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Motormouth: ‘Cold’ tire pressure

Q: I have a 2016 Tahoe and recommended tire pressure is 35 psi. When I start out, it reads 35 but after 20-30 minutes of high-speed driving, all tires are up to 38. Should I reduce pressure to 32 so it will be 35 at speed?

J.C., Key Largo, Fla.

A: No. Not. Never. When driven, tires flex and that causes friction which creates heat which increases the air pressure. Engineers factor this in when establishing the cold tire pressure. Check out the tire pressure sticker on the driver’s door. It says “cold” and that means the pressure before driving 5-10 miles.

Q: Reading your recent column about tire tread depth I have one observation to add. Running tires to minimum tread depth creates a false sense of security. With most tires I have owned, once they get below half wear, they have significant loss of traction especially in wet weather. I had Continental tires that had markings for snow traction, wet traction and dry traction. As they got lower you could see wet traction nosedive. As soon as my tires start having significant loss of grip I replace. My life is worth more than an extra 5,000 to 10,000 miles.

J.K., Whitehall, Pa.

A: Agreed. Relying on the penny test with the [2/3]2 (1/16)-inch depth is the legal minimum at which time the tires must be replaced. A safer test is to use the space between the edge of a quarter and George Washington’s head (4/32-inch).

Q: I have been using synthetic oil for the past three years in my 2004 Grand Cherokee with the 4.7-liter engine. Two months ago, when stopped at a stop sign, the oil gauge dropped to 0. If I put it in neutral and rev up the engine, the pressure returns to normal. The mechanic I have been using for years had no idea except to try using regular oil again, which did not solve the problem. They flushed out the engine before putting in different oil also. The water pump also failed one week after this was all done. Not sure if this was part of the problem or not. When pressure falls to 0 the engine still sounds fine. Got any ideas?

K.B., St. Michael, Minn.

A: The water pump was not a related problem. But the oil pump may be failing. If the pump is worn, it won’t deliver enough pressure when the engine slows down. If it is not a bad pump, engine bearings may be worn, possibly the piston rod bearings. Before replacing the pump, you may want to replace the oil pressure sensor which could be the culprit.

Q: I’ve had a 2015 and now a 2020 Subaru Outback. On both, when I turn the fan off, there continues to be air flowing through the vents. This isn’t particularly helpful when behind nasty car exhaust. Any ideas why off isn’t really off?

R.S., Bloomingdale, Ill.

A: Although you turn the blower to the off position, it continues to run to provide fresh air to the cabin. I am not sure where it is located on your car, but there is another vent that allows the air to escape in order to constantly keep fresh air flowing. There is nothing you can do about somebody else’s nasty exhaust. I guess you just have to suck it up.



Bob Weber is a writer and mechanic who became an ASE-certified Master Automobile Technician in 1976. He maintains this status by seeking certification every five years. Weber’s work appears in professional trade magazines and other consumer publications. His writing also appears in automotive trade publications, Consumer Guide and Consumers Digest.

Send questions along with name and town to Motormouth, Rides, Chicago Tribune, 435 N. Michigan Ave., Fourth Floor, Chicago, IL 60611 or motormouth.tribun



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