Q: We have a 2010 Toyota RAV4 that had the same tire pressure light problem noted in your recent column. Took the car to the dealer as I was mystified — all four tires were within 1 to 2 pounds of each other in pressure. Final solution — the spare tire was low on air!
A: I received a number of similar responses to the question regarding the low tire warning light on the 2005 RAV4 — thank all of you for taking the time to share your experiences.
In the case of the 2005 RAV4, however, the spare tire cannot be the culprit for the simple reason that the spare does not have a tire pressure sensor installed. In fact, none of the tires have an internal pressure sensor.
As I described in my original answer, this vehicle has an indirect tire pressure sensing system that does not rely on individual tire pressure sensors. Rather, it utilizes ABS wheel speed sensors to recognize differences in rotational speed of the wheels while the vehicle is in motion. Assuming all four tires are the same make, size and circumference, significantly low tire pressure could cause a tire to have an effectively shorter radius — distance between wheel centerline to pavement surface — and thus rotate at a different speed. At a certain threshold of differential speed and resonance, the low tire warning light will be triggered.
Q: I have a 2000 Ford Explorer that for the past two years won't start in cold weather. I tap on the fuel tank and it starts. A shop checked all sensors and does not think it is a fuel pump problem because it happens so seldomly. Besides, it's a $600 repair. Any suggestions?
A: Yes, have the shop monitor the amperage "draw" of the fuel pump. The fuel pump of the 4-liter engine delivers 55-65 psi fuel pressure with the key on and the engine not running. As a general rule, electric fuel pumps draw roughly one ampere of current per 10 PSI of fuel pressure, so the pump for this engine should draw roughly 7 amps. Significantly higher current draw is indicative of high electrical resistance in the circuit or pump. As does that "tap" on the bottom of the fuel pump to vibrate the pump enough to get it started — another clear sign of a failing fuel pump.
Q: I have a 2001 Dodge Dakota with V8, 4WD and automatic transmission. When driving down the highway, occasionally the engine RPM will shoot up, then drop back down. There is no slip in the transmission or loss of speed. Would this be a sensor or module for the transmission? Or is the transmission getting ready to quit?
A: If there actually is no sudden slippage or change-of-gear in the transmission, the issue is likely with the instrument cluster and/or tachometer. But if you hear the engine actually rise in RPM, the transmission must be downshifting, unlocking the torque converter or slipping.
Try depressing the brake pedal slightly and/or manually downshifting from overdrive back to drive — do either of these actions replicate what you're experiencing? If so, the PCM may be sensing a need for more power and commanding these actions. A scan tool may be helpful in identifying any issues with the transmission or its control module.
Q: I bought a 2005 GMC Canyon in 2006 with 31,000 miles on it. The Hankook tires on it when I bought it still look new and I can't see any checking. I have driven only 6,000 more miles it since I bought it. Do you think I can still put more miles on them without problems?
A: In my opinion, yes. While tires do deteriorate over time, it sounds like the tires on your vehicle are still entirely serviceable. To make sure, have them inspected by a tire shop annually. Unless there's an identifiable problem, they should remain serviceable at least 10 years with your low annual mileage.