New Tank Cover May Solve Buick’s Stalling

Question: My 1979 Buick Regal V-6 with 60,000 miles is a cream puff of a car, except for a recent problem. After filling the gas tank, the car stalled while going up an 8% grade. After a prolonged wait, the engine started up and I drove home. I had it checked at a dealership and was told nothing was wrong. Everything was fine for a couple of months, when the problem occurred during the same circumstances. Today, it occurred again on schedule three months later. What do I do? --M.B.

Answer: Your problem could be very complex, but my hunch is that it is relatively simple. It sounds like the engine is starved for fuel, which may be caused by the gas tank losing ventilation.

The gas tank is vented so that as the tank empties, air replaces the lost fuel. Otherwise, you would tend to build up a vacuum that would impede the fuel pump from drawing fuel out of the tank.

Some gas tanks are designed to operate with a slight pressure in the tank, which you may notice when you remove the tank cap. The venting normally occurs through a charcoal canister.

When the tank is partially full, your stalling problem does not occur because there is a large amount of air in the tank and any vacuum buildup would be relatively small. But when the tank is full, the drawing out of only a small amount of gas could create a large vacuum if the tank is not vented. The reason the car restarts after a period of time is that the fuel system is not perfectly sealed and the vacuum is lost over a period of minutes.

As for the uphill grade, it is possible that the grade forces the engine to burn fuel at the maximum rate, allowing the vacuum to build quickly. Another possibility is that the fuel sloshes back against a vent tube and blocks the tank from venting.

In either case, the gas tank cap is supposed to have a flapper valve to relieve any vacuum. If this is not working properly, the tank will go into a vacuum condition and the engine will be starved for fuel. So, the next time the stall occurs, try removing the gas tank cap briefly and then see if the car starts up. You might be able to solve the whole problem by simply buying a new gas tank cap.

Q: I have heard that adding a quart of transmission fluid to the gas tank at fill-up time will allow the engine to run more quietly and smoothly. Is this a good idea? --B.S.

A: It is a bad idea that transmission fluid will help clean carbon deposits. There’s no evidence that it can do so. Rather, a whole quart of oil is enough to foul your spark plugs and leave the engine needing a tune up.

Transmission oils, depending on specifications, contain ash additives that can cause engine deposits. If your car is running rough, you can use an additive designed to clean the engine.