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

A: No. Absolutely not. Never. When driven, tires flex, and that causes friction, which creates heat, which, in turn, increases the air pressure. Engineers factor this in when establishing the tire pressure. The tire pressure sticker on your driver’s door says “cold,” meaning the pressure before driving.

Playing it safe

Q: Reading your recent column about tire tread depth, I have one observation to add. With most of the 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 the tread got lower, wet traction took a nose-dive. As soon as my tires start having significant loss of grip, I replace them. My life is worth more than an extra 5,000 miles.

A: Agreed. The penny test with the 2/32 (1⁄16)-inch depth is the minimum at which tires should be replaced. It doesn’t mean that you have to wait that long.

Oil not the culprit

Q: I have been using synthetic oil for the past three years in my 2004 Grand Cherokee with a 4.7-liter engine. Two months ago, when stopped at a stop sign, the oil gauge dropped to 0 (the engine sounded fine, however). I put it in neutral and revved up the engine, and the pressure returned to normal.

The mechanic I have been using for years had no idea except to try using regular oil again. But the problem returned. The mechanic had flushed out the engine before putting in the different oil. The water pump also failed one week after all this was done, but I’m not sure if the two situations are connected. Got any ideas?

A: The water pump was not a related problem. Nor does this have anything to do with the type of oil you’re using. The first thing I would do is replace the oil pressure sensor. Another possibility: The oil pump might be failing and won’t deliver enough pressure when the engine slows down. If the pump checks out OK, then the engine bearings might be worn.

Off is on

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?

A: It has nothing to do with Outbacks. By federal law, there always must be fresh air coming into the cabin. So the fan’s off setting isn’t totally off. As for nasty exhaust, I guess you just have to suck it up.

Bob Weber is a writer, mechanic and ASE-certified Master Automobile Technician. His writing has appeared in automotive trade publications, Consumer Guide and Consumers Digest. Send automotive questions along with name and town to motormouth.tribune@gmail.com.