VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 vapor lock 37MD

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Hi I took my 37MD on a 200mi round trip last Saturday to a car show in 95 deg temps and had some vapor lock issues going and coming. I talked to 3 different older gents about the prob and they said that I should clip wooden clothes pins on my fuel line to dissipate the heat!! Went to town, bought some and clipped them on the fuel line! Low and behold it did not work at all. Do you think I got had? Oh well I now have some clothes pins for sale plus the trip. Hope the pics show on this post, thought you guys would enjoy a good laugh!! Thanks Ray

The clothes pins, wrap with aluminum foil plus a few other suggestions have been touted to eliminate vapor lock for decades. Most suggest wrapping the fuel line between fuel pump and carburetor.

Once I explain what vapor lock is and why it happens you will know that anything between fuel pump and carb is pure malarkey.

Vapor lock occurs when the fuel pump or line leading to the fuel pump gets hot enough to cause the fuel to vaporize due to the heat and reduced pressure. Essentially you create a "bubble" of "gas vapor" instead of drawing liquid into the pump. The result is no fuel is moved from the fuel tank and supplied to the carburetor. Pouring cool water on the fuel pump often temporarily solves the problem.

If the fuel pump is working properly the line between it and the carburetor is under a few pounds pressure. That pressure keeps vapor "bubbles" from forming (except under highly unusual conditions).

I had problems with a '31 Fire Truck vapor locking in slow parades when ambient temps got over 90 deg. The Boss (owner) didn't like lifting the hood, pouring water on the fuel pump, closing the hood and then getting herself back into the truck mid-parade. So I put an electric pump in the middle of the line from the gas tank to the fuel pump. When the engine began to cough & die, hit the switch to the electric pump (pressuring the line to mechanical fuel pump) for a few seconds and everything smoothed out for a while. Kept her in the seat and life was much better for me if you know what I mean?

Now insulating the line between tank and fuel pump, particularly when near the engine, and providing cooling or insulation for the pump will reduce vapor lock. Problem is modern gas has larger percentage of higher vapor pressure components so much more prone to vapor lock. Adding 10-15% diesel to your gas tank will also help reduce vapor lock (raises the overall boiling point or reduced the vapor pressure of the fuel).

In other words vapor lock does not occur between the fuel pump and carburetor but rather between the the fuel pump and the gas tank. That is why some cars had a air deflector mounted over and behind the fuel to help more cooling air contact the fuel pump.

Hey Gene
I read on the CCCA forum and was told by a friend that placing a tee as close as possible to the carb in the fuel line and adding a return line of a much smaller diameter to the fuel line would relieve the pressure caused by the "boiling" gas. Does that make any sense to you?
thanks
Ray

OOps I meant running the return line back to the tank!
Well it's 3AM and I have to go start taking the prep for a colonoscopy, can't wait!!!
Thanks
Ray

Personal experience has been that vapor lock is a symptom of a weak fuel pump. Replacing the fuel pump has solved the problem every time for me. Chip & Gene's input that it occurs between the tank and pump rather then between the pump and carb is new to me. I'll have to file that tidbit away in the grey matter for later reference.

Your observation that worn fuel pumps are more prone to vapor lock is a good one. The greater the vacuum at the pump the better chance that some liquid gas will be drawn in. Also any leak in the diaphragm will add air to the fuel and exacerbate vapor lock.

Yes, you "got had!" of course those old dudes remember doing that way back yonder when vapor lock was common, what they forgot is that it it didn't work back then either, nor did a Grapefruit half crammed down over the fuel pump top.
Ford Flathead V/8s were very prone to vapor lock with a poor grade of gasoline, many of the Flathead Fords that came into the Sinclair
station I worked in near Wichita Falls, Texas (the hottest place I ever lived in), bought Hi-test or premium grade gas to help prevent vapor locking. These V/8s had the fuel pump mounted up topside between the forked 8 cylinders where there was always a good supply of heat, I knew one Ford guy that had two electric fuel pumps on his 50 Ford, one at the tank and one at the front of the engine. It was rarely the case of a Chevrolet or Mopar six that had a bad case of vapor lock unless they were running casinghead or drip gas, or as usual the failing worn out fuel pump. The introduction of the by pass return line back to the tank and the in-tank electric fuel pump has about done away with vapor lock.

AHHH! Them Good old days!

Many professional Baseball Players put a "CABAGE LEAF" under their hat, they swear it keeps them cooler in the sun.
Maybe if you wrap a CABAGE LEAF around your fuel pump it would stop vapor lock....Ed

I used to have this problem with an International/Harvester Scout at elevations above 4,000 ft and slow speeds...I'd toss a bag of ice on the fuel pump and things would be fine...