Got the “Vapors”

Tools Josep Ma. Rosell

Ken Freund
October 15, 2012
Filed under RV Tech Q&A

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We have a 1997 Coachmen motorhome with a Ford 460 engine. We recently returned from a trip west, where we ended up changing our plans and heading away from higher altitudes and even cut our trip short because of the problem we encountered with vapor lock.

The first time happened while driving through Nebraska, where the engine stalled at the end of an exit ramp. We ended up being towed to the campground and the engine started fine in the morning. The next time we were headed to Estes Park, Colo., and two-thirds of the way up we were dead again. After an hour or so it started and we made it into Estes Park, where it quit again. It quit twice more before we were able to make it to our destination.

We now know what causes vapor lock, but how to prevent it is the problem. We were told that automatic transmission fluid (ATF) could be added to the gas tank at 1 pint per 20 gallons, but not to exceed 1 quart, and also to loosen the gas cap to allow some of the vapor to dissipate. Doing that and using gas without ethanol, when we could find it, allowed us to have a trip without further incidents, but only after we changed our travel plans.

We were told Ford had issued a bulletin regarding vapor lock in this engine, but have been unable to find it. What can you add to this and is there a solution?

Carol Bliss l Via Email

You have been the recipient of some old-mechanic tales regarding the ATF, which won’t raise the point at which the lighter portions of the fuel vaporize. However, it will leave carbon deposits in your engine and will harm catalytic converters and oxygen sensors on vehicles so equipped. Loosening the gas cap might help if the venting system was not letting air into the tank to replace fuel consumed. However, if the problem were actually vapor locking in a fuel line, releasing the cap would do nothing.

I started addressing this type of problem with this model back in 1997 in this column and have addressed it numerous times through the years. At first it was thought to be vapor lock, but it turned out to be a problem with the in-tank fuel pump. The pump will intermittently quit working when it gets hot, and often start working again when it cools off. Ford came out with a redesigned fuel pump and wiring harness that seems to solve the problem for most motorhome owners.

To verify the problem, you need to perform a fuel-pressure test when the engine is hot under full load. I also recommend you replace the fuel filter. If you can find gasoline that’s free of ethanol, use that, too.

– Ken Freund


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