Hi -

I'm getting a low pressure readings just about every morning...but my tires appear fine. (We checked them with a guage). By the time I get the car to the dealer, the dashboard warning goes away (twice now). Is this just an idiosynchratic Porsche thing?

I have a '09 4S cab with the "TPMS".

thx. - A

 

The morning tire pressure is based on a cold tire . Once the car is driven the tire hearts up and the pressure reading increases . You get to the dealership and it's all ok which implies that you are borderline . The problem is that when filling the tire it's already warm too . So what i usually do is fill each tire +3 psi above the recommended level. It usually reads at or very near the recommended cold pressure the next day.

 

I've learned to ignore my TPMS. It is a porche quirk...but is also a new tire quirk. Did you just have winter wheels put on?

BTW - what pressure readings were you getting when they were 'low'.?

RJ

 

On the 09's at least, the TPMS takes into account the temperature. The info-pressure readings tell you how high or low each tire is when temperature is considered. In other words, the computer expects the gage pressure of a tire to be below the gage pressure specified for a tire at working temperature. Conversely, when the tires have been worked hard and are hot, the computer compensates again. It expects the gage pressure it reads to be above the specified gage pressure.

The value of this is obvious. First, when setting pressures on a cold morning you don't have to do any arithmetic yourself to compensate . Just ask the computer whether pressure changes are needed. It will specify add one pound to left front, subtract one pound right rear, or whatever, and Bob's your uncle. In the same way, if you suspect a slow leak, you can stop and check the computer. The gage pressure on a right rear (with both at high temps on a desert track like Willow, say) might be 43 pounds. Compensating for the high temperatures, the correct pressure might be 45 pounds. The computer applies the temp/pressure equation and will tell you "RR=+2" or however they phrase it.

Gary

 

thanks, yrlis. Do you have the TPMS option, too?

I understand filling cold tires......but when I read with the hand held gauge, I'm getting the normal PSI (36). My husband checked on Friday, and confirmed it.

 

I have two Porsches :1) an 09 C2S and 2) An 07 Turbo.
Apparently forum member simsgw raised a point that I was not aware of with the 09 cars . My 07 car is more quirky when it comes to the monitoring system .

One thing I have noticed is that the hand held gauge and TPMS aren't always identical but 36 Psi on a cold front tire ought to be fine.

Even though its fine when you make it to the dealership I can understand how it would be annoying . I know it's an inconvenience to leave it there overnight and have a technician see the glitch but like yoirself I also woild want it to read correctly .

What I recently had done with my Turbo --I had them add nitrogen to the tires. I did this last week so I hope to see if it retains the tire pressure better .

Good luck.

 

Took me awhile to figure it out myself. The new owners manual explains it, but not well. Had to experiment to confirm that interpretation was correct.

I agree that 36 psi on a cold morning should be just fine. Is the TPMS telling you your tire pressure is too low, Amy Lynn? Or are you just going by the little tire symbol? I remember seeing some other causes for that warning to go on besides low pressure, but I can't remember what they are offhand. Try getting into the "info pressure" menu item one morning and see what it says.

Gary

 

I have my tires inflated to 33 psi front and 39 psi rear (partial load) but my TPMS always read about 42 front and 40 rear even though I check and recheck it in the morning before driving with a Porsche tire gauge. I mean, the mechanics at my dealer say I am one of the few that always has his tire pressures on the mark. I guess TPMS in an '06 is not reliable unless I can find a way to make it accurate by some calibration procedure etc. Any pointers???

 

TPMS on the screen has been giving me really low numbers.. I've had 20,12,3, 5.... The dashlight warning goes on periodically as well. I'll need to check it again.

The service department child advised me to bring it in and leave it overnight, so they could look at a cold start and check my stems.

 

I have several digital gauges and they all measure within +-0.5PSI. My car's (2010) TPMS tends to read 1PSI lower than the external gauges - when I set front at 34PSI the TPMS reads 33PSI which is fine.

Note: I noted that the cold temps mentioned above are for full load. Light load (2 passengers no luggage) cold temp pressure for 19" rims are:

997.1 - 33F 39R

997.2 - 34F 40R


.

 

It should not be quirky. Also, you cannot go by appearance with low profile tires.

Try setting them as described in the manual, starting on page 146. Do not use the Top Level TPM display (the one with pressures like 36, etc - the one the manual says do not use under any circumstances for setting pressure) , but rather go into the function menu where the +/- offsets are displayed and temperature is taken into account. Page 146 or so in the Owner's Manual for more info.

The reason I suggest using the TPM rather than a gauge is that it should work and if it doesn't, it would be good to know that there's a problem.

I use both the TPM and gauge and it all is consistent and in agreement.

 

Tire pressures should be set cold (first thing in the AM) with an accurate external gauge, not based on TPMS readings. Here is what the manual says (pg.146):

"This display is for informational purposes only. Under no circumstances should the tire pressures be changed based on this display."

I wold not trust the delta +- display for setting pressure either. The proper way is an external gauge.

I prefer the actual PSI display which gives me the actual pressure when the tires heat up. My tires start cold at 34/40 and in normal highway driving move up to 37/42. I like to see that as it gives me an indirect measure of tire temps.

 

Yep --I have read it too . It's too bad that they want the owner to ignore the reading in one sense but consider it valid in another . In my opinion it sends a mixed message but even worse the car itself relies on it and does not care what an external gauge would say . So what happens is a situation like the original poster who gets a warning light and ends up more confused than if she didn't have the TPMS in the first place .

 

I think that the TPMS numbers a re a rough measurement. They should be within 1 or 2 PSI of reality. Mine certainly are. Please note that 997.2 TPMS readings are more accurate than 997.1 TPMS readings.

 

I find the TPMS +/- reading to be accurate within <1lb. The reason there are two type readouts, as I read it, is that one is temperature compensated and correlated to the tire size/type and load settings also - and can only be used when the car is not moving - used for setting the tire pressure. The other is for in motion measurement of uncompensated pressure only. I need to double check that but I am pretty sure those in the +/- display don't change as the tires warm up, but the other Top-level display does.

Unless you are temperature compensating your guage (cold tire temperature is much lower now than it was in July), it may not always agree with the TPMS+/-, or so it would seem.