Of Stem Caps & Sensors

When I got into our Nissan the other day, the little light on the dashboard that looks like a flat tire was on: The air in one or more of the tires was at an unsatisfactory level.

I sighed and nodded. It was one of the first truly cold days of the autumn, and the resultant contraction of the air had lowered the air pressure in the tires. It happens every fall (and the converse, the expansion of air to the point where the tires are over-filled, happens every spring).

So I interrupted my errand with a stop at a service station and got the problem halfway fixed. Two of the caps on the tire valve stems did not want to come off. Annoyed, I completed my errands with the tire light still glowing on the dashboard, and once home, I got a pair of pliers and began to work on the balky stem caps. The first one, a plastic one, came off immediately. The second, a metal one, did not, despite efforts that left it scratched and scarred.

Pensive, I headed down to the nearby convenience store and topped off the tire from which I’d taken the plastic cap and then took a short drive to see if the tire light stayed on, indicating that the currently inaccessible tire also needed air. After a half-mile or so, the light went out. The air in the tires was fine. But we were going to have to get the car into our local tire place to get that metal cap removed.

The shiny metal cap – the one that was badly stuck and now scarred – was new. Sometime last summer, I got tired of replacing the plastic caps on the tire stems. It seemed that every couple months, one of them broke or had its threads damaged. So I headed down to the auto parts shop on Lincoln Avenue, right next door to the building that houses WJON and its sibling radio stations, and bought a set of metal caps for the Versa’s valve stems. As it happened, I installed only two of them, leaving the two remaining plastic caps in place.

Using the metal caps was not a good idea, but I didn’t know that then. That’s something I learned rapidly Monday morning, when I got to the tire shop just up the service road along Highway 10. I told the fellow behind the counter why I was there. “It’s probably mis-threaded, and I’ll need a new tire stem,” I said.

He nodded and said, “Well, yes, but . . .”

And I knew something was wrong.

“It’s not just a stem,” he went on. “It’s also the sensor for the tire pressure. We’ll try to get the cap off the stem, but what’s often happened is that the metal cap bonds to the metal of the sensor stem, and the cap can’t be removed without ruining the sensor.”

If his mechanic could remove the cap without other damage, he said, we’d be talking a minimal charge for labor. But if I needed a new sensor, I’d be looking at something north of a hundred bucks. I scratched my head, then nodded. “It’s gotta be done,” I said. “Go ahead.”

And about an hour later, I left the tire shop with a lighter bank account and a new pressure sensor in my right rear tire. As I’d settled the bill at the register, I’d had a thought. “I did have a metal cap on one of the other tires,” I said.

The fellow nodded. “We replaced that one, too,” he said.

And I headed home with my mind running in two tracks. First, we were lucky: I could have used all of the metal caps and all of them could have bonded with the stems, costing us more than $400. Second, is there any way I could have known better?

Well, yeah, there is, but only if I were an avid auto buff or had thought to do some research. The post about metal stem caps that I found at a forum for owners of the Chrysler Crossfire is typical:

The metal (either steel, stainless, or aluminum) will cause a chemical bond with the valve stem over time. Anytime you put dissimilar metals together you can expect a long term reaction, be it chemically “welded” or the opposite, corrosion. These negative effects are magnified by the fact that our valve stems are actually part of an electric sensor…….think electroplating, electrogalvanizing, etc.

I thought for a second that maybe the fellow at the car parts store might have warned me, but he likely thought I knew what I was doing. I didn’t, of course, and it cost us. The only positive here is that it wasn’t as expensive a mistake as it could have been.

Well, there is one other minor positive. Telling the tale got me digging though my music for something with “wheel” in the title, and I came across a tune by Joe Grushecky & The Houserockers. I’ve liked Grushecky’s work – sometimes credited to his band, the Houserockers (originally the Iron City Houserockers), and sometimes just to him – for years, but I don’t think it’s ever shown up here.

So here’s “Broken Wheel” from their 2009 album, East Carson Street. As sometimes happens, Grushecky and the Houserockers get a little help on guitar from a pal named Bruce Springsteen.



2 Responses to “Of Stem Caps & Sensors”

  1. Paco Malo says:

    The instrumention does sound a good bit like Bruce’s work on “Wrecking Ball” (2012). Fine Saturday single, whiteray.

  2. david young says:

    Why don’t you just get a tire gauge and check your tire pressures manually every week or so? A lot cheaper than replacing the sensor (at least until you have to replace the whole tire).

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