Diagnose Vehicle Overheating Problems

There are many reasons a vehicle can overheat, each of which causes a different overheating symptom. Stop the car as soon as practical if the temperature light comes on or the temperature gauge moves into the red zone. If the temperature rises only when the motor is idling, you most likely have a fan issue since no air is moving through the radiator when the car is not in motion.

Things You'll Need

• Radiator pressure tester
• Common screwdriver
• Phillips screwdriver
• Common set of 3/8-inch drive sockets
• Ratchet
• 2 Alligator clip jumper leads
• Volt meter

Instructions

1. Remove the radiator cap and check the fluid level. If it is full, proceed to the next step. If not, fill the radiator to the top, replace the cap and fill the overflow bottle half full.

2. Install a pressure tester on the radiator and pump it to a pressure of 14 pounds per square inch (psi). Look for a leak at the radiator, hoses, engine, water pump and the heater core. Look under the car. If you find a leak, and follow it to its origin. A heater core leak can be identified by water exiting out of a short hose on the firewall, under the heater hoses. Water pump leaks show at the bottom of the pump. A radiator may show evidence anywhere, so examine it carefully.

3. Check the radiator cap and make sure it has a good rubber seal and that it has a stiff spring. Squeeze the seal. If the spring is not stiff, replace it---it could be the cause of a leak that could lead to overheating.

4. Look at the manual fan if your vehicle is equipped with one. Check for any obvious damage. Rotate the fan and see if there is a small amount of resistance to turning. Spin the fan a little and see if it stops relatively quickly or continues to spin. If it continues to spin, the silicone in the fan clutch may be missing. If the vehicle has front wheel drive and an electric fan, check the fuse. It is located on the driver's side fender well in a relay center. Look for the one designated "engine cooling fan." You should see a strip of metal attaching the two metal legs inside the plastic fuse.

5. Start the engine and let it heat up. In the case of an engine with only a light and no gauge, the engine should not be run for more than ten minutes or until the valve covers get too hot to touch comfortably. If you have a vehicle with a temperature gauge, it can be run until the gauge shows just over 240 degrees (only if necessary for the next test). If too much pressure builds up, it can cause a hose to blow off or split, or it can cause the radiator cap to blow, causing severe burns if you get hit with the water.

6. Test the thermostat function. Allow the engine to cool for at least half an hour. Remove the radiator cap. Start the engine. When the thermostat opens, the radiator fluid will fill up the opening for the radiator cap and may even overflow a little bit, then the fluid will be sucked back into the radiator. If your vehicle is equipped with a gauge, check the gauge to see what the temperature is. Go to Step 7. If this does not occur and the expulsion of fluid becomes stronger, shut the motor off and let it cool as this is an indication that the thermostat is stuck shut and needs to be replaced.

7. Watch for the fan to begin blowing much harder when the thermostat opens. This can be tested by holding your hand behind the fan near the top of the engine. Test the airflow when the engine is first started and again after it heats up so you can recognize the difference. Put your hand in the front of the radiator by the grill and see if you can feel airflow being sucked into the radiator. If the engine continues to heat and the fan makes no noticeable increase in speed and volume, shut the engine off. The fan clutch is at fault and needs to be replaced.

   If the fans are electric, the fan will come on when the thermostat opens. If the temperature begins to climb and the fans fail to come on, there is a problem with the fan circuit or the fans themselves.

8. If the fans fail to come on, remove the plug from the coolant temperature sensor. The coolant temperature sensor is usually located at or very near the thermostat. The thermostat is located at the end of the top radiator hose where it connects to the motor. Removing the plug will set the check engine light, which can be turned off by removing the negative terminal of the battery for two minutes after completion of all tests. Repeat Step 7. If the fan comes on when the coolant temperature sensor is unplugged, the sensor should be replaced. If the fan does not come on, shut the motor down and let it cool so that the pressure build up in the overheated engine does not blow a hose or radiator cap off.

9. Turn the key on and look for the cooling fan relays in the fuse and relay box on the driver's fenderwell. There could be as many as three, two of which indicate speed. Pull them out. Use the voltmeter to test for voltage by holding the negative on the negative battery terminal and sticking the positive in each of the leg-holes on the relay plug-in. If there is voltage, push the relays in slowly. Listen for a click. If they fail to operate (click), switch them with another relay with the same numbers, if available. If not, replace them. If there is voltage and the relay does not click, the problem is with the relay or fans. To test the fans, pull the connector off the engine fan. There will be two wires. Hook one of the alligator clips to one terminal on the fan and to the positive terminal of the battery. Hook the second alligator clip to the other terminal of the fan and next touch it to the negative terminal of the battery. If the fan fails to come on, switch the clips on the battery. If they still fail to come on, the fan is at fault and needs to be replaced. If the fans come on, then the relay is at fault and needs to be replaced.

10. In the event, all the previous tests were passed then the radiator is the only thing left, and it must be replaced as it is clogged up and has insufficient flow.

Tips & Warnings

• Always work with the cooling system when the engine is cold. Removing a radiator cap when the engine is hot can cause burns. It is important to remember to keep your hands clear of the fans and any moving components at all times, as personal injury can occur.