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Old 02-04-2007, 12:30 PM
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Mysterious fuel pressure problem

I am working with a 1981 Corvette. The car is mostly stock with a 350 and stock mechanical fuel pump. The problem is this. I was removing the carb to do a rebuild on it. When I removed the fuel line at the carb I got a shower in fuel. The fuel was coming from the fuel pump side and was coming out at about 5 to 7 psi I would guess. I put a hose on the end on the line and ran it into a bucket. My thought was that it had pressure build up and it would stop after a few mins. I was wrong. The fuel kept coming and at about 5 to 7 psi. I pulled the hose off, got another shower, stuck my thumb over the end, got another shower, so that I could put a rubber cap on the line to stop it. Three days later I went to put the carb back on. I pulled the cap off the line, got another shower. With fuel going everywhere I quickly hook the line back up to the carb and cleaned all the fuel up! I have noticed this problem once before when it was 80 degrees out and thought it was a venting problem, but now it is like 30 degrees so I am not so sure it is a venting problem. Any thoughts about this?

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Old 02-04-2007, 12:48 PM
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re: Mysterious fuel pressure problem

The gas was siphoning out of the tank. Fuel can run right through a mechanical pump, gravity was just draining your fuel into the bucket. (Yes, at a surprisingly high pressure.)
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Old 02-04-2007, 12:59 PM
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Uhmmm

This is the first time I have experienced that! I have worked on my Camaro several times and didn't have that problem. Thank you for the information. I wasn't aware it could still flow like that through the pump!
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Old 02-04-2007, 01:00 PM
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You sure that you dont have an electric fuel pump that you dont know about?. I cant see syphoning having that much pressure, and when you lift the hose above the fuel level, it would stop......Your tank must be building up some pressure.
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Old 02-04-2007, 02:09 PM
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re: Mysterious fuel pressure problem

The pressure really depends on the difference in height between surface of the fuel in the tank and the end of the hose. Yes, if you raise to hose above the level of the tank the flow should stop. If it doesn't, you have something else going on.
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Old 02-04-2007, 03:04 PM
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The tank vent system could also be plugged and creating pressure in the tank, the only place to release it would be through the fuel line...or the cap when you open it. If you take the cap off and the fuel stops, that's probably the problem, otherwise I wonder about an in tank fuel pump as well. It shouldn't be operating with the ignition off though, unless the relay was bypassed at some time. That would probably run the battery down when it set though.
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Old 02-04-2007, 04:22 PM
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It`s a vent issue. Check the vapour line going to the carbon canister, if it`s plugged off or stopped up pressure build up occurs. Next time when you have to remove the fuel hose take the gas cap off first. I had this same problem with my cutlass, it would build pressure all the way up to 14 -15 psi and it was blowing the rubber tip off the needles in the carb.
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Old 02-04-2007, 05:43 PM
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Ok...

I haven't tried to remove the cap yet. I was thinking that with the line above the engine it shouldn't be pulling fuel through. Now that you mentioned it, I think I seen that someone removed the carbon canister when I was working on it. Uhmmm, I better check the to see if the vent lines are plugged off. This carb was only 6 months old and it started flooding the engine. I replaced all the gaskets and installed new needles and seats! Maybe the pressure problem is related to the carb problem!
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Old 02-05-2007, 12:20 AM
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Quote:
Originally Posted by Sir Joab
The gas was siphoning out of the tank. Fuel can run right through a mechanical pump, gravity was just draining your fuel into the bucket. (Yes, at a surprisingly high pressure.)

About 3 1/2 ft of elevation makes 1 pound of pressure.
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Old 02-05-2007, 12:29 AM
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re: Mysterious fuel pressure problem

Quote:
Originally Posted by xntrik
About 3 1/2 ft of elevation makes 1 pound of pressure.
Yeah, you're right. I need to "recalibrate" my "estimator".
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Old 02-05-2007, 12:39 AM
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Quote:
Originally Posted by Sir Joab
Yeah, you're right. I need to "recalibrate" my "estimator".

Hey, a person can blow about 3 psi. Have you ever blown on a small hose full of water? Sure, me too.

1 or 2 psi will shoot a 5/16 stream a looooong way..

So just a little fall makes it look like a lot of pressure to the eye.
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