rjrdomer

So this fangled tire pressure monitoring system that Audi has on the Q5. I know you program the initial pressure and the system lets you know if there is a drop, so it's sort of a relative pressure system.

My question is: how sensitive is the system? I ask because while driving with the wife and sleeping baby in the back of the car, the light came on in 100 degree air temperature. Road likely much, much hotter. I was about 50 miles from home so just kept going as I didn't feel any differences. When I got home, I inspected everything (no nails, scrapes, holes, etc). I checked the pressure and 3 tires were at 33psi and one was at 32.5. Usually not enough to go crazy over and I would just leave it. But is the system that sensitive that if it goes off by half a psi, it alerts you?

Coincidentally, I had my 5,000 mile complimentary service appointment scheduled later in the day so I had them look at it. Audi does a keen thing of not letting you near the service department or talking to the mechanic so I never got a straight answer as to how low they felt the tire was or what the issue was in general. So I just have to accept it for what it is. ny thoughts on to the sensitivity of this?

Reggie

If you got home and checked the pressures - then you need to recheck them as you measured the pressures when the car tires were hot - Low pressures will cause the tire to heat up and increase the tire pressure - hence if one was low you would not know it till the tire cooled down.

scrmorling

Fair point, Reggie but wouldn't the pressure in each tire tend to increase a similar amount?

Reggie

Only if they started with the same pressure. I have seen cases where the pressure in tires were extremely high because the tire had been low - under 10lbs when cold.

This is why one must set pressures when the tires are cold.

I also have heard horror stories of people who had low pressure in a tire(s) and then drove on them and checked the pressures hot and were shocked to see how high the pressure was and let air out to get the pressure down to what the manual said - and thus insuring that they would get a blow out. Air is used absorb the heat the tire generates - take away some of the air that is in the tire - the heat will increase the pressure of the remaining air even more: PV=T

Here is a good article: http://www.arden.org/misc/pressure.html

scrmorling

Agreed.

Reisen

The system is not sensitive enough to register a half psi difference. As someone who has gone a couple of months now with a tire with a slow leak (needs filling maybe once a week), it usually comes on somewhere around 4 or 5 psi difference.

rjrdomer

Thanks all. I will get better about checking them when they are cold.