I bought my 2004 MCS in July. A few weeks into owning this fine piece of machinery my tire pressure light came on indicating low tire pressure. I put air in the tire and motored on. A week later the light came on again but this time it was a different tire that had low pressure. This went on for several weeks before I finally took it to a nearby tire shop. They claimed there was a screw in one tire, patched it and sent me on my way. Low and behold a week later the light came on again and has done so every 50 miles; always a different tire.

I am not a mechanic nor do I know what the different between a ball and a bearing is however this past weekend I did a little experiment. I took the valve cap from the low tire and put it on a different tire to see if perhaps this was the cause. Guess what, yesterday the light came on again and it was the tire I had put the valve cap on. This leads me to believe that the nifty little "low tire pressure" valve caps are actually allowing air to leak out. Has anyone experienced this?

 

when you swapped the wheels, did you pressure them up to 34-35psi before you put the cap on? I know the sensors track the difference in rotation speeds of the abs sensor, but the wheels you swapped may have already had different tire pressures on each corner.

How tight are you torquing the caps? if you torque it too tight it could loosen the valve stem nut inside the wheel when you unscrew the cap.
The cap should only be tightened a little bit...meaning when you start
feeling the friction of the cap to the stem, turn it may be another
1/2 turn until it's secure, but don't crank it.

 

The valve cap should have nothing to do with retaining air pressure in the tire. You can run without valve caps with no loss in tire pressure. Check to see if the cap is screwing down so far that the cap is hitting the stem of the Schrader Valve (the little button in the center of the stem). This may allow air to leak out.

I have heard of people pulling a prank by placing a small pebble inside of a valve cap so that it pushes down on the valve stem when you tighen the valve cap.

 

I think he's talking about something like this




I wouldn't use them - just check your pressure monthly.

 

I assume you have some of the aftermarket valve caps that keep tabs on your tire pressure, and not a "normal" metal or plastic cap, right?

If so, the problem is very likely to be a faulty aftermarket cap. These caps depress the valve in the valve stem, in order to measure the air pressure in the tire. If they have a bad seal, your tire will go flat.

My opinion: Ditch them, and use a normal cap. Rely on the car's onboard tire pressure detection system, as well as occasionally checking them with a tire gauge (monthly is good.)

If you bought them, take them back to the vendor and get your money back....

...david

 

Good one! Maybe xcaligali didn't give her salesperson "all fives" on the customer satisfaction survey, and this is their revenge.

 

My wife's got sparkly aftermarket caps on her MCC. One tire kept losing air over an hour or so. No puncture was visible, and the tires have about 900 miles on 'em. I put her spare on (and was able to show her the value of owning a torque wrench). After having the tire tested, to no avail, I put it back on and eventually heard a hiss & realized the rubber O-ring inside the valve cap was unseated and bunched up. It pressed against the valve core when the cover was tightened, but stayed in the cap when the cap was taken off. Thus the slow leak with no visible problem!!

Ken

 

Tubless valve stems such as those used on Mini's dont have "nuts" inside the wheel. They're just a rubber plug, pulled through a hole from the inside to out.

fwiw.

 

I've had more than one car where overtightening a valve stem cap led to a slow air leak. On occasion, I've even been able to hear it. It always seems to be a single cap out of a set of four, so that moving the cap from one wheel to another also moves the leak.

 

nice, i didn't know that cause i only use aftermarket wheels on my cars.