Help...Post Fuel Pump Replacement Issues

93-331-29psi · Mar 31, 2006

Alright I decided to replace the pump, on my 86 GT, even though it found new life. Pulled the tank, replaced pump, new filter, and I put all back together and now it runs like 'colorful language removed'. I start it up and it appears to idle smooth at around 600-800 RPM. I give it throttle sitting in neutral and it appears to have power. I put it in gear and start to drive and it will die within 50 yards and continue to run like crap until it dies.

I didn't drain and clean out the tank because I figured that was the purpose of the filter and the screen. Anyone have any ideas?

criticman · Mar 31, 2006

Changing my pump today...so subscribing in case I run into the same problem.

jrichker · Mar 31, 2006

You need a fuel pressure gauge so that you can watch the fuel pressure and see if it is dropping off when you drive the car.

93-331-29psi · Apr 1, 2006

Okay, got a fuel pressure gauge on teh car and when I turn the key to on it primes up to 40psi. I start the car and it idles around 38-40psi. Run the car a mile down the road and it adjusts itself accordingly to keep aroun 35-40 psi when on the throttle hard. I turn around and make the return trip and the fuel pressure starts to drop off and surges. I let off the throttle and the fuel pressure returns to the 38-40 psi range. I step on the pedal and it surges and fuel pressure drops off again.

Here is what has been done thus far.
new fuel pump
new fuel filter
new tank to filter fuel lines
tank drained and cleaned

I am at a loss and about as frustrated as I can get. What should I check now???

93-331-29psi · Apr 1, 2006

Are there any sensors that would cause the fuel pump to cut back how much fuel is being supplied to the engine? I replaced the fuel filter again thinking that maybe it got gunked up but to no avail. I am at a loss here and don't know what to do next. I have had the entire fuel system apart about 6 times now and each time no luck.

HISSIN50 · Apr 1, 2006

93-331-29psi said:
Are there any sensors that would cause the fuel pump to cut back how much fuel is being supplied to the engine?

Nope. The FPR can go south and that can cause issues, but otherwise, think of the fuel pump's delivery (itself) as stand-alone (no sensory input to control its output flow).
On returnless style fuel systems, it's a different story (we have a return-style fuel system).

Good luck.

soylentgreen · Apr 1, 2006

If the only thing that changed was the pump, then either the pump is bad or the installation is bad. The install is pretty straight forward and I doubt many ppl could manage to screw it up. I'm guessing the pump must be bad. I think 40psi is in the ballpark...so if fuel pressure is dropping off, the pump must be the problem. Can you put the old one back and check for functionality? I know it's kinda a pain in the ass...but what else can u do?

HISSIN50 · Apr 1, 2006

93-331-29psi said:
Okay, got a fuel pressure gauge on teh car and when I turn the key to on it primes up to 40psi. I start the car and it idles around 38-40psi. Run the car a mile down the road and it adjusts itself accordingly to keep aroun 35-40 psi when on the throttle hard. I turn around and make the return trip and the fuel pressure starts to drop off and surges. I let off the throttle and the fuel pressure returns to the 38-40 psi range. I step on the pedal and it surges and fuel pressure drops off again.

Here is what has been done thus far.
new fuel pump
new fuel filter
new tank to filter fuel lines
tank drained and cleaned

I am at a loss and about as frustrated as I can get. What should I check now???

I missed this post before. Dont overlook a goofy FPR if pressure is doing strange things. The idle setting (with vac line connected) should be 5-9 PSI lower than the WOT setting (the WOT might dip real quick once you step into it, but then it should go back up).

Good luck.

93-331-29psi · Apr 2, 2006

HISSIN50 said:
Nope. The FPR can go south and that can cause issues, but otherwise, think of the fuel pump's delivery (itself) as stand-alone (no sensory input to control its output flow).
On returnless style fuel systems, it's a different story (we have a return-style fuel system).

Good luck.

If I am losing pressure at the schrader valve then is it safe to assume that the FPR isn't the issue?

93-331-29psi · Apr 2, 2006

soylentgreen said:
If the only thing that changed was the pump, then either the pump is bad or the installation is bad. The install is pretty straight forward and I doubt many ppl could manage to screw it up. I'm guessing the pump must be bad. I think 40psi is in the ballpark...so if fuel pressure is dropping off, the pump must be the problem. Can you put the old one back and check for functionality? I know it's kinda a pain in the ass...but what else can u do?

Unfortunately I can't put the old pump back in as it was completely dead. I have mastered pulling the tank so what the hell! I might as well get another new pump and give that a try.

jrichker · Apr 2, 2006

93-331-29psi said:
If I am losing pressure at the schrader valve then is it safe to assume that the FPR isn't the issue?

The FPR regulates pressure by bypassing fuel back to the tank. As the regulator opens up, it passes more fuel to the return line and the pressure drops. As it closes, it passes less fuel to the return line and the pressure goes up. This will effect the pressure measured at the shrader valve. A FPR with an internal leak could cause a drop in pressure when extra volume demand is placed on the fuel system.

93-331-29psi · Apr 2, 2006

jrichker said:
The FPR regulates pressure by bypassing fuel back to the tank. As the regulator opens up, it passes more fuel to the return line and the pressure drops. As it closes, it passes less fuel to the return line and the pressure goes up. This will effect the pressure measured at the shrader valve. A FPR with an internal leak could cause a drop in pressure when extra volume demand is placed on the fuel system.

Fair enough, that makes sense, but still seems strange that if it is the FPR why does the cutting out only occur after a mile no matter how I drive the car (hard or easy).

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