Tires: Tire leak only when it is cold, but the repair shop cant find it, alloy rims, slow leaks

Question
Hi:

I bought a one year old Jeep Liberty about a year ago.  Right away the tires started getting flat over night, but it appeared to be random when it happened.  Some nights it would happen and other nights it didn't.  The dealership checked for leaks and everything they could.  We would leave my car on their lot for a week at a time to prove that the leak existed and wasn't some kids in my neighborhood letting the air out.  They checked for everything, finally replaced two tires but said that's all they could do.  I was ok until winter.  It is clearly cold related.  If I leave my car in the garage it doesn't happen.  Outside on a cold night or even day, one of my tires goes very flat and another goes flat but more slowly.  It never happens in warm weather, and can stay inflated for months other than in winter.  A freeze warning guarantees a flat.  (We don't really get ice or snow where I live.)  Replacing and rotating the tires doesn't seem to help.  I had an independent repair shop check my tires, keep it for several days, etc.  They took all the tires off and checked them one at a time, and couldn't find the leak either.  I have gotten used to traveling with an electric air pump. What might it be and what can I do?

Answer
Penn,

I suspect your rims are leaking - thorough the metal.  The way to tell is to submerge the entire wheel and tire in some water - a kiddie pool works well for this, but you have to add weight to keep the tire from floating.

If the rims are leaking, you will see little bubbles forming on the rim itself - BTW, this would be true of alloy rims, not steel ones.

I'm guessing the reason the shops couldn't find the leak is that they were looking for a fairly fast leak - and that would leave a trail of bubbles when immersed.  Slow leaks through the wheel itself are large area leaks and they don't form bubbles in the same way.

Fix?  I have heard coating the rims works, but don't have any experience with that.  This would mean the tires would have to be removed from the rims, the rims cleaned, followed by several layers of clear coat.  The vehicle would have to be idle for probably a week to allow the coating to dry thoroughly.

Sorry I can't be of more help.