If you are driving and notice that your coolant temperature gauge is getting into the red, you might be tempted to shut your vehicle off. However, you should only do this if you are noticeably leaking coolant. Otherwise, you should just pull over and let let the car idle. Shutting off the car will shut off the cooling system as well, causing the engine to boil over. This is known as heat soak, and you want to avoid this if possible. While idling, turn on the heat to full blast ( this should help remove heat from the engine) and press the gas pedal a bit (this ups your coolant flow). But if you don't see the temperature gauge drop soon, or if you don't feel warm air coming from your vents, you should shut off the vehicle right away; in this case you either have a bad water pump or not enough coolant, and keeping the car idling can cause big problems.
Michel Filion. flickr
Now it's time to diagnose the problem!
If you are only overheating at low speeds, or during city driving, check and see if the fan clutch is working properly. Get your engine warmed up and open the hood. Your fan should be running. Then shut off the engine (or have someone else do it) and make sure the fan stops quickly. It should stop spinning in a few seconds at most. If it takes longer that 4 or 5 seconds, you might have found your problem. A new fan clutch is not too expensive, and should be an easy fix.
If the fan is working properly and you are still overheating, check to see if revving the engine while idling brings down the temperature. If so, you have a problem with coolant flow. Once the engine is fully cooled, check your radiator and coolant reservoir to make sure you have enough antifreeze or antifreeze/water mix in the system. If not, now is the time to check for leaks. The usual suspects here are the seal at the water pump, hose clamps going into and out from the radiator, and the connections between plastic and and metal parts in the radiator. Adding more coolant should buy you time to get replacement parts or fix the leak. Another major culprit is the radiator cap; the seals can fail but it can be hard to diagnose. You need a cooling system pressure tester, but auto-parts stores will have one for rent, and some will test this for you for free!
Blown Head Gasket.
If you have a cracked head or or a bad head gasket, the coolant leak may be internal. White smoke and rough idling are a sign that coolant is getting into the combustion chamber. Run the engine with the radiator cap off and check for bubbles; if you see them then you have a bad head or head gasket, which is leaking pressure into the coolant system. Another way to see if you have an internal leak is to check your oil, if it looks milky or if the coolant in your reservoir looks brown, coolant and oil are mixing. You can determine if you have a head gasket blown by doing a compression test. Available for rent at the store, the tester will show if cylinders are getting enough compression; if not there is probably a bad gasket between them. A final diagnostic method involves sending in an oil sample from your vehicle to a testing facility, like Blackstone Labs (https://www.blackstone-labs.com/). For under $40, they can tell you if your fluids are mixing and you need to replace a head gasket.
