Troubleshooting With Temperature:

The Many Uses of Noncontact Infrared Thermometers

Here’s a tool that will make you wonder how you ever got along without it. An infrared thermometer is a device that measures the surface temperature of objects and liquids by sensing the heat energy they give off. Inside the pistol-shaped tool is an infrared light sensor that measures the "surface emissivity" of the object.

Infrared is the part of the electromagnetic spectrum that’s invisible to the human eye, but can be felt as heat. If you’ve ever stood near an infrared quartz heater, you’ve experienced the warmth of infrared heat. Mosquitoes can see infrared, which makes it easy for them to find warm-blooded prey in the black of night. Likewise, infrared security cameras can see people and other warm objects in total darkness. So having a tool that can measure infrared heat energy radiating from the surface of the object means you can check operating temperatures without having to physically touch anything.

Most of these tools are designed to read temperatures from a distance of several inches to several feet from an object. The closer you hold the tool to a surface, the smaller the area it measures. For example, if you hold the tool within an inch of the surface, you will be measuring the surface temperature in a circle that’s maybe half an inch or less across. As you pull the tool further and further back, the circle expands and may grow to 6" or more at a distance of 4’ or 5’. The optical ratio of the lens determines the tool’s field of vision.

A narrow field of vision is better for most automotive applications because it allows you to take pinpoint readings from up to several feet away. From a safety standpoint, this is a good idea because it keeps your hands well away from fans, pulleys and other moving or hot objects when an engine is running.

Many noncontact infrared thermometers have a built-in laser pointer to help you zero in on your target. Some of the fancier models even have a multi-point laser that shows how the surface area being measured grows as the distance increases.

Compared to the old-fashioned bulb-type thermometers that must be held tightly against a surface or dipped into a liquid to measure temperature, noncontact infrared thermometers are much easier, faster and more accurate to use. If you can see a surface, you can measure its temperature with one of these tools. A heavy coating of grease or dirt on a surface will obviously reduce the accuracy of the reading somewhat, but for most purposes, what you see is what you get.

The temperature range of the tool will depend on the model and the price. The better ones have a wider range and/or offer several different scales from which to choose. A typical infrared thermometer for automotive applications might have a range of 0° to 500° F. A somewhat better model might have a range of -25° F up to 750° F or higher. Accuracy is typically plus or minus 2% with a display resolution of 1°. Temperatures can be displayed in degrees Fahrenheit or Centigrade. The better models also have the built-in laser pointer and may also include a memory hold for freezing the display. Prices range from just under $70 up to $250 or more.

Temperature can be a very useful and powerful diagnostic tool for a variety of purposes. The faster you can diagnose a problem, the quicker you can fix it. So let’s consider some ways you might use a noncontact infrared thermometer in the shop:

Cooling Problems
Overheating can be caused by a variety of things, so checking temperatures at various points in the cooling system can help you zero in on possible problems. A good place to start is the thermostat. By aiming your infrared thermometer at the thermostat housing, you can see if the thermostat is opening. You should see a sudden increase in temperature when the thermostat opens. No change in temperature would tell you the thermostat is sticking and needs to be replaced.

If the thermostat checks out OK, make sure the cooling fan is coming on when the coolant temperature reaches 220° to 240° F. If the fan doesn’t operate, that would tell you there’s a problem with the fan motor, wiring harness, relay or coolant temperature switch.

A clogged radiator can also make an engine run hot by reducing cooling efficiency. By scanning both sides of the radiator’s surface with your tool, you may find abrupt changes in temperature that indicate plugged tubes. If the tubes are plugged, the radiator needs to be cleaned or replaced.

Poor heater output can be caused by a plugged heater core. To check for plugging, compare the temperature of the heater inlet and outlet hoses where they pass through the firewall. Both should be hot, and the inlet hose should be about 20° warmer than the outlet hose. If the outlet hose is not hot, coolant is not flowing through the heater core because of a clog or a defective heater control valve. If hot coolant is flowing through the heater, but the heater is not blowing warm air, the problem is a mispositioned or inoperative blend air door in the HVAC unit.

Low heater output and increased fuel consumption can be caused by a thermostat that is stuck open or has too low of a temperature rating for the application. By reading the temperature of the coolant exiting the engine at the thermostat housing, you can tell if the coolant is within the proper operating range.

A/C Problems
An infrared thermometer is also a useful tool for diagnosing A/C performance. If the A/C system is fully charged and the compressor is operating, but the A/C is not blowing cold air, there may be a blockage in the refrigeration circuit or an HVAC "blend air" door problem.

A quick diagnosis can be made by first checking the temperature at the A/C outlet ducts with the system on maximum cool, recirculating air and the highest blower setting. The A/C outlet temperature should be at least 25° cooler than the ambient temperature. If not, visually inspect the liquid line to the evaporator for icing, which would indicate an internal blockage. Next, visually inspect the condenser for obstructions and make sure the cooling fan is on. Then scan the surface of the condenser to check for abrupt temperature changes that would indicate an internal blockage. Parallel-flow condensers should show an even drop in temperature from one side to the other. Serpentine condensers should show a drop in temperature from top to bottom. If blocked, the condenser should be reverse-flushed or replaced. If no problems are found in the refrigeration circuit, the lack of cooling is in the HVAC unit or controls.

Engine Misfire
You can identify a misfiring cylinder on either a gasoline or diesel engine by measuring and comparing temperatures at each exhaust port. A misfiring cylinder does not produce as much heat energy as a cylinder that is firing normally, so the exhaust temperature on a weak cylinder will be significantly lower (up to 100° F or more) than the other cylinders. To find the weak cylinder, just aim your tool at the exhaust manifold port of each cylinder and compare the readings. Once you’ve identified the weak cylinder, you can check for ignition, fuel or compression problems that would be causing the misfire.

If an engine with a coil-on-plug (COP) ignition system has a misfire, you can also check the temperature of the coils with your tool. A dead coil will be cooler than the others.

Brake Problems
National park rangers at Pikes Peak in Colorado use infrared thermometers to check brake temperatures on vehicles as they descend the treacherous road from the top of the mountain. If the brakes are getting dangerously hot, the motorist is required to wait at the check point until the brakes have cooled enough to continue safely on their way.

By measuring and comparing rotor temperatures after a short test drive, you can often spot problems like a dragging brake, frozen caliper or plugged brake line that can cause one side to run hotter than the other.

Dead Catalytic Converters
On 1996 and newer vehicles, the OBD II system will usually detect a drop in converter efficiency and turn on the Check Engine light. An emissions test is another way to confirm a bad converter. But you also can use an infrared thermometer to spot many converter problems.

Catalytic converters act like an afterburner and should produce a slight increase in the temperature of the exhaust as it passes through the cat once the cat is warm and the light is off. On 1980 and older vehicles with two-way converters, the difference should be at least 100° F at 1,500 to 2,000 rpm. But on 1981 and newer vehicles with three-way converters and relatively clean engine emissions, the difference may only be 20° to 30°. No measurable difference in temperature fore and aft of the converter may indicate a dead converter. An increase of 500° or more aft of the converter would signal overheating due to unburned fuel in the exhaust (check for a leaky exhaust valve or misfiring spark plug). A significant drop in temperature aft of the converter combined with a lack of power would indicate a plugged or restricted converter.

Many Other Handy Uses
Like your coffee a certain temperature? Check it with your infrared thermometer. Feeling a little faint today? Aim your tool down your throat and check your body temperature. Is the shop too hot or too cold for comfort? Take a reading and show it to the boss. If your tool has a laster pointer, you can even pretend it is a phaser pistol and zap the guy in the next bay if he’s getting on your nerves.

An infrared thermometer is a useful and fun tool to own, but they do have one drawback: If you’re the only one in your shop who owns one, it may keep disappearing from your toolbox.