Gauging the accuracy of tire-pressure sensors

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May 26, 2009

Auto Scanner The Salem News Tue May 26, 2009, 12:19 AM EDT

Q: My car is equipped with tire pressure sensors. If the sensors give a different reading than my tire pressure gauge, which should be more accurate, then why would the manufacturer require a 2-pound difference between the front and rear tires?

A: Because tire pressure monitors are a part of today's world of cars, this is a timely question. The manufacturer of the car and its engineers have determined the car handles best at the prescribed tire pressures along with the proper tire size. The concept of different pressures front and rear is not new. When you set tire pressure on a car equipped with a monitor system, it is suggested that a digital gauge be used for accuracy.

Q: I recently purchased a 2005 Chevrolet SSR. It says in the owner's manual that it calls for 91 octane gas. It has the 6.0 liter LS2 Corvette V8. If I use the lower 87 octane gas, would there be any negative effects?

A: If you decide to use a lower octane, you could cause serious engine damage, as well as most certainly a loss in performance. The lower octane gas burns cooler than the 91 octane fuel. Your cylinder temperature will be reduced; your knock sensor will be sending signals to the computer at all times to retard engine timing. When the knocking continues, there is a good chance the check engine light may come on, as well.

Q: My 2004 Jeep Wrangler with 77,500 miles has been trouble-free until recently. Several months ago when I started the vehicle, it would start and then stall after about a second. This happened again and again — it would start and stall after running for about a second. After about a dozen tries, it started and ran. This problem didn't return until yesterday, and then after over a dozen tries, it wouldn't start. Out of desperation, I removed the key from the ignition and turned the key around. It started and ran. Is the problem in the computer chip that is embedded in the key?

A: If the problem is a key chip error, you will have a theft warning light on the dashboard. Since you didn't mention this, I will assume there is no warning light. The most common reasons for your problem are the Maf sensor, the idle air control motor and the throttle position sensor. A thorough check of these devices using the proper tools will reveal the reason for the stalling.

A reader responds to a recent article with this letter:

I have had similar symptoms with the 2004 Mazda-3 in the rain. Heavy rain on the highway, RPMs bouncing up and down and speed showing a sudden increase. I believe, in my case at least, that I was experiencing hydroplaning with front wheel drive. At the time, the front tires were very worn but still legal. As the front tires lifted off the pavement on a wedge of water they spun faster, causing the RPMs to increase and an apparent increase in speed. Taking my foot off the gas slowed the car and the tires contacted the pavement again. Speeding up again caused a repeat of the symptoms, and I then realized what was happening. No error codes to be stored and not repeatable in a garage. I hope this will help that Mazda owner.

Car Care Tip: I always enjoy letters from readers and those that contribute will be printed. Thanks for your readership.

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Larry Rubenstein is a master technician who owns a North Shore service station. His column appears Tuesday in the Business section. Write to Larry at The Salem News, c/o Auto Scanner, 32 Dunham Road, Beverly, MA 01915, or send an e-mail to scanauto@aol.com.

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