what causes vapor locking

[fryedaddy]

i have owned my 66 caliente for 33 years,i have put 4 different FEs in it and i have had vapor locking problems with every engine,mostly in warmer weather.heres what it does. you can go for a ride and get it up to temp,park it for a few minutes or a little while.when you get back in it and go down the road it acts like its running out of gas.then after you drive a little it clears out

[NIsaacs]

Do you use the factory type fuel pump or electric? A good electric will help. The factory steel line runs cooler than black rubber. The best way to stop the vapor lock is with a bypass system and return line to the tank. The gas now days is prone to vapor lock.

Nick

[fryedaddy]

it has a carter mech. factory type.it must be vapor locking from the carb to the tank because it takes a minute to get gas back up to the carb.

[NIsaacs]

The vapor lock is usually from the pump to the carb, that's where the most heat is.

Nick

[fryedaddy]

if the fuel pump is heating up could the heat push fuel toward the tank. it takes almost a full minute to get the carbs working

[machoneman]

How old is your fuel pump? Have you check it for pressure? Couple of ways to do this, just be careful.

Lack of a pressure head can, all by itself, show symptoms of stalling. One way to tell is taking it our for a ride and nailing it hard through a few gears. A weak pump will lean that mixture out and the engine stops pulling hard at top rpms especially. Often it's hard to diagnose true vapor lock versus a weak pulse of fuel.

Also if the car has the OEM sock in the gas tank, it could have become clogged. Take off the line to the fuel pump, crack the gas cap a tad and blow about 20PSI into the line under the hood. Careful here, outside, no smoking! If the car runs better, time to drop the tank for a thorough cleaning.

Finally, any original rubber in the fuel lines should be replaced as guys often skip the suction side (tank to fuel pump) and miss the fact that even a tiny pinhole or crack that doesn't even leak in the rubber lines kills the vacuum from the pump and leans the carb out. 

[fryedaddy]

all of the rubber lines are new even the little one at the tank.i have noticed some trash in my filter and my old carb went south ,maybe clogged.i dont think the pump is weak because i have run it hard for 10 seconds or so at wot,and it didnt lean out,and when i loosen my line at the carb it blows gas out with a lot of pressure

[NIsaacs]

Your original post describes classic vapor lock. Do you have headers that might overheat the fuel line by the frame? An electric pump near the tank will help a bunch, they push fuel better than they suck.

Vapor lock is not really a lock, it just don't pump or fill the carb for crap. It takes a minute to get it out after a warm shutdown. To double check it the next time you have a warm shutdown, pop the hood and leave it open until you are ready to go.

Nick

[Drew Pojedinec]

I live in southern Ga.  It's hot.
Had this problem and many others that were heat related.

Solution:

-Block Exhaust crossover in intake
-Phenolic spacer under carb (even 1/4 inch it good)
-Isolate fuel line from engine heat as best as possible
-electric fuel pump  (this one isn't required but it does help)

[machoneman]

all of the rubber lines are new even the little one at the tank.i have noticed some trash in my filter and my old carb went south ,maybe clogged.i dont think the pump is weak because i have run it hard for 10 seconds or so at wot,and it didnt lean out,and when i loosen my line at the carb it blows gas out with a lot of pressure

Great, then since you have eliminated the other suspects, it is vapor lock.

[FirstEliminator]

     I have a 78 F-150 with a 460. It was pretty prone to vapor lock. Sometimes it got so bad, when I'd stop for a toll booth on the highway, then take back off at a moderate speed it would die out. Usually it would refill before I had to pull over. One of the things that would help the situation was to change the fuel filter. There must have been quite a bit of sediment in the tank of that truck as I went through more fuel filters in that truck any other vehicle....still do. I would guess the clogging filter which was near the carb would slow down the fuel flow allowing it more time to warm up in the pump and line to the carb. Optimally, the best course of action would be to put a fuel pressure regulator right before the carb that has a return line to the tank.

     Another problem I have is with my 69 Cougar 351w 2v. After driving on the highway then stopping and shutting off the engine it will heat soak and you can see the fuel boiling right out of the boosters in the carb. I run a 195 thermostat. Perhaps dropping to a 180 might make the difference. My belief on this is the fuel in the line between the pump and carb heat soaks and is putting enough pressure to blow the needle off the seat. One of my thoughts on fixing this was to Tee into the line just above the pump. From this tee would be a very small restrictor, maybe .062" to .093". This would feed into the line right before the pump. This would allow any  excess pressure to feed back into the open fuel line when an over pressure results from heat soak. The pump will outflow that small oriface by a longshot to feed the Autolite 2100 carb. I haven't done this mod yet. In an effort to keep the bypass as discreet as possible is why I'd do it just around the pump. The car was restored to stock and I don't want any excess hoses, pressure regulators or gauges under the hood. 

  Mark

[cjshaker]

I live in southern Ga.  It's hot.
Had this problem and many others that were heat related.

Solution:

-Block Exhaust crossover in intake
-Phenolic spacer under carb (even 1/4 inch it good)
-Isolate fuel line from engine heat as best as possible
-electric fuel pump  (this one isn't required but it does help)

Everything Drew said.
My old Falcons were really bad about this with the intake sitting on top of the exhaust manifold. It can get you in bad trouble sometimes.
I try not to hug the fuel line close to the engine, but keep it a little more out in the air. Keeps engine heat from soaking into it. Also, if you're at a place where it's ok to do it, pop the hood open a bit when you stop, to let the heat out; those compartments are tight and there's no way for heat to escape.
And if you have headers, make sure the line isn't close to them.

[fryedaddy]

thanks,a lot of good ideas.clean filter,fuel lines away from heat,electric pump,open hood,etc..

[C6AE]

 "Vapor lock" is typically liquid fuel turning to vapor (boiling).  Any reduction of pressure will lower the boiling point. This happens on the "suction" or low pressure side of the system (pump) and with any increase in altitude. This is why electric fuel pumps should be mounted as far from the engine as possible, maintaining pressure all the way to the carburetor. My experience with vapor lock started with flathead V-8's which had to lift the fuel up to the top of the engine (bad, really bad!) Later on Ford caught up and mounted the fuel pump as low as practical, but today with alcohol in the fuel and a combination of things like a small diameter fuel line in close proximity with engine exhaust heat, and even heat reflected up from a hot highway, this is a bigger problem than ever for the old fuel systems. Climbing a mountain pass on a hot day couldn't be a worse set of circumstances.
Ford had a similar problem early on by mounting the water pump on the cylinder head, that centrifugal pump will not pump steam! Big problem, catastrophic - once it boiled you were not going to cool it down using the water pump, and it took them about seven years to move the water pumps down as low as they could ('28-'36) and it was a few more years before they adde a pressure cap to raise the boiling point!

Whoever put the first fuel pump right inside the tank, submerged in the fuel was a genius..

[Drew Pojedinec]

I do love my electric fuel pump, and my Galaxie really benefited from it.
My truck (76 F100) doesn't seem like it cares as much , most likely from the more open under hood environment.  The truck also has a ton more frontal area and subsequently more air flowing through the engine bay.

I've been curious to see how 100% gasoline fares, but honestly with the things I listed above, I just haven't had any more issues, so I quit fussing with it all.