Aside from coolant leaks causing a loss of coolant and air pockets that cause an overheat, a few more things can contribute to your engine overheating. I'm going to call this a breakdown in cooling system efficiency. For this category we're going to include things like:
- a clogged radiator
- a blocked radiator (airflow)
- a blockage in the cooling system
- a faulty water pump
- a cooling fan issue
I know that's a lot, but they all cause the same thing: a lack of cooling efficiency. These are probably the toughest of the group to diagnose since they might not cause a full-blown overheat, but they can cause your engine to run hot.
A cooling fan that isn't working is fairly easy to spot. If you see that you're overheating and the cooling fan isn't coming on, this could be the cause of the overheat. The classic symptom of a cooling fan failure is an overheat while you’re sitting in traffic, but when you start moving the temp starts to come back down. There are about a hundred reasons I can think of that can cause a cooling fan not to operate, including air in the cooling system (see the trend here?). Given that and the fact that different manufacturers handle cooling fan operation differently, I'm only going to speak in general terms here.
Say your vehicle is overheating and it has an electric cooling fan(s) that isn't working. The first thing I often do is check the fuses. A blown fuse is a common problem, and is the easiest thing to fix. If you strike out here, and you've bled the air out of the cooling system, then it’s time to do an electrical diagnosis.
I normally start with the fan itself by running jumper wires to the connector to check its operation. Most cooling fans are simple motors and just need a power and ground to run. 12 volts straight from the battery should do it along with a wire run to battery negative or a good body ground. Just touch the wires to the terminals inside the cooling fan connector to see if it runs. In fact, you can often hook them up backward and the fan will still run, but in the opposite direction. Just know that the black wire is normally the ground side of the circuit, so try to hook that one to battery negative or a good body ground. Body grounds are any bare metal spots on the chassis. Sometimes you'll see wires that are just bolted to the body; these are body grounds and a good place to use as a ground if you need one. For diagnostic purposes, you're just trying to see if it works or not. If it doesn't work, it’s likely that the fan motor is bad and needs to be replaced. If the fan does run when you supply it with power and ground, you need to keep looking. From there, you would need an electrical schematic for the cooling fan circuit and some basic electrical testing equipment. See the electrical section for more information about that.