Been testing out some metal valve stem caps (with little rubber gaskets inside) on my front tires only. After the first month the fronts have held steady whilst (always wanted to say that) the rears have lost 3 lbs each.
It sounds like it may be that the valve inside the valve stem is
not installed tight enough on your tires. Have you tried tightening
the valves? High tire pressure should keep the valves even more
tightly close than low pressure would, so I don't really see a
reason for them to be leaking.
If the valve itself is leaking, you can put a little oil on the valve,
but this has to be done under pressure. An in-line pneumatic
oiler (for air tools) works nicely for this.
Anyway, it sounds like your problem is solved, but I doubt it was
the only solution.
Most Prius tires (in the USA) tend to leak down about a pound a week so they have to be checked often. A while back people using "Tire Minders" were noticing that their tires weren't losing pressure like before. Like you we started to suspect the valves. Pretty weird that all 4 tires leak at the same rate though. Someone suggested using metal valve caps to see if there was a change. I was able to dig up some old metal ones in my tool cabinet that had rubber seals. Tried them on two tires, and they seem to do the trick. One more data point for the Great Prius Overmind in cyberspace.
Pearl's Potenzas are set at 40/38 and haven't leaked down more than half pound a month. Not even enough to get out the tire pump. We're using the standard tire valve stem cover.
We're just coming up on 7,000 miles since delivery 10/15/01. Tires were rotated front-to-back at 3,500 miles (per 2002 manual). Tire wear seems okay; will check again myself before 7,500 mile service and compare my measurements to those of the dealer's service department.
I keep my tires at 42/38 now - ran 42/40 for quite some time before
experimenting and finding a bit less "wander" on the freeway with
the lower back pressure.
I put 42 in the front the day after I bought my car (Oct 31.), and
when checked last week, they've lost 1 lb each, which could be
entirely due to temperature change (or they could have lost more
and it's warmer now). In any event, not fast enough loss to worry
about checking every single week.
I guess I'm just lucky My MPG gauge also reads right on (for
each 1500 mile check I've made) as opposed to being biased
high, so my car experience is not the same as everyones.
I also have a 2002 and have not noticed a significant change in tire pressure, when I first picked up my car in october I pumped my tires up to 42/40 for the first three weeks they dropped half a pound a week, and I pumped them back up, after that I have noticed no drop in pressure
On the Prius forum in Yahoo, the following web site was mentioned with info about tire care:
The info mentions that you should always use metal stem caps without saying why plastic ones are "no good". Also, it is mentioned that tires expand a little until they get 2,000 miles on them. This expansion translates to a need to add more air when tires are new and after 2,000 miles the preceived air loss diminishes.
I believe that difference between plastic and metal caps is this;
plastic is too hard to seal well. The seal has to be at the top of the stem because unless you use teflon tape on the threads neither plastic nor metal caps will seal. The metal caps that I put on have rubber rings inside the cap that seal the top of the stem to the top of the cap. Like the ring of rubber paint on the inside of a pickle jar top that seals the lid to the top of the jar. IMHO me thinks that plastic caps are put on tire valve stems just to keep crud out of the valves.
I have had my 2002 Prius since December 14. I keep it in the garage which is constantly between 45 and 50 degrees F. I have not had to add any air during 2 1/2 months I have had the Car. They are all four at 38 PSI (COLD) If I drive 10 or 20 miles at 60 in 40 degree weather, the front tires will build up to about 40 for a gain of 2 pounds, and the rear will build up only about 1/2 psi to 38 1/2. I would suppose this is because there is a lot more friction wth the front tires due to the front wheel drive plus the extra re-gen braking. I am in S.E. PA, and temp has been generaly mild except the one week around X-mas. I have averaged just about 44 MPG on the 2600 miles to date. Have not had the car out during any temp below 32'F. On the one day (Jan 27) when it got up to almost 65 degrees I drove 155 miles round trip to Gettysburg PA, which included about 1 1/2 hours around the battle-field (stop & Go) under 25 MPH. Got home and gas gauge still on top Line. Showed about 65 MPG on the monitor but kept the speed mostly under 50 mph and tried to stay under the magic 42 as much as possible! Please keep in mind that the outside temp and the speed at which you drive will have a significant effect on the tire pressure.
About crud and plastic caps: I think that is exactly their purpose. The schraeder (sp?) valve in the stem is what is intended to hold the pressure. Apparently in some Potenzas it doesn't. Or maybe for those of us who have the pressure up at 40 or 50 it is more noticable.
In any case, Lux (MY 2002) arrived on Oct 31, and for 2 months I noticed the pressure leaking fairly rapidly. (A pound or two per week would not be wildly out of line, but I did not explicitly record it). After putting on Tire Minders in late December, there has been very little leaking. In fact, my tires are right at 40 psi where I put them in mid January. (Of course the temp is probably a bit warmer, so they may have lost a bit with the temp compensating).
Just a note about caps - The valve stem internal spring loaded valve is the device that holds in the pressure. The cap was never intended to hold pressure, but in some cases where the spring loaded valve is bad, may hold some pressure. The caps were intended to keep out dirt from the actual valve.
I have found over many years and many cars, that the most common cause of slow leakage is a poor tire bead to rim seal. So it is logical to think that when you remount a tire and the leakage is reduced, the tire mounter person did a better job of mounting the tire than the previous mounter did. This may mean using lubricant during the mount and/or cleaning the bead area if on an older rim.
So generally - slow leakage = poor rim seal - assuming no tire defects or punctures. The base of the valve stem also needs cleaning and lubricant to make a good seal and may leak also.
just a note from an old tire installer (I was a mechanic for quite a few years a long time ago)..
'02 super white - 10400 miles - no leakage..
Steve I whole heartedly agree about poor rim seal, but have only experienced it with older cars that I've had. It seems to me that quite a few of us that bought MY2001s have poor sealing valves. I don't recall any posts from owners of MY2002s regarding tires losing pressure. Need to ask my local Bridgestone/Firestone dealer about replacing my valves. Anyone know if the guts can be replaced rather than the whole stem?