Over Heat, Under Heat, Or Somewhere In Between

Vehicle over heating? Vehicle not heating up fast enough? Vehicle heats up just fine but you get no heat in the cabin? This post is just for you! If you have overheated your vehicle, also read this .

One of the first things to note is that the temperature gauge in the dash often does not show an accurate representation of the actual engine coolant temperature. As you drive your vehicle, the coolant temperature fluctuates regularly, and that is normal. In order to convince your average driver all is well, the temperature gauge will sit right in the middle of the gauge as long as the temperature is close to proper operating temperature. In order to known exactly where your coolant temperature actually runs, I recommend purchasing a scan tool such as this one made by Autolink.

automotive repair scan tool

This little device is relatively inexpensive, reads and clears codes, and also displays all your live data (on cars 1996 and later). Along with a scan tool, you will also need a basic instant read thermometer. One with a dial, or electronic display will work just fine.

thermometer, automotive repair

Secondly, when attempting to make a repair to the heating and cooling system of a modern vehicle, you may run into a “Cooling system error” code. These codes are nondescript by nature. Basically, the manufacturer has run a whole lot of tests, and has determined what the normal coolant temperature should be in all situations. If at any point the engine control unit sees an engine coolant temperature that is outside of what it has been told is normal, it stores a generic “cooling system error” code.

Lastly, we must touch on what is normal operating temperature for the engine coolant. Operating temperature will vary depending on the ambient temperature. Furthermore, domestics generally run hotter than imports. A domestic will run somewhere around 208-230 degrees Ferinheight (98 – 110 degrees Celsius) . An import will run somewhere in the range of 185-205 degrees Ferinheight (85-95 degrees Celsius). If in doubt, you can look up normal operating temperature for the engine coolant. You can also find the temperature at which the thermostat should open, and the temperatures where the cooling fans turn on and turn off. All very handy information. That being said …..

One of the most common problems with a cooling system is air pockets. The coolant is moved around the engine based on the principal that liquids don’t compress. If there is air in the system, which compresses rather easily, the coolant will have a tendency to not circulate properly. Also, engine coolant temperature sensors have trouble reading air temperature. If an air pocket works it’s way around to the coolant temperature sensor, the sensor will most likely deliver a faulty reading to the engine control unit. Air pockets can cause over heating, drive-ability problems, or poor heat in the cabin.

The easiest way to remove air pockets form a cooling system is by filling them with an air lift.

An airlift removes air pockets form cooling systems

The air lift is a wonderful device which uses compressed air to suck the air out of the cooling system, rendering a vacuum. Once a vacuum has been achieved inside the cooling system, coolant can be installed leaving virtually no air in the cooling system. I stress virtually, because no air lift will remove all the air, so make sure to still top off the fluid and bleed any air bubbles.

Once the cooling system is full of coolant, the next step is to verify the radiator cap is in good condition. A radiator cap controls the pressure inside the cooling system. As the coolant heats up and expands, pressure builds inside the cooling system. As discussed before, a cooling system requires some pressure in order to circulate the coolant. Too much pressure and parts of the cooling system begin to fail. A radiator cap opens and closes as heat and pressure rises and falls, maintaining the proper level of pressure inside the cooling system. Furthermore, a faulty radiator cap will also leak coolant, causing an air pocket. I recommend replacing the radiator cap on a vehicle every spring. They are generally inexpensive, and nearly every radiator cap in service will fail a pressure test.

Another inexpensive and highly critical cooling system component is the thermostat. A thermostat looks a little like this …..

Thermostat, overheat, underheat repair

A thermostat is basically a heat controlled valve. Until the engine coolant temperature reaches operating temperature, you want the coolant to stay in the engine, in order to warm up more quickly. The thermostat closes the radiator off from the rest of the cooling system. When the engine coolant temperature reaches the proper temperature, the thermostat opens and allows coolant to flow through the radiator, and thus be cooled. I highly recommend putting a thermostat in a pot of water and heating the water to boiling. Seeing a working thermostat open up just do to heat, is like watching magic.

Do not alter the thermostat if your vehicle is overheating. If you replaced the thermostat, and the vehicle sill overheats, there is another problem. Do not remove, drill a hole in, or force the thermostat to stay open.

A faulty thermostat can cause over heating, under heating, poor gas mileage, poor heat in the cabin, check engine lights, or emission failures. If the thermostat stays open, or opens too early, the vehicle won’t reach the proper operating temperature. If the thermostat opens too late, or not not at all, the vehicle will over heat. Again, a fairly inexpensive part. I recommend replacing the thermostat if the car has ever been overheated, and every few years just as a part of maintenance.

Next up, you’ll want to confirm your engine coolant temperature sensors are all working. Most modern vehicles use two engine coolant temperature sensors, one for the gauge and and one for the computer. Use a scan tool to confirm the engine coolant temperature sensor is reading correctly. If the computer does not know the vehicle is running hot, it may never turn the cooling fans on.

Speaking of cooling fans, make sure the are working. Most modern cars have ditched the old drive belt driven fans for a more fuel efficient electric fan. An easy test if the fans themselves work is to turn the air conditioning on. Modern cars kick the fan on any time the A/C compressor is on. If the vehicle is over heating, and the fans are not on, you’ll need to run down the electrical system and find out why. That is unless you have a hydraulic power cooling fan. Some vehicles use the fluid from the power steering system to turn the fan. Make sure the power steering system is full and working properly, and then run down an electrical checklist. If you have an old fashioned fan clutch, make sure it’s in good shape.

An obviously key component to the whole cooling system is the radiator. Sometimes the radiator will become plugged. A plugged radiator won’t circulate coolant.

Sometimes a radiator will not be plugged, but will be calcified, and therefore not exchanging heat. Basically, a thick substance can coat all the walls of the tubes of the radiator, and that substance will insulate, rather than radiating heat. You can often see this coating if you look into the radiator. You should also be able to feel hot air coming off the radiator. Sometime the front of the radiator will become coated in dirt and such. The coating on the front of the radiator will prevent air form flowing through the radiator and thus prevent proper cooling.

The last, and most costly cause of an overheating condition is a bad head gasket. When the head gasket fails, exhaust gas is forced into the cooling system, and the vehicle just over heats.

But what is your vehicle doesn’t overheat, it just doesn’t produce heat in the cabin? First and foremost, ensure the cooling system is full, with no air pockets, and that the thermostat stays closed long enough to reach operating temperature.

Next you’ll want to check your heater core. The heater core looks just like a small radiator, because it is. The heater core radiates heat, just like a radiator and then a fan blows that heated air through the ducts and into the cabin. Check out this picture of a heater box split in half, the heater fan is on the right, the heater core is on the left, and the A/C core is in the middle.

Heater core reair

Just like a radiator, the heater core can become plugged or coated on the inside. If accessible, test the temperature of the hoses going into and out of the heater core. Normally, if the heater system is working, both hoses will be hot (as hot as the coolant in the engine) but a little less hot on one side (because heat has been transfered from the coolant, through the heater core to the air in the heater ducts). If one hose is hot and the other is COLD, the heater core is plugged, or coolant is not circulating through it due to an air bubble or another problem (VW/ AUDI are terrible for the water pump impeller coming off). If both hoses are hot, but no heat comes out of the heater, the heater core is most likely not transferring heat.

While you are checking hoses in and out of the heater core, look for a shut off valve. Many vehicles use a coolant shut off valve in the supply hose to the heater core. The idea here is to close the coolant of from the heater core when heat is not desired. Many vehicles that have these valves will use a cable to open and close the valve. Vehicles with dual climate control will often have two valves that open and close separately to help maintain independent temperatures, each being controlled electronically.

You could also have a heater fan that doesn’t work, or a plugged cabin air filter. The heater fans, control switched and resistors all go out regularly, all of which would result in an heater fan that will not work (or works only on high).  I have also seen cabin air filters so dirty that nearly no air will pass through them, and thus not allowing any air to pass through the heater core and out of the ducts.

Lastly, you could have a problem with the blend doors inside the heater box. Doors inside the air duct system open and close to divert air though the heater core, or to bypass it. If the air is not being diverted through the heater core, then you won’t get any heat.

Cheers-

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