Readers’ Tips—

June 1, 2011 | By Bob Bageant

An Air Shortage Stopped My Truck

I had a 1963 Ford 6-cylinder pickup with a mechanical fuel pump that developed intermittent operating problems like those that have been described repeatedly on your informative Letters pages.

My truck would start and run well for miles but then would quit after coughing a short while. I did not know what to suspect but it seemed to me as though an electrical problem was causing the truck to stop running.

When the trouble occurred, I would lift the hood and check for apparent problems. On one occasion, I opened the points and cleaned them, the engine fired up and I went on.

I could not see burnt spots on the points on that occasion but since that got me going once, I carried spare points and had to perform this maneuver several times. However, that did not always help even though I learned to change points and set a gap pretty quickly while on the side of the roadway.

A more controlled study at a later time showed that the truck would run if I opened the gas cap. That truck had the gas tank behind the seat inside the cab. It depended on the ventilated gas cap to be working but unknown to me it had developed a partial obstruction and was not allowing enough air flow.

When the truck was not running, air pressure slowly became equilibrated between the tank and the atmosphere. Thus it was passing time that allowed adequate gas flow to the carburetor to allow the engine to start running again. So, my time spent changing points actually had been allowing pressures to equilibrate from outside air and the tank to the carb and thus allow it to run again.

I did not pay enough attention to the level of fuel in the tank but presumably, if the tank were full there would be less air space above the fuel in the tank and a vacuum would shut off the engine air flow quicker than when the tank held less fuel.

Before I obtained a new gas cap, I drilled a 1/16” hole through the old gas cap and the engine ran fine.

Bob Bageant Bluefield, Virginia