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Check Air Pressure When Tires Cool

November 26, 1987|RALPH VARTABEDIAN | Times Staff Writer

Question: When on vacation, I frequently drive long distances at high speeds. I never seem to be near an air pump in the morning to check the tires when they are cold. Is there a rule of thumb for how long to wait before the tires are cool enough to check the pressure? Is there any rule for translating a cold pressure to a hot pressure? For example, if I measure the pressure cold in the morning and need three pounds, can I just add three pounds later when they are hot?--C.R.

Answer: The rule of thumb is that you need to wait three hours with the car parked before the tires are cool enough to get an accurate pressure reading. In addition, you should try not to drive more than one mile in getting to a pump if you want to recheck the pressure when you add air.

These rules may sound difficult, but there are some easy short cuts. If your tire pressure is a bit low in the morning, you might consider adding one or two extra pounds at an air pump at the end of the day. Then, the next morning, when the tires are again cool, you can adjust the pressure exactly by letting air out of the tires.

I am not aware of any rule of thumb for translating a cold-pressure rating to a hot-pressure rating. If you are down three pounds of pressure when the tires are cold, then you could probably come close to the right pressure by adding that amount when the tires are hot. Normally, tire pressure rises five pounds per square inch after sustained freeway driving. If you do add air when the tires are hot, you should probably double-check the pressure the next morning.

Q: My 1982 Honda Accord with an automatic transmission occasionally experiences violent shaking upon starting when cold and at other times when it is restarted. In both cases, the car shakes strongly for about five minutes. My Honda does this more frequently when parked on a hill.--M.M.

Q: My 1983 Honda Accord vibrated when backed out in the morning. It was so violent that things fell off the dashboard. It occurred for several minutes and then disappeared. I was so frightened by this problem that I sold the Honda and decided never to buy another one.--D.J.R.

A: Honda has identified a defect in the car that would account for these problems. But there also other possible causes.

The violent vibration and shaking are most likely caused by severe misfiring of one or more cylinders. The cause could range from fouled spark plugs to a problem in the electronic ignition system.

Another possible cause is described in Honda service bulletins regarding an overly rich fuel condition that occurs on restarting a hot car. The rich condition causes misfiring and severe shaking.

The rich condition results when a hot engine is shut off on a hot day. The heat retained inside the engine compartment causes gasoline to expand inside the fuel line and carburetor, causing what is known as percolation. The fuel pressurizes, floods the carburetor float bowl and then spills into the intake manifold.

When the car is restarted, the intake manifold is full of gas and causes the engine to run erratically for a few minutes until it is cleared out. Honda's solution to this problem has been to offer a system that operates the electric cooling fan under the hood for a few minutes after the engine is turned off. This keeps the fuel cool enough to prevent percolation.

Possibly, the two readers' problems are related to this condition, except that the fuel sits around inside the intake manifold until morning. A rich condition doesn't usually cause problems in a cold engine, but with the choke on and the extra fuel in the manifold, this could be the cause.

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