What should you look for when your car overheats when you stop at a light?

Answer 1

This is almost always a fan-related problem. Sometimes a failing water pump or slipping belt can also cause this symptom.

Mechanical Fan:

With the engine stopped and cold, look for obvious damage to the fan or belt. If so, correct the problem by replacing the appropriate part. Never straighten a damaged fan blade, as this can lead to flying shrapnel later. If no problem found, spin the fan by hand and note the resistance. Now, warm the engine fully, and shut it off. Spin the fan again and if it is not much more difficult to spin, replace the fan clutch. Look for any missing or damaged fan shroud.

Hydraulic fan:

Check your Power Steering fluid level and correct as needed. If this is a Chrysler product, verify that incorrect type fluid has not ever been installed, or just replace all the power steering fluid with the correct type fluid. If this a GM truck or SUV, contact your dealer and ask if there are any corporate bulletins requiring replacement of the fan.

Electric fan:

Check all fuses in the vehicle, both in the engine compartment and in the vehicle interior. Consult your owner's manual to be sure you locate all fuse blocks. Some cars have fuse block in the trunk or under the back seat or behind a little door here or there. I am aware of Lexus models with as many as seven separate fuse blocks.

Verify that the engine coolant is completely full.

Visually inspect each cooling fan; there may be only one or as many as four. Warm up the engine until it is very close to overheating (above 240 degrees Fahrenheit.) With the engine running, look to see if the fans are spinning (they should be) and then shut the car off. Keep your hands away from fan blades. Many cars will spin fans even with the key off. Note any non-spinning fans.

Using a long tool, turn any non-spinning fans manually. Difficulty turning the fan or noise or vibration while turning the fan means the fan motor is in need of replacement.

Unplug the fan connector and apply 12V to the wires going to the fan. If it doesn't turn, the motor is bad.

Start the engine and gently tap the fan relay(s) with the handle end of a screwdriver. If this makes a fan spin, the relay must be replaced.

Check the coolant temperature sensor(s). Unplug the sensor looking for corrosion, and bent or missing pins. Start the engine with the sensor unplugged. If the sensor has two wires, use a paper clip to connect the two wires in the connector that is unplugged from the sensor. If it uses one wire, use a jumper wire to connect it to batter negative. If any of this immediately spins the fans, replace the sensor and refill the coolant using approved fill methods.

One or more radiator fans not working. Turn on the A/C and see if they are running. they should run continuously with the A/C on. If not check the fuse first.