Seal on alloy rims likely is causing air leak

Q I own a 1999 Mazda Protegé that has alloy wheels. The car has Cooper tires that were installed in February 2004 and still have some tread life remaining.

This winter, air has leaked from the two front tires. I check the tires monthly, and they have been down about 10 to 15 pounds per square inch (psi) from their normal pressure of 32 psi.

I recently had the tires removed from the rims, the rims power-cleaned and the valve stems replaced. The shop told me that alloy wheels tend to cause air leaks. I've now noticed that the right passenger tire has lost air, while the other three tires are OK. I bought this car new, and have not had this problem in the past. Is air leakage a common problem with alloy wheels?

A I think the best way to answer this is to say that air leaks are not uncommon from alloy wheels. In some cases, it's actual porosity in the alloy casting, but in most cases it's caused by corrosion at the inside edge of the wheel rim where the bead of the tire is seated.

In your case, if the issue were porosity, the problem likely would have been there from the first day. The issue of corrosion at the bead tends to occur on the second, third or fourth set of tires mounted on the vehicle. As the corrosion builds up, it's more difficult for the bead of the next tire mounted on that rim to seal fully.

Thoroughly cleaning the buildup of corrosion from the bead area on the rim is an important step whenever a tire is dismounted or mounted on an alloy rim. In addition, tire shops can apply a special sealer to the rim area to help seal the tire bead and prevent future air leaks.

Q I have a new Acura RL with 500 miles on it. We use it every two or three weeks. The last two times we went to drive it, the battery was dead.

A Actually, there is much truth in their explanation -- although I don't think the battery should be dead in just two weeks. Modern automobiles draw a measurable amount of current from the battery even when everything is turned off.

The computer systems, along with the radio, clock and security system, can draw several tenths of an ampere. Over a long enough time, that would run the battery down to the point that the car would not start. But again, I would think this would take more than two weeks, particularly with a new vehicle and a new battery.

The dealer should perform a "parasitic current draw" test, which will measure how much amperage is being drawn from the battery with the car completely shut off. If it's more than a few tenths of an ampere, I think the dealer needs to dig into the issue further.

Q I have a 2002 Ford F-150 Supercrew with the 4.6-liter engine, automatic transmission and 3.55 limited-slip rear end. It has 122,000 miles.

Recently I brought it to a local dealer to fix a slow drip from the rear differential cover. The dealer fixed the leak and filled it with 75W-140 full synthetic lube but didn't add any friction modifier. I asked the service adviser about not adding the modifier when I picked up the truck. He said none was needed. My owner's manual says to add 4 ounces of modifier with a fluid change.

It doesn't seem any better. Did I do any permanent damage to the limited-slip rear end?

A My Alldata automotive database confirms that Ford calls for both the fully synthetic 75W-140 gear oil and the special limited-slip additive. Typically, the low-speed chatter you hear when turning is caused by wear in the limited slip clutch pack -- perhaps a function of the 122,000 miles on your truck. My old Corvette, which also has a limited slip, has developed much the same characteristic, even with synthetic gear lube and a limited-slip additive.

Here's my suggestion. Make sure the dealership notes and acknowledges an awareness of the issue, but keep driving the truck. See if a few thousand miles of driving allows the additive to do its thing in the limited slip and help the clutch pack slip smoothly rather than chatter.