There are a number of things that can go wrong with your car's cooling system, causing the engine to run warmer than it should. The following is a list of things that are easily checked at home by the do-it-yourselfer, and can always add to your knowledge of cars in general.
Understand that overheating problems may be caused by a low coolant level, the radiator being plugged, the thermostat may be stuck, or other related common cooling system problems.
Check the coolant level at the radiator overflow/plastic coolant reservoir tank to see if you're low on coolant (see "How to Check Your Car's Coolant Level," under Related eHows). Add if necessary.
Open the radiator cap and look inside the radiator when the engine is cold.
Fill the radiator with a 50/50 mix of antifreeze and water if it's empty or low, and close the cap.
Look at both the lower and upper radiator hoses located at the top and bottom of the radiator on the back side and held by hose clamps. Make sure the hoses are securely clamped to the radiator and aren't leaking.
Touch both the upper and lower radiator hoses when the engine is warm and the car turned off. They should both be somewhat warm. If one is cold, you may have a thermostat that's stuck closed.
Use the palm of your hand to feel the outside of the radiator from top to bottom when the engine is warm and the car turned off. It should be warm to the touch evenly throughout. If there's a cold section, you may have a radiator that's internally blocked.
Check under the car, inspect the radiator and look around the engine compartment for telltale signs of a coolant leak: Coolant is usually greenish, slippery and sweet-smelling.
If there is always a puddle of coolant under your car it could be due to water pump failure, or a cracked coolant reservoir.
Visit your mechanic if you know there's a leak but can't find it. A mechanic can detect a slow or small coolant leak by pressurizing the cooling system.
