The Stovebolt Site Forums The Shop Area Engine Shop Stock 1957 235 - Help with Overheating and possibly Vapor Lock

Hi all!

Here's where I'm at after driving the truck for the first time:

The first time, the carb started starving after about 30 min and died on a side road. I checked under the hood and the clear fuel filter was empty. Suspected Vapor Lock.

I rebuilt the carb because the seals were old anyways, and while that helped a little, it still starved at about 40 min.

Next I replaced the steel hose from mechanical pump to carb with a thermo rubber hose and routed it away from heat sources. That helped with two things... #1. My fuel filter always had barely any gas in it, even while cold. Now it's always full. #2 I was able to make a 50m treck home.

However... on that 50m treck not only did her water temp get up to 200, but she started starving again. The gas was bubbling / boiling like crazy in the clear fuel filter before the carb. I poured ice cold water on the fuel pump and the bubbling stopped within 10 seconds. I suspect it wouldn't have made it without the water.



So TL;DR: Stock 235 Engine running water temp is ~200 after even 30m drive (in florida summer at ambient 90 degrees) and this is possibly causing fuel pump to overheat and heavily boil my gas putting it on the verge of vapor lock.

I should mention that I want to keep everything as original as possible, as the truck has sentimental value.

Thoughts or any help would be greatly appreciated!

If it was my truck I'd start with two things. I'd flush the cooling system and put a new fuel pump on it.

If you flush the cooling system be sure to remove the block drain and use an old clothes hanger to scrape as much crud as possible out of the block. I would then buy two bottles of cooling system flush and follow the directions (except for the amount of flush) to flush your system. If that doesn't help the overheating we can go to step two. I used two bottles on my '38.

Vapor lock is very often the first sign of a failing fuel pump.

If you have the fuel lines routed in their normal locations, it is highly unlikely that you have vapor lock.

Do you have a gasket + bakelite-insulator + gasket underneath the carburetor?

Tiny has posted excellent suggestions.

Thanks Tiny! Now that you say it.... the whole system was REALLY rusty when I first got her. I pressure washed what I could reach as much as possible on the rebuild, but a chemical flush might be a great idea.

The fuel pump is brand new, but it did sit for 4 years before it was ever really ran. Might start there.


Tim - I have an insulator and gasket underneath, although I have been checking the carb to see if it was bubbling in the bowl. I've read you can sometimes even hear it.

The bubbling is definitely coming down the line from the fuel pump and into the filter. The hotter then engine gets, the worse the problem gets. What made me lean towards vapor lock was that pouring cold water on the fuel pump made it immediately stop bubbling until it heats up again. I've tried it with a cold wet rag too, same result. Although maybe the heat is making something expand and then it's sucking air?

Check your dipstick to see if it smells like fuel. Sounds like bad /loose fuel pump

You may have trash in the fuel tank partially blocking fuel flow. I recommend that you remove all of your fuel lines to check for crud, and pull the sending unit from the top of the tank and shine a light to see what is in the tank. There should be a sock on the end of the fuel pickup. It could be missing or clogged.

Ditto on the fuel line restriction- - - -the vacuum from the pump will make the fuel boil if it can't flow freely. You can check the pump operation with a vacuum/pressure gauge. The pump should pull 10-12 inches of vacuum on the inlet side with the engine running on the fuel in the carburetor, and the vacuum should stay there for several seconds after shutdown. The pressure side of the pump should make 3-4 PSI at idle, and the pressure should stay there for a short time after shutdown, also. A good pump with a clear inlet line will pump a pint of fuel into a catch container in 30 seconds at idle speed.

To give the cooling system a good cleanout, mix about a pint of muriatic acid (used for masonry cleanup, swimming pools, etc.) with about a gallon of water, and add it to the cooling system. Top off with water and run the engine for about 30 minutes. Drain, flush with clear water, and neutralize the acid by adding a full box of baking soda to a fresh fill of water. Run the soda solution for 10 minutes, drain, and rinse. When you're sure it's clean, run a 50/50 mix of antifreeze and water.
Jerry

200 degrees really isn't that hot. Especially on a hot day. But you may have 195 degree thermostat. If so you can drop down to 180 or 160 degree thermostat for that engine and get it a little bit cooler. I've run 195 thermostat in my old 235 in the past with no issues.

In addition to the others good ideas, make sure the gas cap is vented. If not you can pull a vacuum on the fuel tank and it will quit running.

An easy test for that is to simply remove the gas cap and go for a test drive.

Running lean can cause an engine to run hot too. Both problems may be related to a common cause. Pull the float/fuel pickup from the top of he tank, the sock, if present could be clogged. Disconnect the line at the pickup and at the fuel pump and blow it out, see what if anything comes out.

Thanks, will do!


Checked for this unfortunately. I actually have a fuel filter in-line BEFORE the fuel pump because the type of fuel sending unit I have doesn't have a sock and there's no filter in the gas tank, as it's a below-the-bed aftermarket. Is it possible that the fuel filter could be causing the pressure difference? I've checked it during operation and it looks completely clean / full.



Jerry - Good point, I really should just test it the right way instead of 'backyard mechanic-ing' it.

Roy - Running a 180 thermo, brand new. It hits 180 fairly quick, stays there for about 15 minutes, and then slowly climbs to 200. Also did the gas cap off trick, hoping it was that. No dice



I'm still hung up on why the bubbles stop when I place a cold rag on the fuel pump or pour cold water on it, but I need to make sure everything else is in order anyways either way. So thanks for all the suggestions and input guys!

My Challenger's optimal running temp is ~212 but I was afraid 200 could possibly ruin an older engine. If it's ok for me to run at that temp, I'll just run through all these tests to get the fuel sorted first and also flush the system like Tiny and Jerry mentioned. Basically just run through everything you guys mentioned, step by step and if that doesn't fix it I'll buy a better pump!

Thanks again all, I should make a plaque for this truck that reads "Built with the help of <Insert Stovebolt forum names>".

The cold water on the pump is simply cooling the fuel coming out of the pump, just as you suspect. Fuel flow keeps the pump from overheating, if the flow is restricted, the pump runs hotter.

If the filter before the pump is clean, it will have little effect on the operation.

is it still giving you trouble? it seems with an aftermarket fuel tank below the bed that the tank is below the mechanical fuel pump and may not get fuel to the pump unless the tank is full. I wonder if an electric fuel pump in the tank would be better?

'47/'48 trucks came with tanks lower than the fuel pump without trouble.