Car Talk

December 2003

Dear Tom and Ray:

While driving home from work on a hot day this past summer, the guy who gives the road reports said, "You'll notice a lot of cars off to the side of the road today suffering from vapor lock." He suggested adding a gallon of diesel fuel to the tank to prevent vapor lock. I've never heard this suggestion before, and I'm guessing it would do more harm than good. What do you think? Should he stick to giving road reports, or does he have a legitimate cure? -- Greg

TOM: Well, let's just hope he's not in charge of maintenance for the traffic copter.

RAY: Technically, he's right that diesel fuel will help prevent vapor lock. It's got a higher vaporization point than gasoline, so it would raise the temperature at which the mixture evaporates, which in turn would limit the amount of vapor produced. But it's still lousy advice.

TOM: The potential harm you would do by introducing diesel fuel into a modern gasoline engine is significant. It could damage seals in the fuel system, it could cause problems in your emissions system and it could ruin your catalytic converter. Plus, if you get caught, it'll void your warranty, so you're on your own once you do this.

RAY: And besides, vapor lock is almost never a problem with modern cars. In the old days, the fuel pump was in the engine compartment. So the entire fuel line (from the tank in the back of the car to the fuel pump up front) could become filled with gasoline vapor and prevent the car from running. But now, fuel pumps are in the tank, so the entire fuel line -- from the tank to the engine -- is under pressure, and really can't vapor lock.

TOM: My guess is that this is something the guy learned from his father, long ago. If it were 1961, and there were no emissions systems or fuel injection, the advice wouldn't be so bad. But nowadays it's lousy advice, and I hope most of his listeners ignored it.

RAY: Those cars stalled by the side of the road probably weren't vapor locked. They were probably the poor folks who took his advice the day before. Beat up old car

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