If you are not setting the correct tire pressure for every vehicle that comes in, you could be setting yourself up for failure, and oftentimes, an unhappy customer.
Almost every owner’s manual and door jamb placard recommends the tire pressure to be set when the tires are cold. So what is cold? As a general recommendation, most OEMs say that the tire should be sitting for at least three hours.
Ambient temperature affects tire pressure on every tire. For every increase or decrease of 10° F of ambient temperature, tire pressures will change about 2%, or about 1 psi for the average passenger vehicle tire.
If a tire’s pressure was set on a summer day when it is 90º F and it is now 40º, the tire will have lost 4-5 psi just due to changes in temperature. On some vehicles, this could cause the TPMS light to illuminate. It also works the other way around. If a tire was inflated at 40º F and the outside temperature is now 90º, the tire could now be overinflated.
Always take outside and indoor temperature into consideration when setting inflation. For example, a 40º F discrepancy may require you to add 3 to 4 psi (40º F=4 psi) over the placard pressure to account for cooling when the vehicle has been parked outside. If this is done correctly, the cool pressure should be close to the specified cold pressure.
As a general rule, never set the tire pressure below the specified placard value regardless of tire temperature or ambient temperature. Some late-model vehicles can take the ambient air temperature into account to avoid false activation of the TPMS light.
Find The Leak, Prevent The Leak
Before you top off a tire, take some time to investigate where the air has gone. On late-model vehicles with TPMS, one of the leading causes of slow leaks can be a neglected service kit for the TPMS sensors. Service providers could be thinking they are doing the customer a favor by saving them the expense of installing a new service kit, but that’s not the case.
Often the sensor will be hit during removal or installation and the seal between the rim and the sensor’s valve stem will be disturbed. This can cause a slow leak. In addition, the service parts of the sensor have a limited lifespan because the heat from the brakes and other environmental contaminants can cause the soft components to degrade and the metal components to corrode.
With that said, never reuse nuts, grommets or valve stems. Aluminum TPMS valve stems and nuts are anodized to prevent galvanic corrosion and material deterioration. The nut has a bonded lubricant to help provide the proper torque required for seating a new grommet. If a nut is reused, the anodized surface may be scratched away and corrosion may occur between the sensor, wheel and stem.
Snap-in valve stems and grommets conform to the mating surface of the rim. The instant the nut is torqued or a valve stem is pulled through the rim, it starts to take on the shape of the surfaces against which it is sealing. This memory cannot be erased, so if the seal is reused, it could cause a slow leak.
Alignment & Inflation
Setting the correct tire inflation should be one of the first steps of performing an alignment. Not setting the correct tire pressure could cause a misaligned vehicle that pulls no matter how perfect the toe and thrust angles are set.
If the rear tire pressures are too low or high, they can change the caster setting. This could cause poor steering feel and hurt straight-line stability. If the front tires don’t have the same pressures, they could cause the vehicle to pull in the direction of the lowest tire.
Buy a quality tire pressure gauge and advise the customer to purchase a digital gauge to check the tires in between service. A quality tire pressure gauge should have a tolerance of ±1%. Anything beyond 2% can cause you to set tire pressures too low or too high. Also, treat a tire gauge like an expensive torque wrench. Dropping a gauge from only 4 feet can change the readings — this goes for both digital and analog gauges.
Checking the inflation and the condition of the tires should be part of any maintenance service at your shop. Technicians and service writers should always be looking for the TPMS light when they pull any vehicle into the bay. Not performing these basic steps could cause you to return an unsafe vehicle to the customer.
According to a 2016 survey of 1,000 drivers conducted by the Rubber Manufacturers Association, the average consumer doesn’t have adequate knowledge about proper tire inflation maintenance.
• Only 17% of drivers are “tire smart,” or know how to properly check tire pressure.
• Only 50% of drivers know where to find the correct inflation pressure for their vehicle: The label on a vehicle’s driver-side door or owner’s manual. Fifty percent of drivers wrongly believe that the correct pressure is listed on the tire sidewall.
• One out of three drivers don’t know that tires should be checked “cold” before driving. One of the biggest mistakes you can make is checking tires after driving. As tires roll, they warm up, which causes the pressure to increase. This could give a false tire pressure reading.
• Four in 10 drivers believe they can tell if a tire is underinflated just by looking.
• Two out of three drivers do not check the tire pressure in their spare tire.
Article courtesy ImportCar.