Thursday, May 25, 2017 Detailed Auto Topics
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Tire pressure monitoring system (TPMS) warning lightThe valve stem cap is extremely simple, yet misunderstood and can be the cause of a good many problems. Tires that do not hold air are often caused by missing valve stem caps. Vehicles often come in for wheel alignment only to find a low tire, caused by a missing valve stem cap. On newer vehicles a tire pressure monitoring system (TPMS) warning can be another expensive symptom.

Dry rotted valve stem

The valve stem allows a tire to be inflated and deflated. The stem does this by acting as a check valve. The spring loaded check valve holds air in the tire until more highly pressurized air is applied. It then opens to allow additional air and closes to hold the selected pressure. It can also be depressed to allow air to exit the tire. This type valve is commonly referred to as a Schrader valve.

The Schrader valve is intended to allow for inflation, deflation and as a temporary seal. It is not designed to be a full-time seal. Sealing the stem is the job of the valve stem cap. Without the cap, the tire will eventually loose air.

Another major problem, with a missing cap, is debris. The cap seals the valve core. Without a cap, water, dirt and other debris enter the core. They can cause damage and leaks to the core. Worse, the tire pressure monitoring sensor is normally attached to the stem. Debris forced into the sensor during inflation can damage this expensive component.  Once damaged the warning light will come on and the sensor will need to be replaced.

Tire pressure monitoring sensor (TPMS)On vehicles with tire pressure monitoring systems (TPMS) a special valve core is used. This core may be part of the sensor.  The core will normally will last the life of the sensor, unless run without the protective cap. If the valve core on a TPMS system leaks, it must be replaced with an exact replacement.

Valve stem cores are normally checked with soapy water. Any bubbles that can be seen at the stem opening will call for core replacement. Common valve cores are threaded into the stem. With TPMS cores a special torque wrench is used to properly tighten the core.

Another issue caused by a missing valve stem cap may be damage to a tire pressure gauge. When a tire pressure gauge is used, the debris in the stem can be forced into the gauge, ruining it.

Once debris has been forced into a gauge even the best will give improper readings. If it is necessary to check pressure in a tire that has been used without a cap, compressed air should be used to blow the stem clean, before the gauge is applied. Once cleaned the stem should be checked for excessive leakage and replaced if needed.

Valve stem and protective valve capKeeping valve stem caps on valve stems can help prevent low tires, tire wear, damage to the core, TPMS transmitter and to air pressure gauges. Make sure each wheel is fitted with a valve stem cap.

Checking for dry rot at the stem is another good idea. Remember to also check the spare tire. Simple things should remain simple. With a slight bit of attention, valve stem caps can prevent a great deal of problems.





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