Missing fuel pressure when engine gets hot.
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From: Sindal Jutland
Missing fuel pressure when engine gets hot.
Hello All
Took the Vette out for a long drive today and during the cruise I got struck in some slow moving traffic. After a minute or so the car started to stumble and running very rough. When I came home it did the same again and I popped the hood. I could then see that my in-line fuel pressure gauge said a big 0 in fuel pressure and the fuel filter was almost empty. Upon revving the engine the pressure would rise to about 1 Psi and the filter would start to fill slowly. Going back to idle and the filter was running out of fuel again.
Now the funny part, when engine is cold there is perfect fuel pressure at 6 Psi, as soon as the engine heats up, fuel pressure will go down close to 0 Psi at idle and the stumbling part starts again. This is happening with the gas lid off as well and no pressure is seen in the gas tank.
What is causing lack of fuel pressure when engine is up to normal temp? If it was a defective pump I guess the problem should be there all the time?
Any ideas to solve the issue is very much appreciated
Regards Form Denmark
Karsten Horn
Took the Vette out for a long drive today and during the cruise I got struck in some slow moving traffic. After a minute or so the car started to stumble and running very rough. When I came home it did the same again and I popped the hood. I could then see that my in-line fuel pressure gauge said a big 0 in fuel pressure and the fuel filter was almost empty. Upon revving the engine the pressure would rise to about 1 Psi and the filter would start to fill slowly. Going back to idle and the filter was running out of fuel again.
Now the funny part, when engine is cold there is perfect fuel pressure at 6 Psi, as soon as the engine heats up, fuel pressure will go down close to 0 Psi at idle and the stumbling part starts again. This is happening with the gas lid off as well and no pressure is seen in the gas tank.
What is causing lack of fuel pressure when engine is up to normal temp? If it was a defective pump I guess the problem should be there all the time?
Any ideas to solve the issue is very much appreciated
Regards Form Denmark
Karsten Horn
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From: Sindal Jutland
I do have a mechanical pump and I’m not aware of how old it is as I have only had this car for 3 months – Fuel filter does not look like new but why would it act like that only when hot?
And, by the way no sign of fuel in engine oil.
The problem you had with the filter, did it behave the same way? I.e. fine when engine is cold and stuffed when hot.
Thanks
Karsten Horn
And, by the way no sign of fuel in engine oil.
The problem you had with the filter, did it behave the same way? I.e. fine when engine is cold and stuffed when hot.
Thanks
Karsten Horn
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Rich
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Sounds like you have a case of vapour lock.
Do you have the stock GM with 3 ports on it? Many people make the mistake of substituting a 2 port pump.
The third port is required for the return/bypass line, makes sure that the pump has cooler fuel at all times.
Do you have the stock GM with 3 ports on it? Many people make the mistake of substituting a 2 port pump.
The third port is required for the return/bypass line, makes sure that the pump has cooler fuel at all times.
Last edited by Mike Ward; Apr 15, 2007 at 09:19 PM.
Team Owner
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From: Crossville TN
I think you have a flexible fuel line that is collapsing and restricting flow when it gets warm (rubber softens as it heats up...probably from exhaust system). I assume you have an electric fuel pump and have not placed it near the fuel tank. Check the line between the tank and the pump.
Le Mans Master
Joined: Jul 2005
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From: Austin Texas
Replace the pump and make sure none of the fuel lines are near (very close) to the exhaust system. Replace the fuel filter also.
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i have the same issue with my vette runs great cold..get it hot runs like crap..still working on the problem
ToolMan
Joined: Mar 2006
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From: Magnolia Texas
I have the same problem when hot but it is infrequent. 1970 with 350/350. I replaced the fuel pump and got the return line functioning again (it had been plugged off). That helped but did not eliminate the problem. I am considering an electric fuel pump. Rob
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From: Sindal Jutland
Thanks for all the good replies.
Regards
Karsten Horn
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From: Sindal Jutland
Changed the fuel filter today and absolutely no difference, did some more measurements with the infrared and the hottest spot is the bottom of the carb and that was around 165 F (top of carb around 125 F) and the fuel line/fuel pump around 120 F. I can hear fuel returning back to the fuel tank via the return line (Splashing) so do I have vapour lock or is it the pump?
A quick question, will any mechanical fuel pump for a 350 SB fit on my 400?
I can only get a pump listed for a 350, not a 400
Thanks in advance
Regards
Karsten Horn
A quick question, will any mechanical fuel pump for a 350 SB fit on my 400?
I can only get a pump listed for a 350, not a 400
Thanks in advance
Regards
Karsten Horn
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From: Sindal Jutland
Regards
Karsten Horn
4th Gear
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I'm in the process of changing up carbs and considering an new fuel pump. Edelbrocks and others have 2 lines vs. 3 (no vapro return). Is there a way to use a high permformance mechanical pump on the C3's with no return?
Le Mans Master
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Pumps are the same with the exception of the number of ports.. They ARE interchangeable..
Le Mans Master
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From: one night in Bangkok you are never lonely
4th Gear
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I just changed my Holley performance mechanical pump for an in tank electric with a return from the carb bowl to the tank. I have had big vapor lock problems ever sense removing the original return line stock pump. The RPMs would drop to nothing when it is hot in city driving.