Fuel pressure droppping instead of rising
Thread Starter
Senior Member
Joined: Sep 2000
Posts: 560
Likes: 1
From: Austin TX
Car: 91 RS Convertible
Engine: 355
Transmission: 700R4
Axle/Gears: 2.73 peg leg
Fuel pressure droppping instead of rising
Hey guys. I have the vacuum adj. fpr which is holding at about 11-12 psi at idle. When I open the throttle, the pressure drops slightly (maybe 1 psi at WOT). What could be causing this? Did I hook up the vacuum wrong? I ran the line from the regulator to the front driver's side of the throttle body. Is this the correct port. Oh yeah, I just put in a new Holley in-tank fuel pump and replaced the fuel filter, checked for gas leaks, and everything else seems ok. Thanks
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91RS convertible 355 TBI 65# inj. custom compcam grind .480/.480, Edelbrock TBI manifold, open element, hedman hedders, Y-pipe, cheap cherry bomb muffler, Hypertech cop car thermomaster, Accel coil, wires. 2.02/1.6 cast iron heads. Engine specially made by Pflugerville machine shop. Forged bottom end, balanced. Coming soon: Vacuum controlled FPR, better exhaust, Torque converter, DIY PROM, bigger TB.
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91RS convertible 355 TBI 65# inj. custom compcam grind .480/.480, Edelbrock TBI manifold, open element, hedman hedders, Y-pipe, cheap cherry bomb muffler, Hypertech cop car thermomaster, Accel coil, wires. 2.02/1.6 cast iron heads. Engine specially made by Pflugerville machine shop. Forged bottom end, balanced. Coming soon: Vacuum controlled FPR, better exhaust, Torque converter, DIY PROM, bigger TB.
Senior Member
Joined: Mar 2001
Posts: 849
Likes: 2
From: MA
Car: 93 GM300 platforms
Engine: LO3, LO5
Transmission: MD8 x2
You probably have it hooked up wrong, because no one else's behaves that way.
When you connected the vacuum port on the vFPR to the front DS of the TBI, is the vacuum hook up on the TBI BELOW the throttle plate?
You need to have the vFPR plumbed into intake manifold vacuum, and it sounds like you might not be well enough into the manifold by using a port on the TBI itself.
Can you use one of the NPT capped ports in the manifold? You'll need a brass male NPT thread -to- hose nipple adapter, to screw into the manifold. You could also use a T-fitting into the vacuum hose that feeds the power brake booster (assuming you have power brakes).
The other possible problem is that the hose you are using to connect the vacuum source to the vFPR is too flexible, and it's collapsing at idle and cruise (high vacuum conditions). You either need to use vacuum-specific hose, or hard line (fuel or brake line) with small hose connections at the manifold and vFPR ends.
When you connected the vacuum port on the vFPR to the front DS of the TBI, is the vacuum hook up on the TBI BELOW the throttle plate?
You need to have the vFPR plumbed into intake manifold vacuum, and it sounds like you might not be well enough into the manifold by using a port on the TBI itself.
Can you use one of the NPT capped ports in the manifold? You'll need a brass male NPT thread -to- hose nipple adapter, to screw into the manifold. You could also use a T-fitting into the vacuum hose that feeds the power brake booster (assuming you have power brakes).
The other possible problem is that the hose you are using to connect the vacuum source to the vFPR is too flexible, and it's collapsing at idle and cruise (high vacuum conditions). You either need to use vacuum-specific hose, or hard line (fuel or brake line) with small hose connections at the manifold and vFPR ends.
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