Vapor lock in an M37

[HWooldridge]

Like a lot of folks, I use my M37 to work around the farm and it hauls a hay wagon every season. About once or twice a year, it vapor locks after long sessions of low speed pulling. This past Saturday, I drove around for a couple hours picking up bales and just as I was about to head to the barn, she stuttered and quit. It wasn't terribly hot, only about 95 with a nice breeze, so I put the hood up and headed to the house for an ice tea. Apparently, I cool off at about the same rate as the truck so when I went back out 30 minutes later, she fired right up and off to the barn we went. I'm reasonably sure it's vapor lock because this only happens when it's hot and low speed - any other combination of work and temps doesn't produce the problem. The truck is pretty close to factory but I do have a rubber hose and in-line fuel filter just ahead of the fuel pump.

So my question is whether this is common and can anything be done to eliminate it? I was considering putting some holes in the heat shield to help with ventilation or maybe fabbing another shield out of aluminum but wanted to ask before going down a bad path.

[Gordon Maney]

I have seen people make aluminum heat shields or extensions to heat shields.

Is there any reason why the carburetor might be getting too hot?

[HWooldridge]

I have seen people make aluminum heat shields or extensions to heat shields.

Is there any reason why the carburetor might be getting too hot?

Well, I don't really know about the carb but I think the line out of the fuel pump is getting too hot because it is usually empty or nearly so when this happens. The truck doesn't run hot at the gauge and the water pump is about 3 years old. Now, I can say that I've always run it on straight water with no anti-freeze so maybe it is getting too warm. In addition, the internal baffle in the exhaust manifold is frozen in the half-open position so that would increase the retained heat.

[Matthew Ziegler]

fuel line

I had a similar problem, I made up a new steel line that routed farther away from the exhaust, I also purchased some shiny aluminized cloth tubing at a local speed shop, I don't remember the brand but it was designed to protect lines and wiring from high heat. Might be a good idea to check your float hight in the carb as well.

[Gordon Maney]

Well, I don't really know about the carb but I think the line out of the fuel pump is getting too hot because it is usually empty or nearly so when this happens. The truck doesn't run hot at the gauge and the water pump is about 3 years old. Now, I can say that I've always run it on straight water with no anti-freeze so maybe it is getting too warm. In addition, the internal baffle in the exhaust manifold is frozen in the half-open position so that would increase the retained heat.

The straight water aspect is not a problem; if it was, it would be boiling it.

[HWooldridge]

The straight water aspect is not a problem; if it was, it would be boiling it.

What do you think about the baffle in the exhaust? Should I attempt to work at getting it fully open?

I may have to reroute the line - that would certainly help.

[Gordon Maney]

It can't hurt to open it. I do know the valve on the truck I drove for many years was stuck in the center position because someone put it there, as was often done, and it froze in place. It was that way year round.

Rerouting the line is a good idea. Also, you can make up a big fin out of thin aluminum sheet to go between the carburetor and the manifold. Experiment with a cardboard pattern to see how big you can make it.

[HWooldridge]

It can't hurt to open it. I do know the valve on the truck I drove for many years was stuck in the center position because someone put it there, as was often done, and it froze in place. It was that way year round.

Rerouting the line is a good idea. Also, you can make up a big fin out of thin aluminum sheet to go between the carburetor and the manifold. Experiment with a cardboard pattern to see how big you can make it.

All sounds reasonable - thank you.

[CSCameron]

I did as others have suggested and extended the routing of the fuel line on my 55 FFPW as shown in the pics. No problems and I'm in Phoenix.

[pwrwagonfire]

At work, we run two WM300s very heavily as fire apparatus.

They have each had a "flow through" electric fuel pump added in line, and whenever the trucks vapor lock you turn on the pump, and within a second or two the problem is done.

The '57 WM300 we have has a 230 like the M-37, and is much more prone to vapor locking in hot temperature then our '64 with a 251...this is my experience anyhow!