One of the most aggravating problems with a car, especially in the summer, is cooling failure. There are a number of causes, some obvious, some not so obvious.
Coolant level
Check at the radiator if possible, not just the reservoir. It may need to have the air bled from the system.
Some cars have bleeder screws on the top of the radiator or high point in the cooling system; some are just a pain in the neck to bleed. After you think it’s full, run it long enough for the thermostat to open, and watch for bubbles. Keep topping up until the bubbles stop coming.
If there is an air leak anywhere in the system, it will suck in air when cooling, rather than the coolant from the reservoir. In other words, the reservoir can be full but the engine and radiator are not necessarily full.
Note on Audis 1985 and later: The coolant in most German cars is pink (if has factory coolant). If you find a light green leak similar in color to most common anti-freeze, it is probably power steering HSMO! (See HSMO) Don’t use economy coolant in aluminum heads/engines!
Coolant type
For cars with aluminum heads or engines (many European and Japanese cars) we recommend coolant meeting Mercedes specifications. We have seen too many aluminum heads — especially Mercedes and Jaguar — that have turned into big aluminum sponges, with coolant oozing through tiny wormholes in the aluminum.
Read the label! Remember that coolants are most efficient at a 50/50 concentration. Again, read the label — some coolants are sold as “premix” which is usually cheaper, for the simple reason that they are already half water.
TIP: When adding water to coolant, use distilled if you plan on keeping the car. Distilled is not that expensive, is readily available, and import cars don’t use that much of it. It won’t make any difference now, but it will later! (Thanks to David Doiron)
Radiator
The radiator should be uniformly hot when the car is running. Cold spots can indicate clogs.
Air should flow freely through the radiator — Is it full of debris in the fins, or between the radiator and A/C condenser?
Radiator cap
Check for cracks on the seals. The typical failure pattern is for the car to spit out hot fluid and bubbles when you come to a stop, or turn the car off, when it’s hot. The temp gauge often doesn’t get over 3/4, not even to the “red”, but it can still be scary with the steaming and spitting.
Cars with reservoirs usually have caps with rubber seals both at the bottom and at the top, right under the top lid. Some vehicles with reservoirs have a pressure cap on the reservoir and a flat filler cap on the radiator (example: Volvo, Jaguar).
Electric fan
Electric fan (if fitted) should come on with both temperature and A/C. If the motor has been changed or removed, make sure that it was refitted with the fan blowing toward the engine. (Sounds obvious, but we’ve seen them in backwards!)
Hoses
Hoses may collapse, or have cracks or pinholes which are not obvious.
Thermostat
Some cars can cool OK without a thermostat, but most do not flow properly.
The thermostat is a flow control as well as a heat gizmo. Most thermostat recommendations are fairly high temperature, as the factories want them to come off of cold start as quickly as possible. In hot climates, it might be OK to go to a colder thermostat.
Remember that when the thermostat is fully open, all thermostats flow the same amount of fluid. If your car requires a thermostat with an extra plate on the bottom, make sure that the replacement has the plate of the proper diameter. This controls another coolant passage. The little “jiggle valves” are there to help trapped air escape.
On Nissan and Toyota, some models require a thermostat that has its working guts offset to fit correctly in its housing. On VW Rabbit-family cars, the little water outlet that looks like a baby thermostat housing isn’t; the thermostat is in the water pump housing.
Water pump
It’s tempting to blame the water pump when a car overheats, but this is not real common. Try to find a way to see if the water is actually moving. If the radiator is hot, the pump is probably working.
Once in a while a pump will shear off an impeller (the little finned thing that actually moves the water) and appear normal from the outside, but this is rare. Usually a bad water pump will leak through the little weep hole in the side of the casting (don’t try to plug this, it will just force the water in the bearing), or the bearing will be obviously bad — wiggly or noisy.
Water pump bearings can fail in such a way that the shaft breaks and the hub, fan, and whatever else is attached to them fly into the radiator. This is quite spectacularly horrendous; don’t ignore a noisy water pump.
Some cars have engine-driven fans that are not attached to the water pump; examples are Mercedes 300E, TR3 & 4, TR7, Saab 99. They have a separate fan bracket that has bearings that can die, but have nothing to do with the water pump.
Fan clutch
A wiggly fan that looks like a dying water pump could be a dying fan clutch. Fan clutches come in several varieties; most are viscous drive like a little torque converter. They should be fairly firm at idle and low speeds, and as the airflow hitting the fan blade drives it with more force than the engine does, it de-clutches and freewheels, eliminating fan noise and slightly increasing gas mileage.
German and some other viscous-drive clutches have a “heat button” on the outside center of the fan. These do not lock up until the airflow hitting the heat button is hot enough to expand the goop inside and lock it up.
Make sure that the airflow through the radiator is free and the auxiliary fan is working properly. A few models (Mercedes 190E, for example) have fan clutches that are big magnets that lock up the fan with an electrical signal.
Thermal fan switches
Cars with electric fans or magnetic fan clutches have fan switches, usually in the radiator, sometimes on the engine. Most are fairly large with a two-prong plug. Many Mercedes are smaller with a single round prong and red plastic around the base of the prong.
Some fan motors only run with the ignition switch on, some don’t. If your fan keeps running after the engine is shut off, it obviously doesn’t go through the switch. Don’t panic if you hear the fan after the engine is off; engines will get hotter temporarily (“heat soak”) when the coolant circulation stops.
You can test a fan switch by jumping the two terminals in the car side of the plug, with the ignition switch on if necessary, and see if the fan comes on. If if does, that indicates that the rest of the system is working and the fan switch is possibly bad.
Clean the plugs and see if it works; sometimes the problem is as simple as corroded terminals. (If it’s a single-prong switch, jump the car side of the plug to ground. This should bring on the fan if everything else is OK.) The fans usually are also connected to the air conditioning system, so that if the A/C comes on, the fan also comes on.
Timing
Insufficient advance can make a car run hot. Some emission systems have overrides to restore advance if the engine runs hot. A loose vacuum line or malfunctioning vacuum advance can cause overheating.