Mysterious pressure drop………Help needed!

[thehueypilot]

I have been fighting this issue for a long time and have not resolved it yet. If anyone has had the same experience and resolved it I would love to hear from you! When the car starts to warm up the fuel pressure starts to drop. The regulator (replaced twice with no effect) is adjusted to 7.5 psi cold and once the car reaches a temp of 180 degrees or hotter the pressure drops to about 2 psi. I have a mechanical fuel pump (1st one was Holley Volumax 170 gals/hr at 15 psi) (2nd one is Elderbrock 130 gal/hr at 12 psi) with 3/8” SS lines (pre-bent) coming from the fuel tank. I have #6 fuel line going to the carb. I have bypassed the gas tank and lines by running a separate, short 3/8 fuel line from a gas can to the inlet of the fuel pump with no improvement. I have ran cold water on the fuel pump, regulator and fuel lines to include to gas tank with no effect. All rubber transition lines have been replaced. All lines have been wraped with insulation to keep the radiant heat off them. The car is starving for fuel at 6000 rpm? Help!

 

[2bav8]

What fuel pressure gauge are you using, and have your verified your fuel pressure with another gauge? Also, have you actually checked the float bowls to make sure they are actually out of fuel at 6000 RPM? 6000 RPM is pretty healthy for a 427.

 

[pabear89]

Sounds like it is sucking debris up to the pickup in the tank.
You have seemed to have replaced everything, But the filter?
Next step would be to pull the tank and sending unit and clean it.


PB

 

[bud4660]

Looks like you have tried everything. Might as well give this a shoot. Try running a fuel pressure regulator that allows the fuel to continually flow in the fuel line, via a bypass valve and run a return line to the tank. See if that solves the problem.

 

[thehueypilot]

The tank/sock filter and sending unit are all brand new. I just changed to a 20 gallon tank in place of the 16 gallon stock unit so I could skip every other gas station. The problem existed before the new tank. The tank is vented as best as a stock unit can be vented at the gas cap. I can't say that the fuel bowls are running dry, but the spark plugs are extermely white to almost melting. I could triple the main and sec. jets and still not turn the plugs black so I don't think it is carb. related. The liquid filled fuel pressure gage was swapped out with the same results. I could try the bypass regulator, but I was hoping that someone else had the same issue and solved it without running a return line. Please keep the suggestions coming!

 

[brianj5600]

If you bought a quick fuel carb, give 'em a call. They should be more than happy to help.

 

[thehueypilot]

Actually, I have emailed them with little response. I think their stand is that since this is not a carb. problem they are too busy to help.

 

[bud4660]

I'm gonna ask just for the sake of asking. Have you left the gas cap open and tried it? Man this is a tuff one. You have done everything I can think of already. Ok. I just went and reread what you wrote. Have you tried an electric pump at the tank to push fuel up to the mechanical one yet. If none of that works then I think I would go back to a stock carter fuel pump with no regulator and see what that does.
Unless the pickup tube is sucking air someplace? But if it did that before the change, then that shouldnt be the problem.

 

[GaPonyFarm]

Bad fuel pump eccentric maybe? Also check the routing of your fuel line to ensure its not near a heat source anywhere... it could be a fuel vaporization issue.

 

[WORTH]

swap it to an electric pump, it could be floating at high rpm or the cam could be worn and simply not moving the pump arm enough.

 

[WORTH]

Stupid question #1, you said it runs 2 lbs after it reaches temp, is that also at idle? or just at 6000 rpm's??

 

[thehueypilot]

Stupid question #1, you said it runs 2 lbs after it reaches temp, is that also at idle? or just at 6000 rpm's??

In this case their are no stupid questions! It could be something so minor that I am over looking the problem. The fuel pressure drops to 2 psi at idle and stays there. Since the fuel pressure gage is under the hood I do not know what it is at 6k. I assume since the plugs are extremely lean after a run the pressure is staying constant at 2 psi at any RPM. The gas cap has been left open and I have never heard it making a hissing noise when ever I have opened it. I even enlarged the vent indentation to get more volume of air through the tank. The cam/engine is fairly new so if it was the fuel pump eccentric it would always be low no matter the temp. I do have an electric pump in the back for the NOS setup, but have not tried it yet because it only flows 100 gal/hr. Going back to the stock setup is interesting because it may be a quirk of these high volume pumps? Please keep suggestions coming.

 

[WORTH]

If you have a vacuum guage, "T" it into the pump suction line. That will tell you if it's a suction problem or a pressure problem. that's half the battle, then we'll take it from there.