Howdy

I haev a 2006 997, 35k miles.

From what I understand, it needs about 33 psi in the fronts and 39 in the back.

And they are showing this on the onboard digital guage thingy.

Yet it is now telling me -- when starting the car -- to add 4 psi to the right rear tire.

Should I just add some air?

 

The TPMS is good for letting you know of a relative change in the tire pressure. It is not necessarily accurate. Use an accurate tire pressure gauge to check the tires. Note what the TPMS says at that pressure for future reference.

I only adjust the tires based on a real pressure gauge, not the dash display.

 

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i had this same issue on my 07 C2S. Using a high quality Mac Tools tire gauge (as in accurate), I was 33/39 all around, yet every idiot lamp on my dash told me I was low in 2 tires and needed to add 4 psi of air. These inidacators were on all the time and I grew tiresome of looking at them. Researching here on 6Speed, I found the TPS is not very accurate and you should trust a good gauge over the onboard system.

So... I schedule a service appt and bring my car in stating there are errors in the TPS system. All codes check ok and all TPS batteries still have 36 mos life left. The delaer adds air to the tires and all warning lights go off. I felt like an idiot to bring my car in b/c my tires needed air.

Service manager tells me to listen to the TPS system, it's more accurate than any gauge (I call BS to myself but didn't want to argue and upset our developing relationship). Whatever...

 

So after the dealer added air what did your Mac Tools tire gauge say the pressures were. Unless you checked dealer may have put 40+ all the way around to get the lights to go out.

 

My handheld gauge reads 42 psi in the rears when cold. I've verified this against another gauge I have. TPS states 39 at the same time.

Also, while at the dealer, I was instructed to not follow the normal dash TPS menu screen (as can be paged and displayed while driving) when filling the tires but review the screen that can only be displayed through the dedicated TPS menu when the car is parked. This screen shows +/- values for each tire, not the overall presssure. Apparently this more detailed screen is compensated for temperature which the main screen (which you can access while driving) is not temp. compensated.

 

I have TPMS on a Suburban and a Lexus in addtion to the Porsche and none of them provide the same accuracy as the hand-held gauge. Fill your tires and measure pressure with your hand held then check the TPMS - e.g., you say your rear tire measures 42 psi with the hand held and 39 psi on the TPMS so now you know there is a 3 psi difference and you can monitor your tire pressures knowing this differential. If 42 psi is your desired psi then if TPMS shows 37 psi (when cold) then you know your down 2 psi. I've never depended on TPMS for absolute pressures on any car.

 

I never had a question here, I was just describing what I went through and how inaccurate TPS is. I'll always rely on a good handheld pressure gauge over an onboard system, despite what my dealer tells me.

If I keep an actual recommended 39 psi in the rears, TPS freaks out and tells me I'm 4 pounds low and flashes 2 warning lights on the dash that do not go out.

Running an actual 42 psi (tps reads 39), and there are no TPS errors. I'm okay with this...

 

I run my pressures the same and got the same message but for my LT rear. I checked my tires with my gauge and everything looked fine. I reset the TPMS and the next day got the same message. This time I inflated fronts to 35 and rears to 42 psi then brought the fronts and rears back down to my original ( 33 frt 39 rear) pressures and reset the TPSM no message now for the past 5 days. Maybe this method would work for you.

 

Set your cold pressures (68F) at 33/39. If the 'cold temp' is not 68F adjust pressure up or down by 1PSI/10F diff. Afterwards reset your TPMS making sure all parameters are correct (type of tire and load).