The Stovebolt Site Forums The Shop Area Engine Shop Is this vapor lock?

I have a 1955.1 Model 3100 with an all stock 235.

Last weekend I drove about 80 miles and then sputtered to a stop. Plenty of gas. I have clear gas filters both before and after the fuel pump. The one on the fuel tank side was filled with gas. The one on the carb side was bone dry. Cranking accomplished nothing. After waiting about 45 minutes, fuel slowly dribbled on its own into the upper filter. She restarted but only ran a few miles before the same thing happened.

A local "oldtimer" garage looked at it and said it was vapor lock and there was nothing to be fixed.

I picked it up today and she did the same thing after about a 35 mile run. It was not very hot (80's), the vehicle temp was normal and all the fuel lines are original and in the right places. So, I don't believe the vapor lock idea.

Do you think the fuel pump could be weak (perhaps a small tear in the diaphram) so its OK when cool but losing power as it gets hotter? I am inclined to just go right to putting in a new fuel pump. Am I missing something?

Also, any tricks to fuel pump replacement? How do I keep all the gas from running out when I disconnect the tank side fuel line from the pump?

Thanks,

Trooper Jeff
(who's tired of sitting by the side of the road)

One possibility is that the float valve is sticking closed. It has nothing to do with heat although a change in temperature can cause it to unstick. if you want to test, the next time it happens give the carb body a couple of sharp raps close to the fuel intake. That will usually free it up. It will usually happen with increasing frequency from now on.

If the filter between the pump and carb is dry, then it's not the float.

Sounds like a weak fuel pump...how old is it?

Mike B

If the fuel started to flow on it's own to the higher filter, and the filter is above the level of the fuel in the tank, The fuel pump diaphragm was held down by something and when conditions changed it pushed fuel up.
It sounds like old fashion vapor lock.
Found this:

"The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily."

A blocked gas tank vent could cause the same symptoms. The filter before the pump wouldn't be necessarily be empty because the vacuum in the tank would hold the pump diaphragm down and not allow it to pump the fuel through.
When the air temp changed or some tiny venting occurred in the tank side of the system the vacuum would have reduced allowing the diaphragm to slowly rise and push fuel. It would only take a few pumps of the fuel pump to recreate the vacuum and stall the engine again.

I have found that making sure that the fuel line has a gradual "up hill" slop to it all the way to the carb helps prevent vapor lock. I had a similar problem until I eliminated the "high" spots in my fuel line path. I still sometimes get vapor lock on really hot days.

You may also be experiencing cheap gas syndrome. I have found that cheap gas seem to vapor lock sooner the a national brand gas -- lower boiling point. You really are not saving anything by buying bargain gas.

These are my thoughts. I hope they help.
Walt L. Williams
La Verkin, UT

Jim,

This is a great suggestion.

Can I test this by operating the truck without the gas cap (or removing the gas cap when it stalls)? If I removed the gas cap after a stall, would I hear the vacuum pressure release?

That's the easiest way to test it but my money is on the weak fuel pump. Your description is spot on for that condition. Take it out & when it stops pop the gas cap. If the front filter doesn't immediately start to fill by cranking the engine it's likely the pump.

54 Belair has a good point about no high spots in the line to the carb. I always put the fuel filter in the vertical line after the pump, not over the engine.

Ive had this exact thing happen a few times on the 216 and 235's. Always ends up being check valve in fuel pump. Float probly not stuck.

Brings back old memories. I remember my Grandpa had a whole row of wooden clothespins on the fuel line to the carb.
I guess this was to dissipate heat?

yes, the clothespins were pretty standard on old F flatmotors in a hot climate - worked OK

Bill

I also remember you could not get the old Plymouths wet either. If it ever rained on them, they would not start.

Same thing happened to me a few months ago- going along and motor just starved for fuel and eventually died. I've had the truck 8 years and it's never done that before. Filter (up near the carb) had no fuel, and pump wouldn't pump with engine turning over. Plenty of fuel getting to the pump. After getting it home I took the pump apart, put it back together, and it ran fine. Until the next trip, then the same thing again. Fuel pump was 2 years old.

I drained and cleaned the tank (just a small amount of rusty gas in the bottom), added another filter between the tank and the pump, and put on an electric fuel pump, bypassing the mechanical one. So far so good, but I'm still not happy not knowing for certain what the real problem is/was. If you figure anything else out please post- thanks

vapor lock is a product of heat (and you Virginia boys dont know heat come to Texas and Hell's Half Acre to see heat). and usually the only hot spot on a old chevy pickup is where the fuel line passes real close to the thermostat housing (and sometimes when it gets close to a radiator hose top or bottom), but with a good operating fuel pump you should have enough pressure to raise the boiling point of the fuel to keep it a liquid and avoid that problem. It well could be that the fuel pump's spring has gotten tired.
to really test that theory try wrapping some asbesto (try finding some asbestos) around the fuel line at those hot places. If it runs vapor lock free, then leave it for conversation's sake or put on a new fuel pump.