Thread: Vapor locking issue please help

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Since summer started I have been dealing with a major vapor locking issue on my 76 monza. It was so bad that after it got to operating temp ( between 190-205 degrees) and kill the car, you could literally see the air making the fuel in my clear inline filter bubble. It almost looked like it was boiling. See the car originally had single exhaust (going out the passenger side) the stock fuel line goes down the frame rail on the drivers side and goes up and across the engine compartment to my carb. but I had duel exhaust put on the car or better yet my dad did over 15 years ago and i know beleive that the exhaust pipe in too close to the metal fuel line heating it up and causing it to vapor lock.

To remedy the problem I wrapped my fuel lines in a insulation wrap ( normally used on headers but i heard it worked well for that) and replaced my factory carter electric fuel pump for a High output Holley (7 psi)

See the car ran fine unless you killed it after geting it to operating temp and after that it didnt want to crank so i was very pleased today when i got it up to operating temp and killed it and waited 5 mins and it cranked and ran like a beast

I drove it around the block after getting hot and again it did great. But there is about a 1/2 straight stretch from where i turn onto my rode to my driveway.... .... I hammerd on it and after it shifted it died ( acted like it ran outta gas which is why i know it vapor locked) and was vaporlocked because the inline was bubbling( not near as bad as it was) but it cranked rite back up after i let my pump run for idk bout 30 seconds.

I need some advice on what else i could do. Is this still going to be a problem or is it just because i hammered on it? any advice will do

thanks,

Slade


p.s I have 5/16 fuel line...idk if it matters but it is smaller than it should be

Where is the fuel pump located? Electric pumps dont do very good at sucking gas, they like to push.

in the trunk

In the trunk is not good. Your pump needs to be below the tank so that it has a net positive suction head, read that to mean positive pressure on the pump inlet. I expect your pump instructions told you that.

pump instructions said mount it in the trunck

how would you even mount it below the fuel tank ?

Originally Posted by Slade1134

how would you even mount it below the fuel tank ?

Mine is mounted on the frame rail ahead of the axle. I'd mount the pump as low as possible relative to the bottom of the tank - i.e., mount the pump low on the frame rail flat section ahead of the axle.

i think its too big for that but ill take a look at it

Originally Posted by Slade1134

i think its too big for that but ill take a look at it

Your pump may be built to suck fuel from the tank, and pressurize it to the engine. Most electrics don't like negative suction head. Go with what you've got....

Does the fuel line from the tank to the pump pass any heat? Usually, the negative pressure lowers the boiling point so the vapor lock occurs upstream of the pump.....

Is the clear fuel filter full when the engine is off? When you see bubbles in the fuel filter, is the fuel pump running?

IF your fuel pump is a 7psi Holley, what is the pressure at the carb inlet? IF you have 7psi at the trunk, you have mucho less at the carb.....perhaps too little....just a thought.

Troubleshooting via the internet is a hit/miss shot.

Originally Posted by rspears

In the trunk is not good. Your pump needs to be below the tank so that it has a net positive suction head, read that to mean positive pressure on the pump inlet. I expect your pump instructions told you that.

very few pumps like to be above the tank lower or below center of the tank .most fuel pumps do not like to prime there self. pulling fuel up hill is hard on most of them. just that in it self could heat the pump up .i alway thought that gravity was helpful the pump never will run dry or over work to draw fuel the holley or the china knock off vein pumps will not take many dry runs

wood ... phenolic.... some plastic are your friends some cost more then others but work good for carb spacers help to hold heat down. i have made up heat shields out of .060 alumium from fuel bowls

I’ll throw my 2 cents in for what it’s worth. First off, if I’m reading this right you are describing two different issues.

“......See the car ran fine unless you killed it after geting it to operating temp and after that it didnt want to crank......”

If you are saying that after shutdown, the engine wouldn’t turn over then that issue is likely heat soak on the starter motor (pretty common on older chevy SBs especially in a tight engine compartment like the Monza/Vega. The cure for that problem is usually to wire in a remote starter solenoid.

The second issue, vapor lock is caused by the fuel boiling in the lines prior to reaching the carburator. While the problem is caused by heat, it is aggravated by poor quality fuels (the gas with ethanol tends to boil quicker that good old plain gas), altitude and outside temperatures.

Having an electric fuel pump at the tank to pressurize the fuel system (raising the boiling point of the fuel) often helps, but it doesn’t seem to be enough in this case.

The solution that I have found that is most effective unfortunately is also the most work. That is to run a fuel return line from as close to the carburator back to the fuel tank. You will need a fuel pressure regulator (which you probably should have anyway with the 7 PSI pump) and then run the return line back to the tank and tap in a return hole to the tank (usually thru the sending unit). The pressure regulator will lower the excessive pressure at the carburator by bleeding off fuel that is returned to the tank. In the process it will purge any vapor in the line and more importantly keep the fuel constantly circulating so it doesn’t sit in the fuel line long enough to absorb enough heat to vapor lock.

As far as fuel pump mounting as mentioned you really want the pump mounted as close to the tank as possible and ideally lower than the fuel level in the tank. All of the pumps I have dealt with will create at least some vacuum to pull fuel to the pump, however if they lose their prime it can take some time to pick fuel back up again. In those cases where I have to mount the pump in a location that is higher the fuel level I make sure that I run the fuel lines to and from the pump so they are higher that the pump (basically the pump sitting in a low place in the fuel lines. This insures that even when the engine is shut off the fuel remains in the pump instead of draining away from it and it doesn’t have to worry about picking up prime.

yep i go along with mike . my old 100 over in the black gto would heat soak the starter .many times we would pour a big gulp of water on starter or let sit.i pick up a real ac delco solenoid fixed it. heat sheild would help you to . my blower car would run fine then would seem like it ran out of fuel but really what was happing the ignition switch would over heat .it could not take the amps of the holley pump .made up two thin 1/4 plastic spacers up for the carbs never had any more issues till i put a big fuel pump in and my problem came back .so i knew it was not heat and fuel deal?

Pat brings up another good point. Did you install a relay to power the electric fuel pump? You don't want to be pulling that large amp draw through your switched circuit. Things can get too hot too fast. Likely doesn't have anything to do with your vapor lock problem, but a necessary feature of the installation.