Overheating

Overheating can have many causes so it pays to always check the basics before tearing things apart. First, let your car cool down a bit. Is there enough liquid in the cooling system? Have you bled the air pockets from the system? Is the water pump belt slipping? Are the radiator fins blocked with debris? Are all the ducting panels in place? If the fan is engine driven, does it have an electro- magnetic clutch? Does the clutch engage the fan? Do the electric fans cycle? (On systems with double electric fans, one fan is usually triggered by the air conditioning controls, the other by a thermostatic switch in the radiator.) Are the brakes dragging?

Overheating can have many causes so it pays to always check the basics before tearing things apart. First, let your car cool down a bit. Is there enough liquid in the cooling system? Have you bled the air pockets from the system? Is the water pump belt slipping? Are the radiator fins blocked with debris? Are all the ducting panels in place? If the fan is engine driven, does it have an electro- magnetic clutch? Does the clutch engage the fan? Do the electric fans cycle? (On systems with double electric fans, one fan is usually triggered by the air conditioning controls, the other by a thermostatic switch in the radiator.) Are the brakes dragging?

The all-time favorite attempted cure, of course, is to replace or even eliminate the thermostat. Replacing the thermostat on a hunch is not always very practical. Thermostats for many models can be expensive and hard to find. An alternative is to remove and test the thermostat. Most thermostats are stamped with a number corresponding to the temperature at which they open to let heated coolant flow through the radiator. The number should be between 70 and 90 to show degrees Centigrade or 160 to 190 to show degrees Fahrenheit. If you don’t find a number, don’t worry about it. Let’s test the thing. Find a glass pickle jar or mayonnaise jar, fill it half way with water and set it on the stove. You don’t need Pyrex, as long as the jar has water in it while being heated. lay a pencil across the top of the jar and fashion a wire “S” hook so that you can suspend your thermostat under water. Make sure the thermostat is completely submerged but not touching the bottom or sides of the jar. Hang a thermometer in the water but don’t let it touch the bottom or sides of the jar either. Set the stove burner to high and watch the temperature climb. After the thermometer passes the 150 degrees Fahrenheit mark, pay close attention to the little round door in the center of the thermostat and record the temperature at which it begins to open. Now make sure it opens to at least a 3/8″ gap.

The figure you recorded should be somewhere between 160 degrees Fahrenheit and 190 degrees Fahrenheit (70 degrees Centigrade and 90 degrees Centigrade) and should correspond to the number stamped on the thermostat. The conversion formulas are:

  • degrees Fahrenheit = (degrees Centigrade x 1.8) + 32
  • degrees Centigrade = (degrees Fahrenheit – 32) x 0.56

If you can’t find a stamped number but your figure falls within the range, the thermostat should be useable. 180 degrees Fahrenheit is ideal. If your thermostat opens a little on the high side and you like to experiment, try drilling 2 1/4″ holes in the rim to act as a by-pass. You won’t get warmth from the heater as quickly in the winter but it can make life easier in the summer.

Here’s another tip: While the thermostat is hot and open from testing, jam it open with an aspirin tablet. Be careful that you don’t damage anything. Just place the aspirin in the opening and the little door will quickly close on the aspirin as it cools. The open thermostat will make filling and bleeding the cooling system much easier and the hot coolant will melt the aspirin. If the old thermostat tested OK, you may have more serious problems requiring professional help such as a clogged radiator, blown head gasket, or even problems with ignition or fuel mixture.

If the old thermostat has a problem you will have to replace it. Running without a thermostat is not a good idea. The engine can take too long to warm up or never completely warm up and it can even cause overheating. If the coolant runs through the radiator too fast it sometimes doesn’t get the chance to shed enough heat.

Install the thermostat, refill the cooling system and bleed any trapped air from the high points in the system. Make sure all hoses are in good shape and that all clamps are tight.

by Wil de Groot



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