the Aeromotive AFPR is a very well-known, certified POS.
The fuel pressure at idle, with the vacuum line off the regulator should ideally be 43.5 PSI. GM spec accepts anything in the range of 41-47 PSI. When you reattach the vacuum line the pressure should drop proportional to intake manifold vacuum, about a 8 to 10 PSI drop below the “no vacuum” pressure. With a more aggressive than stock cam, the pressure drop will be less.
When you move the throttle, intake manifold vacuum changes, so fuel pressure changes. As the throttle is opening, vacuum drops, fuel pressure rises. At WOT the fuel pressure should be within 1 to 2 PSI of the “no vacuum” reading. Snap the throttle closed at high RPM and intake manifold vacuum reaches its peak. Fuel pressure can drop more than 10 PSI below the “no vacuum” reading.
- faulty check valve in the fuel pump
- faulty fuel pressure regulator
- leaks in fuel line, including the flex-line in the tank
That said, the Aeromotive AFPR is a very well-known, certified POS.
The fuel pressure at idle, with the vacuum line off the regulator should ideally be 43.5 PSI. GM spec accepts anything in the range of 41-47 PSI. When you reattach the vacuum line the pressure should drop proportional to intake manifold vacuum, about a 8 to 10 PSI drop below the “no vacuum” pressure. With a more aggressive than stock cam, the pressure drop will be less.
When you move the throttle, intake manifold vacuum changes, so fuel pressure changes. As the throttle is opening, vacuum drops, fuel pressure rises. At WOT the fuel pressure should be within 1 to 2 PSI of the “no vacuum” reading. Snap the throttle closed at high RPM and intake manifold vacuum reaches its peak. Fuel pressure can drop more than 10 PSI below the “no vacuum” reading.
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Registered User
Joined: Nov 2004
Posts: 100
From: Waynesville I.L.
Re: Fuel PSI drop??
Fuel psi drops down to 0 in a matter of 30 to 60 seconds. I no longer have any leaks on the outside that I can see with a mirror or dripping any where. Is there a way to test check ball or line in tank without dropping tank and removing?
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Joined: Nov 1998
Posts: 71,094
From: Hell was full so they sent me to NJ
Re: Fuel PSI drop??
Have you checked the regulator vacuum line for wet fuel?
I also should have listed a leaking injector as a possibility. You can pull-up the rails with the injectors intact, prime pump to pressure the system and look for drips.
The factory manual procedure to test for fuel leaking past the regulator into the return line is to install a factory test tool consisting of a valve in the return line to shut off potential flow through the regulator. Hard to do without the valve. Some people have tried that test by pinching the nylon (plastic) portion of the return line, but that’s a bad idea because it can weaken the line. If you can figure a way to cap off the return line at the quick-connect, that would allow you to rule out the regulator.
After you rule out the regulator (and I will repeat - the Aeromotive AFPR is a known problem), rule out leaking injectors, and rule out leaks in the supply line from the tank to the rails, you are left with the tank internal components.
The only down side of rapid pressure loss due to the pump check valve is an extended cranking time when starting the engine. If you cycle the hey rapidly several times before turning to “start” the cranking time should be reduced. Once the engine starts, the pump check valve no longer plays a part.
The alternative to dropping the tank is to cut a “hatch” in the package shelf sheet metal right above the fuel sending unit. There are instructions in a “sticky” thread on the “Fuel and Ignition” forum.
I don't know if the check valve in the pump is serviceable. I have a complete sending unit, with my original factory pump, but the pump is enclosed in a plastic bucket. If you read the details on the Racetronix website, the bucket can also affect cranking time:
https://www.racetronix.biz/product.asp?ic=fpa-003a
I also should have listed a leaking injector as a possibility. You can pull-up the rails with the injectors intact, prime pump to pressure the system and look for drips.
The factory manual procedure to test for fuel leaking past the regulator into the return line is to install a factory test tool consisting of a valve in the return line to shut off potential flow through the regulator. Hard to do without the valve. Some people have tried that test by pinching the nylon (plastic) portion of the return line, but that’s a bad idea because it can weaken the line. If you can figure a way to cap off the return line at the quick-connect, that would allow you to rule out the regulator.
After you rule out the regulator (and I will repeat - the Aeromotive AFPR is a known problem), rule out leaking injectors, and rule out leaks in the supply line from the tank to the rails, you are left with the tank internal components.
The only down side of rapid pressure loss due to the pump check valve is an extended cranking time when starting the engine. If you cycle the hey rapidly several times before turning to “start” the cranking time should be reduced. Once the engine starts, the pump check valve no longer plays a part.
The alternative to dropping the tank is to cut a “hatch” in the package shelf sheet metal right above the fuel sending unit. There are instructions in a “sticky” thread on the “Fuel and Ignition” forum.
I don't know if the check valve in the pump is serviceable. I have a complete sending unit, with my original factory pump, but the pump is enclosed in a plastic bucket. If you read the details on the Racetronix website, the bucket can also affect cranking time:
https://www.racetronix.biz/product.asp?ic=fpa-003a
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