Mysterious leaky tire

The rear tire of our 1995 Honda Civic has a leak. In about a week, the pressure goes from 30 psi to just about nothing. But I can't find the leak.
The Honda dealer checked all 4 tires and declared that none leak. But still, after a week, the rear tire is flat.
Today I checked it myself with soapy water. No bubbles. I removed the wheel from the car and now it is in our bathtub upstairs and there are STILL no bubbles.
How can this be? Any ideas?
R.
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Well, Elle wants me to always mention Tegger's useful website, so here it is
http://www.tegger.com/hondafaq/faq.html
I could not find anything there about leaky tires, so drawing on my own experience I would suggest two possibilities:
1. The tire bead is not properly seated. Off the car, the tire does not leak, but on the car, as the tire flexes in normal driving, air escapes. Causes could be a bent or damaged rim, or just a bad tire, or dirt under the bead, etc. Dismounting the tire and examining the rim would reveal any damage.
2. The rim itself could be leaking. You say you have the tire in the bathtub, but is the rim submerged? Modern alloy rims cast as one piece rarely leak, but steel rims sometimes do. I once long ago had a steel rim that actially had a small pinhole (manufacturing defect, I guess). Silver solder fixed that rim.
3. The valve stem could be leaking, around the base or through the valve itself. If you have not already checked to see if the stem is leaking (submerged?) this would be my recommendation as the first place to look. But you have probably already checked it.
Hope this helps
Note to Elle: The signature lines that appear below are not a title. I earn a substantial part of my incomeg as a freelance writer. It is what I do. I am not sure why it offends you, but since it does I would suggest you place me in your permanent killfile.
Elliot Richmond Freelance Science Writer and Editor
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alloy rims can crack and leak,or become porous.
--
Jim Yanik
jyanik
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It may be a poor seal. I once had a rim that was very slightly bent as a result of running over something in the road. You could not tell that the rim was bent. However, the tire kept losing air--slow leak. There was nothing wrong with the tire. They were still making use of intertubes at that time so I told them to use an intertube inside the tire. It solved the problem. I don't believe that they still make or sell intertube. In some cases, the tire is the problem and in other cases--the rim is the problem. If it is a poor seal between the rim and tire--perhaps you could try making use of a can of solution that might be called "stop leak" or something like that. They probably sell it at tire shops and auto shops like NAPA. Perhaps someone that drives the car ran over something in the road and did not tell you about it. Jason
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Either the rim-bead seal is leaking, or you have small hole in the tread or more likely the wall. As the tire flexes, the hole opens up and lets out pressure. Just standing still (unless in just thr right place to open the hole) won't leak. Happened to me last year.
Have the tire remounted and inspected inside and out.
Stewart DIBBS
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A lot of good suggestions here.
Another possibility is a cracked valve stem. Centrifugal and inertial forces flex the stem all over the place at road speeds. I'd try bubble checking again while flexing the stem around with your fingers.
It's not all that common, but I've seen a couple of these through the years. If I'm right, it's an easy fix for your local tire shop. You really should have them re-balance it too. Insist they install a 'long' stem if hubcap/air chuck interference is an issue. It's also a good idea to always get new stems whenever you buy tires...
Good Luck, let us know how it turns out.
Erik
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