JimPap

You know a problem with cold tire measurements is when your car has been sitting overnight and your day starts with a temperature of 45 degrees, such as today and by noon, it's 85 degrees. That makes getting the tire pressure right a bit tough.

Then you have the difference between ambient temperature and the temperature of the tires. On one day that started off at 74 degrees and got up to 99 degrees, I checked the tires temperature with an infrared thermometer after having driven them for 30 minutes and they all read 120 degrees. Black asphalt probably is likely hotter than the ambient air. That's a 46 degree swing in temperatures.

jctevere

Ok, I finally got it figured out after a long stay at the dealer for this and a few other minor issues. It turns out someone was right in that the tires can't be inflated by more than 4PSI past what the recommended pressure is on the door jamb, even if you reset it to current values... I know, silly. It is coded in and the dealer said he could change it for me, but I decided to leave it for now.

He said it was a safety feature where if you had the issue come up and you looked at your tires and they were fine, and the issue kept popping up but the leak was slow; and each time you reset the values, you could keep lowering the allowable tire pressure to dangerous levels without even knowing. I guess its a good thing, but our cars should stop treating us like mororns... haha.

Markom

Hello, everyone ... Sorry for dredging this thread up again, but I've searched the site otherwise and this appears to be the most thorough discussion on the subject.

I too have recently had issues with tire pressure, change in ambient temperatures, and the TPMS; however, the trickiest aspect of this issue is obtaining the proper "cold" tire pressure, as the door label indicates. I have a 2010 Luxury (my first MB), so the front and back tires are exactly the same, but should be inflated to the recommended cold tire pressure of 30 psi in the front and 35 psi in the back. I recently had my 10,000-mile service and I believe they corrected the tire pressure to these specs as part of the tire rotation I had done.

BUT, at 30/front and 35/rear cold pressure, the ride and handling have degraded considerably in my opinion. When the tires warm up (after the 1/2-mile+ distance), the pressure now reads 33-34/front and 38-39/rear. Prior to the 10,000-service, the pressure after driving briefly, was nearly always at 28-30/front and 34-36/rear.

Anyone have any standard guidance for a more comfortable ride and handling as they relate to tire pressure? Would I be needlessly wearing the tires quicker at a lower pressure?

Thanks.

craiger13

I have been to the dealership to get my routine maintenance about 2 months ago. I had the re adjust the tire pressure because we were going for summer to winter and I know that I would have gotten a TP alarm. Now my issue is the pressure in the back is always off by 2. after spirited driving I can get up to 38 in the rear, but the opposite (passenger side) will say 36. this is repeated for whatever measurement is on the driver side rear. sometime its even off by 3. there is NO slow leak because my tire would have been empty by now. note: it remains the same PSI for two days or so and then it goes back to being off by two. Anyone have any suggestions of what could be wrong?

Tom in Austin

Mercedes' TPMS system is another of the mysteries of my car ... it was coming on every time I got in the car and the air pressure was off by one PSI on any wheel. I couldn't even get my tires all within one PSI of the target pressure using my cheapo $5 air pressure gauge, so I just ignore the damn thing. Then, a couple weeks ago it just shut up and haven't seen it since.