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    Question on TPMS and Valve Stem Caps

    This sounds crazy, but I thought I'd ask Rennteamers if they had an answer.

    Several weeks ago, I lost (or someone stole) one of the valve stem caps on my 911S, which are just some cheap plastic. Not wanting to run without one, I bought a set of cheap aftermarket metal ones and put them on. Originally, I seemed to be getting reasonably accurate readings on the TPMS monitor.

    I just checked the Porsche Tequipment site, and they offer a nice-looking set of metal ones with the Porsche crest on the top, but the description said that the metal caps were not to be used with cars equipped with the TPMS.

    Does this mean that metal caps will interfere with the TPMS transmitting accurate tire pressure readings?

    On another posting, TPMS readings were discussed, and I thought I would have to get my system recalibrated to get accurate readings. Currently, I'm generally getting readings that are 3 lbs. per square inch lower than the level to which I have inflated the tires.

    Are the metal caps the reason I'm getting low readings?

    Jim

    Re: Question on TPMS and Valve Stem Caps

    Just a thought . Why don't you take the caps off and see if the readings change ?

    Re: Question on TPMS and Valve Stem Caps

    I doubt it. Most TPMS readings seem to be off; mine are. My caps (stock) are grey plastic. How'd you get the metal ones?

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    MMD said:
    I doubt it. Most TPMS readings seem to be off; mine are. My caps (stock) are grey plastic. How'd you get the metal ones?



    I sort of thought I was on a wild goose chase regarding this, but checking out the Tequipment site started me off on this tangent. I thought that the metal caps, for whatever reason, might have interfered with or altered the signal that is sent from each tire to the TPMS. Elsewise, why would the Tequipment site say that the good-looking metal caps they offer should not be used on cars with the TPMS? I would think that the valve stems are all the same size.

    Regarding the metal caps, I bought them at an auto supply store (Advance Auto Parts). I can't seem to find any like the grey plastic ones on the Porsche, which, by the way are actually made in Germany (you can see "Germany" stamped on the top of the cap), compared to the caps at the auto store, which are all made in China, even the metal ones.

    I guess it's off to the dealer to get the TPMS properly calibrated.

    Jim


    Re: Question on TPMS and Valve Stem Caps

    Quote:
    Jim48 said:
    Quote:
    MMD said:
    I doubt it. Most TPMS readings seem to be off; mine are. My caps (stock) are grey plastic. How'd you get the metal ones?



    I sort of thought I was on a wild goose chase regarding this, but checking out the Tequipment site started me off on this tangent. I thought that the metal caps, for whatever reason, might have interfered with or altered the signal that is sent from each tire to the TPMS. Elsewise, why would the Tequipment site say that the good-looking metal caps they offer should not be used on cars with the TPMS? I would think that the valve stems are all the same size.

    Regarding the metal caps, I bought them at an auto supply store (Advance Auto Parts). I can't seem to find any like the grey plastic ones on the Porsche, which, by the way are actually made in Germany (you can see "Germany" stamped on the top of the cap), compared to the caps at the auto store, which are all made in China, even the metal ones.

    I guess it's off to the dealer to get the TPMS properly calibrated.

    Jim





    Why don't you just take jcs's advice?

    Read tire pressures shown on display when tires are cold anyway, switch off ignition, remove all valve cups, get yourself a coffee, and check display readings again a half hour later to see if they are the same.

    It'll cost you the price of a cup of coffee, which you might need anyway.

    If pressures are the same, metal caps made no difference; if pressures differ by more than 0.1 bar or 1 - 2 psi, then opposite conclusion could be valid.

    I would guess that metal caps would not cause lower pressure readings to be shown, as you fear.

    I'm no expert, but I suspect that metal caps are not recommended because they could shield the radio link between sensor antenna and receiver antenna, so that in unfavorable conditions you would get no pressure being displayed at all. In other words, interfering with the signal is not the same as altering the value of the signal.

    I sometimes get individual wheels showing pressure as "-.-" briefly after a cold start, and I think a metal cap might cause this to happen more frequently.

    You don't say what you are "calibrating " the readings against, but your comparison gauge would also have tolerances.

    Keep us informed.

    Re: Question on TPMS and Valve Stem Caps

    Owner's Manual (page 232) says only use plastic caps - there must be a reason for that statement??

    Re: Question on TPMS and Valve Stem Caps

    So does anyone like having TPMS. I ask because I just oderd mine with and I wonder if I shouldnt have???

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    rhino said:
    So does anyone like having TPMS. I ask because I just oderd mine with and I wonder if I shouldnt have???



    ABBBBBBBBBBBBBBSOLUTELY a good idea (worth every penny). Can save us$1500 worth of tire/wheel. Safer too.

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    jhbrennan said:
    Owner's Manual (page 232) says only use plastic caps - there must be a reason for that statement??


    It was not my intention to suggest that there was no reason for the statement. My suggestion was that metal caps might interfere with the radio signal to the extent that it would not be received at all, rather than to cause the display to show a lower reading than the wheel sensor measures.

    My non-expert take on this is that digital radio signals either get through or they do not get through, but they do not get "degraded" like analog signals.

    So, in my view it would be wise to follow maker's recommendation and use plastic valve caps, but the "low" display readings complained of above are probably not caused by use of metal valve caps.

    I may be wrong, but I wouldn't be surprised if I'm right.

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    Jim48 said:
    Elsewise, why would the Tequipment site say that the good-looking metal caps they offer should not be used on cars with the TPMS?



    Thanks for the info/clue. That's probably the reason.

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    rhino said:
    So does anyone like having TPMS. I ask because I just oderd mine with and I wonder if I shouldnt have???



    I definitely like having TPMS, and would now not voluntarily do without it.

    Re: Question on TPMS and Valve Stem Caps

    Great news!!!

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    fritz said:
    Quote:
    Jim48 said:
    Quote:
    MMD said:
    I doubt it. Most TPMS readings seem to be off; mine are. My caps (stock) are grey plastic. How'd you get the metal ones?



    I sort of thought I was on a wild goose chase regarding this, but checking out the Tequipment site started me off on this tangent. I thought that the metal caps, for whatever reason, might have interfered with or altered the signal that is sent from each tire to the TPMS. Elsewise, why would the Tequipment site say that the good-looking metal caps they offer should not be used on cars with the TPMS? I would think that the valve stems are all the same size.

    Regarding the metal caps, I bought them at an auto supply store (Advance Auto Parts). I can't seem to find any like the grey plastic ones on the Porsche, which, by the way are actually made in Germany (you can see "Germany" stamped on the top of the cap), compared to the caps at the auto store, which are all made in China, even the metal ones.

    I guess it's off to the dealer to get the TPMS properly calibrated.

    Jim





    Why don't you just take jcs's advice?

    Read tire pressures shown on display when tires are cold anyway, switch off ignition, remove all valve cups, get yourself a coffee, and check display readings again a half hour later to see if they are the same.

    It'll cost you the price of a cup of coffee, which you might need anyway.

    If pressures are the same, metal caps made no difference; if pressures differ by more than 0.1 bar or 1 - 2 psi, then opposite conclusion could be valid.

    I would guess that metal caps would not cause lower pressure readings to be shown, as you fear.

    I'm no expert, but I suspect that metal caps are not recommended because they could shield the radio link between sensor antenna and receiver antenna, so that in unfavorable conditions you would get no pressure being displayed at all. In other words, interfering with the signal is not the same as altering the value of the signal.

    I sometimes get individual wheels showing pressure as "-.-" briefly after a cold start, and I think a metal cap might cause this to happen more frequently.

    You don't say what you are "calibrating " the readings against, but your comparison gauge would also have tolerances.

    Keep us informed.



    jcs and Fritx,

    Good advice. I'll check it out now and remove the plugs to see what happens.

    But I did buy a set of cheap (under $1 US) plastic caps (black ones) and took the metal ones off. I checked the pressure, turning on the ignition switch but not starting the car, and the numbers came up the same as when I had the metal caps on, 31 lbs. in front and 36 lbs. in the rear (I set the pressure using a reasonably accurate digital electronic gauge at 34 lbs. front and 39 lbs. rear - the gauge supposedly is accurate to 0.1 lb). So the TPMS readings understate my pressure, when the tires are cold, by about 3 lbs. After driving for 30-40 minutes, the pressure readings on the TPMS usually rise to 34 lbs. in front and 39 - 40 lbs. in the rear.

    Fritz, I think you're correct about the metal caps possibly blocking, but not altering, the signal, as there are times when I'm not getting a reading on the tires until 5 minutes or more after starting to drive, and that is with the metal caps.

    Regarding calibration, I'd have to get a matchup of actual measured pressure with the TPMS readings, doing it while the tires are "cold." As I have no way of matching the TPMS readings to the actual readings, I'll have to get my dealer's service shop to do it.

    I took the plastic caps off the tires about ten minutes ago and will check them after 30 minutes has elapsed. I'll report my results in about 30 minutes.

    Jim

    Re: Question on TPMS and Valve Stem Caps

    I took off the caps, let them off for 30 minutes, and then turned on the ignition to check the readings. They were the same as with the metal caps and also the same as with the plastic caps on (not the original Porsche caps, but plastic is plastic).

    I think fritz was correct about the metal caps interfering with the actual signal and not affecting the pressure reading.

    Jim

    Re: Question on TPMS and Valve Stem Caps

    Jim, you mention that "the gauge supposedly is accurate to 0.1 lb" That is not really it's accuracy, only its resolution. Accuracy in generaly stated in percent i.e. 1% accuracy. Just wanted you to be aware of the difference. You can be very 'precise' but 'not accurate'.

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    Jim48 said:
    I took off the caps, let them off for 30 minutes, and then turned on the ignition to check the readings. They were the same as with the metal caps and also the same as with the plastic caps on (not the original Porsche caps, but plastic is plastic).

    I think fritz was correct about the metal caps interfering with the actual signal and not affecting the pressure reading.

    Jim



    So how was the coffee?

    Naturally pleased to hear that my intuition was right. So your initial concern was caused by assuming that your hand-held digital pressure gauge was correct and jumping to the conclusion that your TPMS display had to be wrong!

    You should have more faith in German engineering.

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    fritz said:
    Quote:
    rhino said:
    So does anyone like having TPMS. I ask because I just oderd mine with and I wonder if I shouldnt have???



    I definitely like having TPMS, and would now not voluntarily do without it.



    Same here, a must IMO. And must say that my TMPS readings have always been sport on when double checked with gauges, always.

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    1st997 said:
    Jim, you mention that "the gauge supposedly is accurate to 0.1 lb" That is not really it's accuracy, only its resolution. Accuracy in generaly stated in percent i.e. 1% accuracy. Just wanted you to be aware of the difference. You can be very 'precise' but 'not accurate'.



    Good point. Thanks for the explanation of the difference between the resolution of the gauge and its actual accuracy.

    Jim

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    fritz said:
    Quote:
    Jim48 said:
    I took off the caps, let them off for 30 minutes, and then turned on the ignition to check the readings. They were the same as with the metal caps and also the same as with the plastic caps on (not the original Porsche caps, but plastic is plastic).

    I think fritz was correct about the metal caps interfering with the actual signal and not affecting the pressure reading.

    Jim



    So how was the coffee?

    Naturally pleased to hear that my intuition was right. So your initial concern was caused by assuming that your hand-held digital pressure gauge was correct and jumping to the conclusion that your TPMS display had to be wrong!

    You should have more faith in German engineering.



    fritz,

    I actually had some scotch whisky (late night in the U.S. when I was doing my posts), but I wasn't drunk.

    What does worry me is that I may be running the tires at too low a pressure if the TPMS is giving accurate readings. It means that the various pressure gauges I have been using, both the mechanical type (with a needle on a gauge) and the electronic type (a digital readout on a gauge), give readings that exceed the TPMS reading, usually by about 3 lbs. per square inch. I guess I had better inflate my tires to assure that I don't damage a wheel or ruin a tire or both!!

    Jim

    Re: Question on TPMS and Valve Stem Caps

    I found that pressure gauges themselves vary by a pound or two either way and of course temperature/load affects the tyre temperature and thus pressure. I'd be surprised if 3 psi is going to damage anything?

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    fritz said:
    So your initial concern was caused by assuming that your hand-held digital pressure gauge was correct and jumping to the conclusion that your TPMS display had to be wrong!

    You should have more faith in German engineering.



    I got two different digital guages (nice ones, not junk) they both agree with eachother. They indicate the TPMS is wrong by about 2-3 lbs. I need ALOT of faith to conclude the guages are wrong and the TPMS is correct. Will test again and report back... .

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    fritz said:
    Quote:
    Jim48 said:
    I took off the caps, let them off for 30 minutes, and then turned on the ignition to check the readings. They were the same as with the metal caps and also the same as with the plastic caps on (not the original Porsche caps, but plastic is plastic).

    I think fritz was correct about the metal caps interfering with the actual signal and not affecting the pressure reading.

    Jim



    So how was the coffee?

    Naturally pleased to hear that my intuition was right. So your initial concern was caused by assuming that your hand-held digital pressure gauge was correct and jumping to the conclusion that your TPMS display had to be wrong!

    You should have more faith in German engineering.



    fritz,

    You're right about having more faith in German engineering.

    I just re-read the owners manual description of the TPMS and realize that I had not read it carefully enough. For that, I apologize to the group.

    From what I now understand, you should fill the tires to the proper level when cold (using your regular air gauge to monitor the pressure), and then get into your car, turn on the ignition without starting it, and use the computer control lever to enter the TPC function. Enter the "info pressure" section and this will tell you whether your air pressure is above or below the level recommended for your car, given the settings you have made (for 18 or 19 inch wheels, for summer or winter tires, for partial or full load).

    The pressure display that can be accessed on the computer display below the digital speed readout, below the tachometer, is useful while actually driving. It will tell you the tire pressure as the tires warm up, but it is not to be used for setting pressure (page 110, U.S. version of the owners manual).

    Now I just completed this process, and when I insert the key and turn on the ignition, after getting the oil level reading, my tire pressure reading says 30 lbs. for the front and 36 lbs. for the rear. That is 3 lbs. less than the recommended level to which I filled the tires. But when I turn on the computer TPC section, for the accurate reading when the tires are cold, I get "0" readings all around, meaning that my pressure setting is accurate.

    It all sounds a bit confusing, particularly why the regular reading, that you can get without accessing the TPC info pressure section, wouldn't seem to be accurate, but I will take a guess. The basic info measure you can easily access only gives accurate readings when you are actually moving.

    Anyway, I think this explains it, but I would like to get MMD's take on things, as he and I seem to have been having the same concerns with the monitoring system.

    By the way, I would not go without the TPMS. The option price was only $590 in the U.S., and this is a very inexpensive way to prevent damage to the wheels and possibly tires.

    Jim

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    Jim48 said:
    ...
    I just checked the Porsche Tequipment site, and they offer a nice-looking set of metal ones with the Porsche crest on the top, but the description said that the metal caps were not to be used with cars equipped with the TPMS.
    ...



    The simple reason is that the Tequipment metal caps with long sleeve won't fit
    I have the metal caps (actually, they seem to be aluminium or silver colored plastic) on my 993 and the TPMS on my 997S. The diameter of the TPMS valve stems is much greater than the diameter of the non-TPMS valve stems and they have no conic shape. So, the Tequipment metal caps that are made for the non-TPMS valve stems just won't fit.

    I have seen pictures of different metal caps in this forum that could be used for the TPMS, too. These caps are without sleeve and are real metal with colored crest. Unfortunately, IIRC, according to other Rennteamers that have them these caps seem to bend the non-TPMS valve stem and scratch the rim at higher speeds. Maybe this is the reason why these are not available through Tequipment. Hmm, I also don't know whether these caps are original Porsche parts or aftermarket.

    Re: Question on TPMS and Valve Stem Caps

    Quote:
    Jim48 said:
    I think this explains it, but I would like to get MMD's take on things, as he and I seem to have been having the same concerns with the monitoring system.



    Missed my chance to get to the car this weekend. Will investigate when I do and report back.

     
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