Why would a car overheat if there's plenty of coolant?

Answer 1

You got those 3.8 (60,000-80,000 miles) Blues. Could be a thermostat. Are you going through coolant? If so, the technician is probably right. The 3.8 motor wants head gaskets every 70,000 miles or so. Plan on machining the head flat again too. If you are not going through coolant, it would sound more like a thermostat or possible blockage in the system.

Answer

Check the condition of the radiator, the water pump, the thermostat, and the pressure cap. I had a similar problem with my 1991 Chevy Cavalier (4 cyl, 2.2L). When all is well it normally runs about 180-200 degrees F (the electric fan clicks on at about 200 and shuts off at 180, fan runs continuously w/AC on). Because I neglected to flush the system for about 3 years, the radiator clogged and the water pump was on the way out (all but three of the blades had eroded away). The car was running 230 degrees (about 3/4 of the way up the gauge). Even after the water pump was replaced, it still ran hot even though the T-stat and cap were ok and there was plenty of coolant. I flushed out the system and replaced the radiator. This cured the problem instantly. Running temp fell from 230F to 180F (about 1/4 of the way up the temp gauge on my car). I don't know many particular problems that occur with '90 Cougars, but these are several generic problems that cause overheating. Check the T-stat by removing it and boiling it in water on the stove. If it is good you will see it open as the water temp in the pan increases. While the t-stat is out of the engine, start it and run it for the length of time it takes for it to overheat. If the t-stat is the problem, with it removed, the temp gauge won't even lift off the peg. (Don't run the engine without a thermostat longer than it takes to verify the problem). Also check water flow through the system by warming up the engine with the cap off and looking into the radiator fill pipe. With the engine at running temp, there should be a strong water flow out of the tubes (water turbulence for a vertical radiator). Also make sure the fan works.

Answer

Bear in mind that any air (as well as gunk) trapped in the coolant lines will cause overheating, particularly higher up around the head gasket. This overheating can definitely cause warping and result in a blown gasket. This is especially common in cars with aluminum head gaskets. (I don't know how the Cougar was designed.) Always "burp" your coolant system after flushing.