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Saturday, October 31, 2009

Vapor Lock

I bought a used Dodge-powered RV with 27,000 miles. My son and I have been repairing things that have happened to it over the years. It was used in California for awhile, then to Phoenix for years and sat out in the sun.
The engine appears fine but on a trial run a transmission seal gave out. We had it repaired and the tranny rebuilt. Recently we were going to a racing meet where the temperature was ninety-three degrees. Eight miles down the interstate the engine started cutting out, but we finally limped home. A serviceman said it was undoubtedly the fuel pump vaporlocking and that it was normal for RVs in hot weather. He said to put an electric fuel pump in it. Is this problem solved by installing an electric fuel pump? Another fellow also told us that lousy gas diluted with alcohol also causes vapor lock. He said to add five gallons of diesel fuel to our forty gallons of gas. He said it will smoke a little, but will get you out of troubles. I also have a kit of a woven insulating material to cover the metal line from the fuel pump to the carburetor to insulate against the heat. We are also putting in a new fuel pump, since it has been setting dry for a long period. I also thought about installing a windshield washer which sprays the front of the radiator to help cool it. We just don't want to get trapped on the interstate this summer if we can help it, so will appreciate any tips!
Dr. Ron, (Niwot, CO)


Dr. Ron, indeed adding an electric, pusher-type auxiliary pump will help rectify vapor lock problems on most older Dodge chassis motorhomes. Installing the pump as close to the tank as possible usually provides the optimum performance. 

Insulating the fuel line, as you suggest, is also a worthy practice, but consider instead going to a larger size fuel line the entire length of the run. Some OEM applications found the original 5/16" fuel tubes mounted inside the frame rails close to exhaust components which simply amplified the problem. Installing 3/8" fuel lines and mounting them on the outside of the frame rails will help tremendously.

It is also advised to replace all rubber hose sections found in the fuel line including the hoses at the tank takeoff tube and all vapor return lines. Since the RV sat for so long and you have already experienced rubber seal deterioration, it's a sure bet all rubber products have deteriorated to some degree. 

Aside from vapor lock, you may be suffering from fuel starvation; literally sucking air through cracks in the rubber sections. Look closely, there are quite a few rubber sections in the system. Normally I shy away from backyard mechanic tricks like adding diesel fuel to the gasoline. Aftermarket products exist that would be a much better choice. And unless you are experiencing extreme overheating there is no viable benefit to spraying water onto the radiator.

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