auto maintenance February 07, 2018

Suburban Legends

Insidious Vapors

When I started working in auto repair in 1981 I noticed quite a few cars with clothespins clamped onto the metal fuel lines under the hood. When I inquired as to the purpose of this to the car owners, they always said it was to prevent vapor lock. “Vapor lock” is actually a misnomer, made worse by the fact that some auto manufacturers use the term. Poorly designed, inadequate or faulty fuel delivery system would be a more accurate description.

This problem occurred almost exclusively on the carbureted vehicle of the early to mid-eighties, and disappeared more or less when fuel injection replaced carburetors. A fuel line placed too close to the exhaust, or a hot spot, allows the fuel to boil in it, creating a gas bubble through which fuel cannot flow. How clothespins on the fuel line were supposed to stop this I don’t know, but every tech bulletin I ever saw on the subject involved putting an in-tank fuel pump in, correcting the weak or inadequate fuel system, and there wasn’t any locking about it. Since modern fuel-injected cars have in-tank fuel pumps that efficiently push fuel instead of inefficiently sucking fuel, this problem has in effect disappeared.

Gas Pains

I often hear people attribute the problems with the way their car runs to “bad gas.” Now, there was once a time when a gas station’s underground supply tanks may not have been maintained as they should be. Leaks may have developed, allowing ground water and contamination to enter the tanks, causing fuel filters to clog and cars to stop. But this was long ago. Environmental laws require that underground gas storage tanks are strictly monitored and regularly replaced. Hint: water and gasoline don’t mix. They remain separate even within the same tank. Water being heavier sinks, and gasoline being lighter rises.

When the United States’ oil tanker Ohio arrived in Malta with desperately needed high octane gas for the Spitfires in mid-1942, they were afraid the badly damaged ship would break up from the change in buoyancy as they pumped the aviation fuel out. So they pumped sea water into the bottom of the tanks as they drew gasoline out the top to maintain the same buoyancy. And they didn’t have any problems using the gas to blast the Luftwaffe out of the sky.

Lesson: if you have enough water in your gas tank to actually get into the fuel lines and up to the fuel injectors or carburetor, your car will stop and not run at all until the problem is corrected. Water has too high a viscosity to pass through the passages in a fuel injector or a carburetor. So water in gas is highly unlikely. As far as the quality of any given brand of gas, the chemical makeup of the gasoline is by and large predetermined by law. I can go to the big tank farms nearby and watch five different gas suppliers pull their gas from the same tank. Only a small bottle of additive poured into the gas at point of distribution gives it its brand flavor. So your car isn’t running badly because of bad gas.

Discharged with Prejudice

Every now and then a customer will come in with an electrical problem, either starting or charging, and tell me he knows the alternator is working because he unhooked the negative battery terminal while the car was running and it didn’t shut off. Apparently the look on my face must be sufficiently horrified because after a few seconds they usually say, uh, did I do something wrong?

I suppose this alleged test dates back to the Model T Ford and probably wasn’t valid then. It is rather the equivalent of setting a woman on fire to determine if she’s a witch or not. The answer won’t much matter. As the negative battery cable is pulled away, a tremendous voltage arc occurs. The alternator starts to both overcharge and burn with nowhere for the voltage to go, and every circuit in the car is desperately hunting for ground. Think of all the computer processors, diodes, solid state circuits, relays and modules all getting an enormous voltage spike. You may as well fly a kite into a thunderstorm and hook it up to the back of your PC. Heck, BMW says you can blow the radio in their car with just a jump start.

So please, if you believe nothing else I’ve written, do not disconnect your negative (or positive) battery cable while the car is running. It is not a valid test, it tells you nothing, and is likely to cause future electrical problems.

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