Learning To Diagnose Problems With Our Cooling System

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You pull up to a traffic light and see steam coming from under the hood of your vehicle. Now the coolant temp light has come on. Perhaps you are driving down the road in cooler weather and you note that the car seems to be using more gasoline and the heater does not work. Both of these scenarios are indicative of cooling problems. The good news is that it does not take a trained mechanic or special tools to diagnose a cooling system problem and it only takes an hour or so to check it out.


  1. Notice when your car runs hot. This can be due to the failure of a number of different cooling components.
  2. Allow the engine to cool before you begin to examine your engine. Never open the cap when the engine is still hot
  3. Check for leaks.This should be the first action you take to determine if you have a cooling system problem.
    • Make sure the car is full of coolant, that it is at operating temperature, and that it is running under correct pressure.
    • Find the different water hoses with the hood of the car raised. There is an upper and lower engine hose and two heater hoses. Grab the upper hose and give it a squeeze. It should feel like it has pressure inside. Similar to squeezing a bicycle tire, if there is no pressure there might be a leak or the radiator cap is bad.
    • Check to be sure that there are no leaks around the hose clamps or a split in the hoses.
    • Look around the radiator to make sure that it hasn’t sprung a leak and isn’t, therefore, the source of your problems.
  4. Remove the radiator cap after the car has cooled.Now, restart the engine and bring the car up to operating temperature.
    • Look into the opening. You should see coolant flowing if the thermostat is working properly. If you don’t, then the thermostat is stuck closed and you will need to replace it.
    • If you are having heater problems, then you may have a problem with the heater core, which can become blocked and need to be replaced. Find the two hoses that are smaller than the engine hoses, they run to the back of the engine compartment (bulkhead). One of these hoses should feel slightly warmer than the other when the engine reaches operating temperature and the heater is turned on. If one is significantly cooler than the other, the heater core is closed off and may need to be replaced. However, some cars have a valve to shut of the flow of coolant to the heater core. Depending on where you grab the hose, the hose might be cold.
    • Have the radiator cap checked at an auto supply store to be sure it is holding pressure. Replace it with a new one.
  5. Look for other problems.
    • A water pump can also be the cause of a cooling problem. Reach down and check for a weep hole at the bottom of the pump (with the engine off).
    • The water pump is failing and needs to be replaced if you feel coolant or dampness.
    • The water pump may have broken impellers. In this case, it does not pump enough coolant through the system and needs to be replaced.
    • Another problem could be a stopped up radiator. This can happen in vehicles where the cooling systems are not routinely flushed or coolant replaced. Coolant should be replaced at least on a yearly basis. The fix is to replace or rebuild the radiator.
    • A final overheating problem on vehicles with electric cooling fans may be that the fans are not working. With the vehicle parked and running at operating temperature, check to see if the fan comes on and pulls cool outside air over the radiator. If not, it could need replacement. Before you do, take it to a trusted mechanic and have him verify it isn’t a blown fuse or bad connection.
    • Overheating isn’t the only problem a vehicle coolant system can have. Some vehicles may run too cool. While it is not as rough on your engine as overheating, it can result in poor gasoline consumption. Optimum operating temperatures assure proper combustion. This problem is similar to overheating, except the thermostat is staying open.
    • Check the thermostat while the car engine is cold. Remove the radiator cap and start the engine. Look down into the radiator. The thermostat is stuck open and needs to be replaced if you see coolant flowing. You may also have a problem with the heater core. Check for heater core problems following the steps outlined above.

Article provided by wikiHow, a wiki that is building the world’s largest and highest quality how-to manual. Please edit this article and find author credits at the original wikiHow article on How to Diagnose a Cooling System Problem. Content on wikiHow can be shared under a Creative Commons License.

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