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Discussion Starter · #1 ·
I’ve run into this problem a couple times now if I’m idling too long or driving it too hard. I’m not too sure if it’s vapor lock as I’ve never encountered this problem yet. Starts running like it’s out of gas. Open the gas cap to find some “blow your hair back” pressure in the tank. Give it gas while starting again and it’s fine.
Got a Holley electric fuel pump and rubber hose from tank to carb. New gas tank last year as well.
So I guess my question is…
-Do I heat wrap the fuel line?
-Drill a hole in the gas cap?
-Use ethanol free or higher octane?
-All of the above?
I’m surprised something hasn’t blown up yet with how much pressure builds.
 

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87 Caballero Amarillo, original 305/200-4R, QJ
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Pressure in the gas tank shouldn't be an issue, lack of or a vacuum usually is. Vented gas caps are there to let air in, to prevent a vacuum condition, not fumes out, that's the cannister job.

Fuel delivery and starting are 2 different things, although related. A vapor lock is where the gas in the line/carb gets so hot it vaporizes before it gets to the jets, not necessarily a line issue, but can also be a hot carb issue where the bowl gets so hot that you literally are trying to pump a gas through the carb not fuel.

What pressure is the electric fuel pump set to? Any kinks in the fuel line? Filter or filters between pump and carb?
 

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Discussion Starter · #4 ·
Pressure in the gas tank shouldn't be an issue, lack of or a vacuum usually is. Vented gas caps are there to let air in, to prevent a vacuum condition, not fumes out, that's the cannister job.

Fuel delivery and starting are 2 different things, although related. A vapor lock is where the gas in the line/carb gets so hot it vaporizes before it gets to the jets, not necessarily a line issue, but can also be a hot carb issue where the bowl gets so hot that you literally are trying to pump a gas through the carb not fuel.

What pressure is the electric fuel pump set to? Any kinks in the fuel line? Filter or filters between pump and carb?
It’s a 7 psi Holley Red pump. No regulator or filter before the carb. Also seems to only do this when it’s low on gas. It’s got one of those oil filter style filters after the pump.
Thanks for clarifying that pressure is ok. I was thinking it would be the opposite.
 

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Like Karadjgne said, vapor lock is a condition where some or all the fuel in the line essentially boils itself into vapor before reaching the carb or inside the carb, causing stumbling or stalling; this is usually because some portion of the line is too close to an exhaust pipe or header. But, I don’t know that this would cause the tank pressure you’re seeing.

Since the fuel pump is always drawing fuel from the tank, the tank vent connected to the vapor canister has to allow air in to replace it or else you’ll get negative pressure, causing it to implode; but somehow you’ve got the opposite problem. Is there an exhaust pipe or anything that could be overly heating the tank itself up, to where you’re not only getting vapor lock, but also causing it to pressurize? Like, if you’ve ever left a gas can in the sun, it blows off pressure when you open it-
 

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Your saying give it gas when starting could be giving it air. Might be a flooding condition. Cracking the throttle stops it. Pressure in the tank won’t help.
Tom
 

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87 Caballero Amarillo, original 305/200-4R, QJ
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Check the plumbing and especially that oil type filter for any leak. Once you turn off the ignition, the only part of the line that now has any pressure is the line behind the pump, being pushed there by the pressure in the tank. However, if there's even a pinhole leak, that'll allow the gas between the pump and carb to dissipate, especially if the line is close to a heat source such as heads or exhaust. As the gas sits and heats up, it'll expand, pushing fuel into the carb bowl, which then evaporates, or pushing it out the leak, albeit slowly.

Given enough time, that line dries out, and you'll crank for a minute or 5 to fill the bowl up, or just sit with ignition on for a bit so the pump can fill the bowl.

If you look at most qj filters, they'll have a check-ball inside that blocks up the fuel line when the pump isn't running. Prevents it from siphoning back down. Kinda like holding a finger over a McDonald's straw prevents the coke from dropping out the bottom.

Because you are missing that, most likely engine heat, exhaust heat, lack of decent thermal spacer gasket under the carb, something is cooking your fuel line and the gas inside.

Try turning on the ignition and wait 30 seconds or so, then 2x pumps on the gas pedal to @ 50%, the first sets choke, the second sprays gas via accelerator pump. Don't touch the pedal, crank it.

With an electric pump in-line, really should have a pressure regulator with dial. Unless you are running high horsepower, like 400ish+, a QJ usually runs @ 4-6psi, so 7psi is maybe a bit much, and as said, you may not be vapor locking but flooding. To check for that, floor the gas pedal and hold it there, don't let up. Crank it. That kills the accelerator pump totally, clears any flooding. Should start within @ 10 seconds at most.
 

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Discussion Starter · #8 ·
Check the plumbing and especially that oil type filter for any leak. Once you turn off the ignition, the only part of the line that now has any pressure is the line behind the pump, being pushed there by the pressure in the tank. However, if there's even a pinhole leak, that'll allow the gas between the pump and carb to dissipate, especially if the line is close to a heat source such as heads or exhaust. As the gas sits and heats up, it'll expand, pushing fuel into the carb bowl, which then evaporates, or pushing it out the leak, albeit slowly.

Given enough time, that line dries out, and you'll crank for a minute or 5 to fill the bowl up, or just sit with ignition on for a bit so the pump can fill the bowl.

If you look at most qj filters, they'll have a check-ball inside that blocks up the fuel line when the pump isn't running. Prevents it from siphoning back down. Kinda like holding a finger over a McDonald's straw prevents the coke from dropping out the bottom.

Because you are missing that, most likely engine heat, exhaust heat, lack of decent thermal spacer gasket under the carb, something is cooking your fuel line and the gas inside.

Try turning on the ignition and wait 30 seconds or so, then 2x pumps on the gas pedal to @ 50%, the first sets choke, the second sprays gas via accelerator pump. Don't touch the pedal, crank it.

With an electric pump in-line, really should have a pressure regulator with dial. Unless you are running high horsepower, like 400ish+, a QJ usually runs @ 4-6psi, so 7psi is maybe a bit much, and as said, you may not be vapor locking but flooding. To check for that, floor the gas pedal and hold it there, don't let up. Crank it. That kills the accelerator pump totally, clears any flooding. Should start within @ 10 seconds at most.
Just bought a new gas gauge and sending unit today, maybe I’ll just redo all the lines. The rubber hose for fuel is 15 years old and kinda cracked. I put a new tank in it last year and one thing I’m confused about is how it’s getting pressurized in the first place because the filler neck to the tank is cracked and drips gas out of it when getting filled. So I’m not sure how gas can drip out while keeping air pressure?
One more thing. So the guy told me he hooked up the fuel pump incorrectly when I bought the car, so the pump is always on with the key on. Not sure if that’s a contributing factor. Do vented gas caps go both ways or are they a 1-way valve? I can’t seem to blow or suck any air through it.
 

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Discussion Starter · #9 ·
Like Karadjgne said, vapor lock is a condition where some or all the fuel in the line essentially boils itself into vapor before reaching the carb or inside the carb, causing stumbling or stalling; this is usually because some portion of the line is too close to an exhaust pipe or header. But, I don’t know that this would cause the tank pressure you’re seeing.

Since the fuel pump is always drawing fuel from the tank, the tank vent connected to the vapor canister has to allow air in to replace it or else you’ll get negative pressure, causing it to implode; but somehow you’ve got the opposite problem. Is there an exhaust pipe or anything that could be overly heating the tank itself up, to where you’re not only getting vapor lock, but also causing it to pressurize? Like, if you’ve ever left a gas can in the sun, it blows off pressure when you open it-
Yeah, the hose rides pretty close to the exhaust and headers.
 

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87 Caballero Amarillo, original 305/200-4R, QJ
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Vented gas gaps are single direction, inwards. If you've ever left a gas can in the sun, it expands with the pressure, that'll go to the cannister. But if you release the pressure when hot, when it cools it'll shrink, hard and that's where the vented cap comes in, allows air to enter the tank to stop the vacuum affect.

Drilling a hole in the gas cap would work, but in the middle of summer the rear end will constantly stink of gas as the pressurized vapors escape through the hole instead of getting absorbed by the cannister.
 

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You say it only does this when it's low on fuel, how low? Fuel sender might not be reading correctly and it might be lower than you think. I would also rewire the pump to operate a relay off of a oil pressure switch for safety reasons. If you get into a accident you want the pump to stop if the engine dies. No reason to die in a preventable fire.
 
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