If the engine seems to only run hot at idle or when traveling at very low speeds, like in stop-and-go traffic, I would suspect a possible cooling fan problem. Depending on the specific vehicle, the cooling fan is either blowing or pulling air across the radiator to help cool down the engine. This extra air flow is needed at low speeds and especially when the air conditioning system is on--adding an extra heat load to the engine.
If the engine runs hot only at freeway speeds, I would suspect an antifreeze restriction or low flow problem. Since air is being forced across the radiator due to the wind that is created at freeway speeds, the assistance from the cooling fan is not needed. A clogged radiator, a worn water pump, or a stuck thermostat are some of the items that could cause this type of overheating.
Now with that said, the one major ingredient that is needed to keep the engine cool is antifreeze, or engine coolant which is the same thing. If the radiator is low or empty of coolant then obviously the engine will overheat. The coolant removes the heat from the engine, and the air that blows across the radiator cools the coolant. So consider the liquid coolant a heat transfer agent that is constantly heating up (removing engine heat) then cooling back down in the radiator as fresh air blows across the radiator.
In either instance described above, the first thing you should do is check the level and condition of the coolant inside the actual radiator. Don't just rely on the level of coolant in the plastic jug usually located on the fender well; this container is just an overflow reservoir. Of course, the coolant inside the radiator should only be checked when the engine is cold to avoid scalding yourself.
If the coolant inside the radiator is low, then the system should be pressure tested to look for a potential leak. This is a simple step that many people overlook. They top off the radiator with coolant and drive the vehicle, which does not overheat for a day or soâ?¦.until the coolant leaks out again. Sometimes a coolant leak can be tricky to locate and may not necessarily leak while the engine is sitting still in your garage. The cooling system operates under pressure, and the leak may only be present when the engine is hot and the system is fully pressurized.
So how do you locate a coolant leak? You locate the leak using a coolant pressure tester. A cooling system pressure tester is like a small bicycle tire pump that attaches to the radiator neck and allows the mechanic to pump air into the cooling system, thus mimicking a fully-pressurized cooling system. This is a very simple and inexpensive test, but is almost always necessary to properly diagnose a coolant loss/overheating problem.
Ok, now on the head gasket dilemma. People seem to jump to the head gasket conclusion way too fast, and overlook a simpler problem that could be found using the pressure testerâ?¦and some common sense. The head gaskets are super strong gaskets form a seal between the top and bottom halves of the engine. Theses gaskets usually do not fail unless the engine has significantly overheated, and usually for a prolonged period of time (caused by driving an overheating vehicle for a few miles). If the engine has overheated to the point where the head gaskets have ruptured, the coolant will usually leak internally and not result in a visible external coolant leak.
Some symptoms of possible head gasket failure are:
White smoke from the tailpipe (caused by steam)
Engine running problem, like an engine misfire
Water in the engine oil or oil in the radiator
Coolant loss with no external signs
Overheating
If you are experiencing symptoms 1-3, then it is possible you have a head gasket problem, and further testing by the mechanic should be done. A compression test and a chemical test that checks for combustion gases inside the radiator should be performed next. If you do suspect a head gasket problem, do not continue to drive the vehicle as you will just compound the problem further.
If you are not experiencing any of the symptoms described above and your mechanic thinks you have a head gasket problem, you might want to get a second opinion from another mechanic. Replacing head gaskets can be an expensive job and is not a task that I would recommend a non-seasoned mechanic attempt. Proper checking beats guessing!
