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An Infrared Thermometer As Diagnostic Tool

Infrared Thermometer

Most professional mechanics, but only a few driveway mechanics are aware of the use of an infrared thermometer to speed up the diagnostic process on engine cooling problems. For the DIY mechanic, the infrared thermometer may seem like a costly tool that will only rarely get used, but with prices for them coming down to become equivalent with many of the hand tools most mechanics take for granted, it's worth having one.

As an example, we've talked about the process of testing a thermostat suspected of being faulty. You can greatly simplify and reduce the time required to do that testing if you use an infrared thermometer instead. Simply get a heat reading on points before and after the thermostat, without removing or draining the coolant or removing any parts, and you can immediately tell if the thermostat is working properly.

That is just one example of using an infrared thermometer to speed up diagnostics, but it's probably the most common.

Similarly, the thermometer can be used to test cooling fan engagement, radiator efficiency, heater core operation, A/C performance, and more.

Keep in mind that the reflectivity of the surface being tested will determine some temperature flux, so metal housings are likely to give slightly warmer readings than are black rubber hoses. For this reason, do not consider the readings on your infrared thermometer to be exact during diagnostics, but very close instead. Thus if the thermostat opens at 225 instead of 220 degrees, for example, there is probably nothing wrong with the thermostat.

Checking the Thermostat
We mentioned checking the thermostat with an infrared thermometer. The process can be done by either bringing the engine up to temperature and checking temperatures immediately before the thermostat housing and just after it or by reading temperatures as the engine heats up and gauging whether the thermostat opens by noting whether the flow begins to regulate temperature to either side of its housing.

Checking the Cooling Fan
Again, reading temperatures as hot coolant enters the radiator and the ambient air near the fan (if mechanical) or very near the heat sensor (if electric) should tell you whether the engine's heat is sufficient to be engaging the fan.

Checking the Radiator
Using the infrared thermometer, take readings of the radiator when the engine is hot. Read from the input point (usually the top) and across in segments, taking three or four readings before you end on the exit point (bottom) for the coolant. The radiator's temperature should get progressively cooler as you progress. Abrupt temperature changes should be investigated further and likely indicate a clog or other issue.

Checking the Heater Core
Begin by checking the temperature of the core's input hose (from the firewall) when the engine is up to temperature and the thermostat is open. The exit hose should be roughly the same temperature, usually within ten degrees (often cooler) if the core is functioning properly. If not, you have a blockage that is not allowing the hot coolant to circulate through the core.

Check the A/C
Several parts of the air conditioning system can be checked using an infrared thermometer. Coolant lines, the evaporator, condenser, etc. can all be checked, of course. So can the cold air ducts inside the car before and after the fan to determine whether the fan is operating. Individual ducts can also be checked to ensure that no blockage or blend door actuators are affecting airflow.