An engine that runs consistently hot can age prematurely, even if it doesn't show classic signs of overheating. This makes good cooling system maintenance particularly important, and your program should go well beyond the occasional flush-in-fill. Excessively high temperatures can cause engine knock, which shortens the life of virtually all key engine parts, including pistons, pins, crankshaft and bearings. High temperatures also increase friction (and engine wear) by thinning out engine oil and causing parts to warp, affecting engine sealing.

Make sure you've really filled the system. Many late-model cooling systems are prone to trapping air, which not only reduces heater output, but can create harmful hot spots in the engine.

If the car manufacturer has a specific bleeding procedure, including the opening of air bleeder valves on the engine block or cylinder head, be sure to follow it when you fill up. If the radiator cap is not the highest point of the cooling system, jack up the front of the car and raise it, which helps it self-bleed.

Make sure the reservoir is filled to the proper marked level, and recheck the level periodically.

Check the clutch fan on a north-south engine. Most accurate: Measure the air temperature when the clutch locks up. Slip the probe of a pyrometer (temperature-measuring meter) into the fan shroud and run the engine until it warms up. When the engine is warm, the fan should virtually lock up. Fan noise will increase noticeably and you can note the temperature on the meter which should be between 150° and 170°F.

No pyrometer? Try to spin the fan with the engine fully warmed up (but not running). If it turns easily, the clutch is bad. Also, run a finger around the bearing area at the rear of the clutch assembly. If it picks up a blob of goo, that's silicone, and it indicates a leak from the clutch assembly. Unbolting the clutch fan assembly and installing a replacement is a straightforward job.

Test the radiator electric fan, or fans. Within a minute of turning on the a/c, at least one fan should turn. Refer to a manual for your car to see when a second fan and/or the high-speed range of a single fan should come on.

You may have to block the front of the radiator or a/c condenser with cardboard to raise coolant temperatures high enough to fully check a multispeed fan system.

Check the fan shroud. If it's loose or missing, the efficiency to the fan drops significantly. Don't remove the thermostat. It won't cure overheating (unless it's caused by a bad thermostat), and it prevents proper engine warmup. Most engine wear occurs on cold starts before the engine is warm. Cars without thermostats may never warm up fully - confusing the computer into running in it's overly rich warmup mode forever.

Look under the front of the car. Many modern cars draw in cooling air from the underneath. If an air dam or chin spoiler is missing, the car may run too hot, particularly in highway operation. Cars with limited grille area may use underbody airflow, and cooling problems occur if they're missing.