Before you can fix a cooling problem, you first have to diagnose the underlying cause, which is often the most time-consuming and challenging part of the job. One way to speed up the process is to use temperature checks to zero in on critical components.
Overheating, as you know, can have many possible causes: a stuck thermostat, a clogged radiator, a cooling fan that is not coming on, low coolant, etc. By a process of elimination, you can eventually figure out what is causing a problem. But how long will it take you? And will you be absolutely sure of your diagnosis before you replace any parts?
The "old fashioned" way of checking a thermostat is to remove it from the engine so it can be dipped into a bucket of hot water to see if it opens. Most technicians do not have that kind of time to waste so they simply replace the thermostat if they have reason to suspect it might not be working properly. Sometimes they are right and sometimes they are not.
A better way of testing a thermostat is to measure the actual temperature at which it opens on the engine, not in a bucket. That way, if the thermostat is good you have not wasted valuable time replacing it unnecessarily. You can proceed to other diagnostic checks or repairs.
A good way to do this is with a noncontact infrared thermometer such as Raytek's Raynger ST that measures heat energy radiating from the thermostat housing. An ordinary contact thermometer relies on surface conductivity to take a temperature reading. Consequently, it is slow and may not give an accurate measurement if there is poor contact between the thermometer and thermostat housing. But a noncontact infrared thermometer does not rely on direct physical contact. It measures infrared heat energy radiated from the thermostat housing and gives you instant results. Note: The accuracy of an infrared temperature reading depends on the reflectivity (emissivity) of the surface.
The Raytek Raynger ST accurately measures surface temperatures from -25 degrees F all the way up to 750 degrees F with an accuracy of plus or minus one percent! All you do is point the pistol-shaped Raynger ST at the surface to be measured and pull the trigger. It is that easy. There is even a built-in laser pointer on some models to help you aim at the spot you want to measure.
Using a noncontact infrared thermometer also eliminates the danger of coming into contact with hot surfaces, high voltage plug wires or moving parts such as fans, belts or pulleys.
If the thermostat checks out okay, make sure the cooling fan is coming on when the coolant temperature reaches 220 to 240 degrees F. No fan would tell you there is a problem with the fan motor, wiring harness, relay or coolant temperature switch.
If the fan is working properly but the engine is still running hot, scan the entire surface of the radiator to check for clogs. Temperature readings should decrease evenly from one side to the other on a crossflow radiator, or from top to bottom on a downflow radiator. If you find an area where there is an abrupt temperature change, the radiator is plugged and needs to be flushed, cleaned or replaced.
When troubleshooting a low heater output problem, use your infrared thermometer to check coolant flow through the heater core. Compare the temperature of the heater inlet and outlet hoses where they enter the firewall. Both should be hot, and the inlet hose should be about 20 degrees warmer than the outlet hose. If the outlet hose is not hot, the core is clogged or the heater control valve (if used) is defective. If reverse flushing the core fails to open the blockage, you will have to replace the heater core.
Low heater output can also be caused by a thermostat stuck in the open position or the wrong temperature range thermostat. If the coolant temperature at the thermostat housing or radiator inlet is unusually low (140 degrees or less), the thermostat needs to be replaced.
An infrared thermometer can also help you diagnose poor A/C cooling 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 a 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, recirculate air and highest blower setting. The A/C outlet temperature should be at least 25 degrees 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.
Some problems such as engine misfire can be time-consuming to troubleshoot. Isolating a misfire to a specific cylinder usually requires a power balance test (which is not easy to perform on some of today's distributorless ignition systems) or observing ignition patterns on an oscilloscope (which requires an expensive scope and making the necessary hookups). But now there is an easier way. Just use an infrared thermometer to measure and compare the temperature of the exhaust at each exhaust port. A misfiring cylinder does not produce as much heat energy as a good cylinder, so the exhaust from a weak cylinder will not be as hot as that from cylinders that are firing normally.
To find the misfire, aim the gun at each exhaust port on the manifold and squeeze the trigger. Note each reading and compare the results. Any cylinders that are misfiring will read significantly lower than the others.
Once you have identified the misfiring cylinder, you can zero in on the underlying cause. Remove and inspect the spark plug. If wet, the plug may be misfiring because of a bad plug wire, cracked distributor cap or bad coil on a DIS system. If the spark plug is fouled, the cylinder may have an oil consumption problem due to worn valve guides and/or rings. If the spark plug appears to be okay, the air/fuel mixture may be too lean. Check for a dirty or inoperative fuel injector, or an air leak around the injector seal. On older engines with carburetors, check for intake manifold air leaks. Another possibility may be a leaky valve. Check compression.
This same procedure also works great on diesel engines. Reading and comparing exhaust temperatures can help to identify weak cylinders and diesel injector problems.
You can also use a noncontact thermometer to check the function of your catalytic converter. The converter may not be working if the catalyst has been poisoned by lead (leaded fuel), silicates (from an internal engine coolant leak) or phosphorus (oil burning), and your Check Engine light may be on with a P0420 Trouble Code. By measuring and comparing the converter inlet and outlet temperatures while the engine is idling, you can determine if the converter is doing anything or not. This type of quick temperature check won't tell you how efficiently the converter is working or whether or not it can pass an emissions test, but it will tell you if the converter is still functional.
On older vehicles with two-way converters the difference should be at least 100 degrees F. But on 1981 and newer vehicles with three-way converters the difference may only be 20 to 30 degrees because computer-controlled engines normally run very clean. No difference in temperature indicates a defective converter or no air from the air pump (check the air pump diverter valve & plumbing). An increase of 500 degrees or more indicates converter overheating due to a rich fuel condition (check the fuel system) or misfiring spark plugs or compression leaks.
You can use your infrared thermometer to diagnose certain brake problems, such as overheating and pulling. A difference side-to-side in front brake temperatures after stopping may indicate dragging or sticking calipers. Many race car teams use an infrared thermometer to check brake and tire temperatures to chck brake balance and tire loading.