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Has anyone experienced increased vapor lock problems lately. With the temperatures getting warmer, every time I take my truck out it vapor locks. it will run fine until it gets up to warmer operating temp then no more fuel and it dies. If I pour cool water on the fuel pump and lines going to the carb it will start again. I have replaced the line from the fuel pump to the carb with rubber and rerouted it over to the fender well. It didn't help. The odd thing is, I am having the same problem with my M725 ambulance. I have bought gas at different places just in case. I am wondering if some places are adding ethenol or something to the gas. I believe ethenol would have a lower boiling point and may be the cause of the problems. I have owned my M37 for three years and have had no problems in the past, even in long 4th of July parades. I have owned my M725 for eight years and have not had any problems with it either. Any thoughts guys?

sure my truck done the same thing when I got it , most of the time it is a simple fix, look on the fuel pump, there should be a heat shield attached to the top of the pump. if it is missing put one on , should solve the problem.

My motor has a civilian style fuel pump on it. I am going to try to rig up some sort of heat shield for it. Is this a stock part and does anyone have a photo?
I still can't understand why I just started having this problem. Have I just been lucky these past couple of years?

Yes, there is a stock heat shield.

Yes, you've probably just been lucky for a couple of years.

Thanks. That makes me feel a little better. Someone also told me about putting spring clothes pins on the line from the pump to the carb will keep it from vapor locking. Anyone hear of or try that?

I think that's an "old wives tale." The only way to prevent vapor lock is to keep the fuel line from getting overheated. Somehow, I don't think the clothespins will do the job.

I put a new nos fuel pump on my truck, IF you are doing a complete resto. like I am, dont forgett that the wiper motors run off the fuel pump. I put new wiper motors , new fuel pump, new hoses, guess what? the wipers work as good as they did when the truck was new, even when the truck is going up hill or at idle. well satisified with the results!

The heat shield mounts to the top of the pump but the line running from the pump to the carb is near the manifold. Ideally, there should be a heat shield between the manifold and the line, but maybe the air gap is enough? As a last resort, shielding the line might be in order but I think we'll wait until a summer heat soak to guess.

Can someone post a pict. of the heat shield ?
Thanks,

My truck has an industrial engine in it and the fuel pump is rearward, and very close tothe exhaust downtube. Since there was no other way I could figure to shield the pump, I had to wrap the downtube with that stuff that motorcycle buff's use to insulate the exhaust so they won't burn their legs, etc. Seems to work as I have not had a vapor lock while driving, although I did have a problem with the truck being hard to restart after driving for a while then shutting down, then restarting a few minutes later. Had the same problem when i went to restart the next morning when the engine was cold so don't know if that was related. I had to use some ether down its throat and it let out a cough of greysh smoke then has been starting fine ever since, so maybe I just had something clog a jet etc momentarily, I don't know.

The shield attaches to the forward bolt on the exh manifold and there is a small stud on the top of the pump that aligns to a hole in the shield. I made the mistake of using one of the nylon lock nuts... heat and nylon don't like each other.


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Thanks for the photo. Maybe I can get this problem cured before the 4th of July parade.

Here is another thing to look at. On your gas filler cap, if it is original, it should say - vented cap, vent open normally and closed for fording or vapor lock. allowing a little pressure to build in the tank will help push the vapor throgh the fuel pump. I have driven my M37 through some really hot weather and have not had any vapor locking. But, the heat shield is in place plus a closed vent valve in the gas cap. Sid

Thanks for the tip Sid!

Thanks for the picture.