85 GT Mechanical Fuel Pump Flow Issues

[Mgino757]

I'm working on figuring out my fuel delivery issues on my 85 GT 5 speed. I might have a defective fuel pump I'm struggling with, but I need some pointers to determine if I'm on the right track. Last time I went to the track, I was running out of fuel at the top of 3rd gear and into 4th. Since then, I have changed both fuel bowls to the center hung 4150 style to make sure I have plenty of capacity. It still starves for fuel up top, even in 2nd gear now. Come back home, pop the hood, and my fuel filter housing barely had any fuel in it.

So, I tested the fuel flow with the supply hose off of the filter and had it spraying into a gas can so I could see what the flow was like. Running the engine at idle, I get good, strong sprays of fuel. However, when I increase the engine speed to, say, 2500 rpm or higher, the fuel stream gets noticeably weaker and appears to lose volume big time.

Is this normal or am I seeing signs of a weak fuel pump? The stock fuel pump should be plenty of fuel delivery at 30 gph. Stock, this engine only needs maybe 18 gph if my calculations are correct. This pump is a stock replacement Carter that I bought from NAPA 5 years ago. The car does sit a lot though.

[graphicdesigner80]

I'd suspect the pump, however, if your rubber sections of fuel line are still original, they could be collapsing internally. That would really show up with increased rpm/load.

If it were me, I'd try those first. Be sure to use quality fuel-rated rubber hose and try to reuse the band-style clamps if possible. I've also seen the rubber lines get pinched between the tank to trunk floor... If the tank was down recently that would be a place to check as well. Good luck.

[Mgino757]

My tank has never been dropped. Fortunately a new pump is only $30. If that doesn't fix it, then I know what to do next.

For what it's worth, I performed my test with the fuel cap off to eliminate any possibility of having any vacuum in the tank.

[Broncojunkie]

You could always get a new section of hose and hook it up to the fuel pump....opposite end down into a fresh can of gas. Run it from the gas can and see if it does any better. Basically, you're bypassing the car's tank fuel lines.

[Mgino757]

Yup, it was the fuel pump. New one in, no issues other than floating valves at high rpm. lol

[erratic50]

what are you revving it to? Stock stuff with an HO cam is good to around 6500 usually. Watch it on floating valves as it can cause catostrophic failure in a big hurry!

[Mgino757]

Around 5500-6000 rpm. The valve springs have over 250,000 miles on them. lol