astrotoy

I need an explanation. This morning (it was a balmy 52 degrees in the SF Bay Area) I checked my tire pressure. The car had not been driven for about 14 hours. The pressure display showed 37 35 35 35 35 (with 37 being the spare). After driving for about 90 minutes, mostly freeway (it was now up to 57 degrees outside) all of the tires were at 39 psi, including the spare. In the past I have seen the spare go up to 40. Why does the spare increase in pressure - especially when the ambient temp is not changing much. What is going on? Thanks, Larry

DrComputer

Although the ambient air temperature didn't change much, the spare tire is in the trunk next to the exaust pipes under the car which do get very hot. This will cause the pressure in the spare to rise too.

rominl

iTrader: (4)

your gas tank is about to explode?

j/k. you have the exec package right? is the rear AC on? exhaust? that might generate some heat. i don't recall much change in my spare pressure, it's always been around 39 and stay that way throughout my drive

astrotoy

I have the exec package. I don't think my rear A/C is on. I'll check. Otherwise, I think I need to call the dealer. Anyone else have this experience? Thanks, Larry

rominl

iTrader: (4)

man, i am not sure if this is something worth going to dealership about. like drcomputer and i said, exhaust could add some heat too

Johnny

There is a heat shield above the muffler that absorbs much of the heat radiating from it in addition to the air gap, so I doubt much of it would be transmitted to the trunk. Most of the heat rise in the trunk comes from the sun's energy. Heat in an enclosed space like the trunk accumulates and when the sun isn't a factor, it can actually get cold in there, such as on a rainy day. The kind of heat that would cause a spare tire's air pressure to rise would need to be significant enough to be felt when touching it. I have found the TPMS are a little bit fickle on this car, and the readings don't always correspond or match air pressure readings at the tire. No pun intended, but I wouldn't read too much into this phenomena.