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Wednesday, April 23, 2014

Doug's Domain

Doug Vetter, ATP/CFI

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Monday, December 18, 2006

CSL Replica Valve Stem Issue

Last Saturday I was finishing my weekly car washing ritual and cleaning tons of fresh brake dust off the new CSL wheels for the first time when I discovered a potentially serious problem. As I pushed the sponge into the crevice between the valve stem and the wheel, I heard a hissing sound. I pulled the sponge away and the hissing stopped. I then grabbed the valve stem and rocked it back and forth. Hiss. Hiss. Hiss.

A quick pressure check found not one, but BOTH, rear tires at around 14 PSI, while the fronts were near 33 lbs as I would expect. I couldn't believe my eyes, so I checked it again. Same results. 14 PSI is way too low for a low profile tire. And in fact it's low enough to permit the tire to dismount from the wheel under load. I looked more closely at the sidewall of the rear tires and found they bulged outward a very slight amount as compared to the front tires, but not enough to communicate the severity of the problem. That's when I realized low-profile tires do not exhibit the signs of underinflation the way the 16" or smaller tires do.

The valves were obviously the cause of the lower pressure, but I couldn't immediately explain the disparity between the front and rear tire pressures. If this was a simple matter of centrifugal force, as a function of speed, pushing the stem outward and causing a leak, why didn't the front tires lose pressure?

I then remembered a similar case involving the E46 M3. People were bitching about the fact that the valve stem caps were contacting the wheel and marring the finish, and the problem was concentrated on the rear tires. It turned out that it had more to do with the wheel hop that occurs as the vehicle quickly accelerates and the tires claw for grip, or when the tires go over a bump in the road. The solution involved, from what I recall, installing fixed metal valve stems.

Later that night I sent email to the wheel distributor as well as to e46fanatics.com in search of information. The consensus was that the valves were improperly installed -- they simply weren't tightened enough. When I asked the distributor for a torque spec, he confessed he didn't have one. It appears the stems are installed "tight, but not too tight" and with a delicate sprinkling of pixie dust for good measure. I emphasized the critical nature of this problem and the liability it represented, but the distributor pointed out that he had not heard of anyone's stems leaking. One guy on e46fanatics admitted that while the stems on his car leaked in the same way as mine, he hadn't lost any pressure other than the normal and expected 1-2PSI per month. Guess I'm the first to experience this problem. Lucky me.

I went to the dealer first thing this morning and discussed the matter with my technician. He apologized for the error, but pointed out that lacking a torque spec, he tightened the stems as far as he thought prudent, as he didn't want to crush the o-ring and destroy the seal they're designed to create. Truth be told, the stems were tight and barely moved...but just weren't tight enough. We discussed the option of replacing the stems I provided with metal and rubber stems he installed on a daily basis, but we were also conscious of the fact that changing stems would affect the balance, and neither one of us wanted to rebalance the wheels unless absolutely necessary.

My technician agreed to address the issue as soon as I could get the wheels to him, so the plan is to drop them off tomorrow morning. For logistical reasons I may not put the wheels back on the car for some time, but I'll report on the fix in a few days.

Parts Order

I just learned that my company is shutting down during the week of Christmas, so I plan to leverage the extra time to do some needed work on the car. I'm planning to do:

I used realoem.com to find the myriad parts required for these tasks and put them in an email to Jason at Tischer BMW for a quote. Due to shipping costs, however, I'm planning to pick up the oil from my local dealer.

The last oil service was what I call a "mid-cycle" change, in which I don't reset the oil service indicator. This time I'll need to clear the indicator, so I figured it was time to acquire my own means of doing that. I've known for some time this amounted to grounding pin 7 of the diagnostic connector (under the hood) for a specific period of time (3 seconds for oil service, 9 for inspection), but I never bothered to concern myself with this because my dealer always took care of it.

Based on some great information I learned that BMW sells a short length of wire with a pin crimped on one end designed to fit the diagnostic connector. At less than $2, I figured this was better than buying those ridiculously overpriced $50 commercial service reset tools, so I included that part in the parts order as well. All I'll have to do is attach an alligator clip to the end of the wire and I'll have a $5 solution to this problem. I don't have a category in my "bottom line" reports for "tool savings", but if making my own tools becomes a regular thing around my garage, I may just add one.

The parts quote came back at $280. While that's a lot of money, it's a great deal for what I'm getting. Labor for the equivalent work would probably be at least twice that, so I think doing this work myself will make good fiscal sense. The parts should be here the middle of next week.

Mileage: 137000, Parts: $280