Fuel Pressure Mystery - Team Camaro Tech
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post #1 of 55 (permalink) Old Jul 3rd, 09, 02:55 PM Thread Starter
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Fuel Pressure Mystery

This is driving me crazy, I need some help!!! I am running the same combination in my 67 for quite a few years now without any problems. It is a mildly built 350, 4 speed with a Holley 1850 600 cfm vac secondaries. The car sat for a couple of years with some treated fuel in it. I have recently started driving it and had a fuel starvation problem where I almost didn't make it back home. The old pump was all wet on the outside and I figured that it was going bad, so replaced it with a new Holley mechanical unit and installed a see through filter and pressure gauge between the pump and carb. Fired it up, filter is full of fuel and 5.5 psi, then it slowly starts loosing pressure until it wouldn't stay running. Figured it must be the sending unit, since it was the original one. Pulled it out and the filter/sock was all deteriorated so I thought that was the problem. New tank, new sending unit, new steel line from tank to pump and all new rubber fuel line sections. Fire her up and same problem. Has to be the pump right? Summit sends me a replacement. I install it and same problem. I fire it up, 5.5 psi and fuel filter is full of fuel, but it slowly starts loosing pressure and volume. What the heck is the problem. The carb is the only part of the fuel system that I have not changed. So does it just need to be disassembled, cleaned out and rebuilt or is the problem somewhere else?!?!?
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post #2 of 55 (permalink) Old Jul 3rd, 09, 03:28 PM
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Re: Fuel Pressure Mystery

Is your gas cap vented? It could be that you are developing a vacuum in the fuel tank from the fuel pump which is causing fuel flow to come to a halt. A quick test is to just take the gas cap off and see if your problem persists. Taking the cap off will create a massive air bleed into the tank which will not allow a vacuum to form in the tank.

Imagine a quart of beer with a straw and the straw is hermetically sealed to the cap. You will only get so much beer before you just can't suck anymore thru the straw.

alan
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post #3 of 55 (permalink) Old Jul 3rd, 09, 05:18 PM
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Re: Fuel Pressure Mystery

That could very well be the problem.
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post #4 of 55 (permalink) Old Jul 3rd, 09, 06:17 PM
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Re: Fuel Pressure Mystery

You have a sluggish running fuel pump push rod due to it being partially bent!....Purchase a new, straight push rod and you'll be all set!
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post #5 of 55 (permalink) Old Jul 3rd, 09, 06:38 PM Thread Starter
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Re: Fuel Pressure Mystery

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Originally Posted by alanrw View Post
Is your gas cap vented? It could be that you are developing a vacuum in the fuel tank from the fuel pump which is causing fuel flow to come to a halt. A quick test is to just take the gas cap off and see if your problem persists. Taking the cap off will create a massive air bleed into the tank which will not allow a vacuum to form in the tank.

Imagine a quart of beer with a straw and the straw is hermetically sealed to the cap. You will only get so much beer before you just can't suck anymore thru the straw.

alan
Stock, vented gas cap. I have run it both with and without the cap, no difference.
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post #6 of 55 (permalink) Old Jul 3rd, 09, 06:45 PM Thread Starter
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Re: Fuel Pressure Mystery

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Originally Posted by Badbird View Post
You have a sluggish running fuel pump push rod due to it being partially bent!....Purchase a new, straight push rod and you'll be all set!
I've had this same motor in the car for a decade or more and just replaced the fuel pump for the first time. How does a push rod get bent during normal operation? It seemed free, when I had the old pump out, the push rod just slid right out. I had to use the bolt hole in the front of the block to hold it while I installed the new one, making sure not to mar the surface by only snugging it by hand.
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post #7 of 55 (permalink) Old Jul 3rd, 09, 07:04 PM
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Re: Fuel Pressure Mystery

Well, if the push rod is not bent then it's time to disassemble the carb and install a renew kit.....Most likely the floats needle and seats are leaking!
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post #8 of 55 (permalink) Old Jul 3rd, 09, 08:17 PM
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Re: Fuel Pressure Mystery

Install a deadheaded vacuum gauge, gauge only at inlet, at the fuel pump inlet and with a full carb of fuel, see how much vacuum the pump creates. Should be a min of 15 inches of vacuum.

If it passes, then blow shop air into tank sealed with a rag at the filler neck and see volume of fuel coming out of inlet hose, should be quite a bit. No fuel, then a rubber line has been sliced when installed or kinked.

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post #9 of 55 (permalink) Old Jul 3rd, 09, 09:02 PM Thread Starter
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Re: Fuel Pressure Mystery

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Originally Posted by Everett#2390 View Post
Install a deadheaded vacuum gauge, gauge only at inlet, at the fuel pump inlet and with a full carb of fuel, see how much vacuum the pump creates. Should be a min of 15 inches of vacuum.

If it passes, then blow shop air into tank sealed with a rag at the filler neck and see volume of fuel coming out of inlet hose, should be quite a bit. No fuel, then a rubber line has been sliced when installed or kinked.
I'm not sure if I follow you. First part of test, install T-fitting before pump and install vacuum gauge and read while running? Second part of test, disconnect hose to carb inlet and see how much fuel comes out when blowing into filler neck?
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post #10 of 55 (permalink) Old Jul 4th, 09, 05:51 AM
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Re: Fuel Pressure Mystery

Yes, disconnect the inlet hose from the tank and install a vacuum gauge on the pump inlet, crank up the the engine with a carb full of fuel and see how much vacuum the pump creates.

You can 'T' the gauge with the inlet hose if you like after doing the above test and watch the gauge. There should be no vacuum created if pump is primed and no faults with the fuel supply.

Blowing air into the tank filler tube, no more than 10-15 PSI, short squirts with the air nozzle, pressurizes the tank alittle and forces fuel out the tank through the fuel line and to the pump inlet, if the inlet hose is disconnected at the pump. If there is a good flow, all is well from the inlet hose back to the tank.

Does the inlet hose at the tank have a kink in it?

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post #11 of 55 (permalink) Old Jul 4th, 09, 01:23 PM Thread Starter
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Re: Fuel Pressure Mystery

Ok, ran the test as described. Steady 15" Hg. Only let it run about 45 seconds, wasn't sure if the pump diaphram would get damaged if being run dry for too long. Forced air into the tank and had a steady stream of fuel coming out of the supply line. What next?!?!?!?
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post #12 of 55 (permalink) Old Jul 5th, 09, 08:41 AM
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Re: Fuel Pressure Mystery

Did you clean out the tank when you had it apart? Those socks plug pretty easy. Also, did you dump the old gas and start with fresh?
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post #13 of 55 (permalink) Old Jul 5th, 09, 08:48 AM
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Re: Fuel Pressure Mystery

Good vacuum and no restrictions in the line. Now hook up the inlet hose to the pump and outlet line/hose to a qt jar or gas can and pump should fill qt jar in approx 15 seconds. Try it with tank filler cap on & off.

What happens to the see-through filter if you pressurize the fuel tank while running?
Does the engine die or keep running?

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post #14 of 55 (permalink) Old Jul 5th, 09, 09:48 AM Thread Starter
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Re: Fuel Pressure Mystery

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Originally Posted by deerhunter View Post
Did you clean out the tank when you had it apart? Those socks plug pretty easy. Also, did you dump the old gas and start with fresh?
Everything is new:tank, sending unit, fuel lines, pump and even the gas!!!
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post #15 of 55 (permalink) Old Jul 5th, 09, 09:50 AM Thread Starter
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Re: Fuel Pressure Mystery

Quote:
Originally Posted by Everett#2390 View Post
Good vacuum and no restrictions in the line. Now hook up the inlet hose to the pump and outlet line/hose to a qt jar or gas can and pump should fill qt jar in approx 15 seconds. Try it with tank filler cap on & off.

What happens to the see-through filter if you pressurize the fuel tank while running?
Does the engine die or keep running?
Thanks Everett, I'll run these tests today after I get done with my honeydos!
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