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Is Your Car's Thermostat Failing? Warning Signs And Testing Tips You Need To Know

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The coolant in your car's radiator plays a critical role in engine protection and operation. After all, if your engine doesn't have sufficient or effective coolant, it is likely to overheat and may cause serious structural damage. The thermostat controls the flow of coolant through the engine, and many overheating problems can be traced back to its malfunction. Here's a look at what you need to know about thermostat trouble and how to fix it.

How it Works

The thermostat installed in your car is built to operate at a specific temperature. When the coolant in your engine reaches that temperature, the thermostat opens up to allow the coolant to flow through the engine and cool down. This cooling process helps to keep the engine cool. When the engine cools below the temperature of the thermostat, it closes and blocks the flow of coolant through the system. If the mechanism sticks and doesn't open or close the way that it should, it can cause problems in the engine.

Recognizing Thermostat Failure

If the thermostat sticks closed and blocks coolant flow, the engine will overheat. If the temperature of the engine gets into the bright orange, or unsafe, range on the car's temperature gauge, it could potentially damage the head gasket and possibly the engine block. If your car starts running warmer than it usually does (as evidenced by the temperature on the dashboard gauge), or it's overheating, the thermostat may be faulty.

Fluctuating engine temperatures can also indicate a failing thermostat. This happens when the thermostat is failing gradually. You'll see fluctuations in the temperature on your dashboard if this is happening. If your temperature gauge isn't steady and consistent, it indicates that the thermostat isn't maintaining consistent coolant flow.

How to Test and Replace a Bad Thermostat

You've narrowed down your car's symptoms to what appears to be a bad thermostat. The next step is to test the thermostat to see if it's the culprit. You'll want to have a new thermostat, gasket and gasket sealant on hand for this. If it turns out that you don't need the new thermostat, you can usually return it.

  1. Removing the Existing Thermostat: Let the engine cool completely, then remove the cap from the radiator. This prevents an air lock in the system when you remove the thermostat. Place a large bucket under the radiator's drain plug Then loosen the plug to drain the coolant level enough that it is below the upper hose on the radiator. This ensures that there's no coolant backed up against the thermostat when you remove it. Follow the top radiator hose from the radiator end to the aluminum housing at its other end. That housing holds the thermostat. Disconnect the hose from the housing by removing the hose clamp and pulling the hose off. Then, remove the bolts on the housing with a small wrench. Pull the housing loose and remove the thermostat from the housing.  
  2. Testing the Existing Thermostat: Look at the thermostat to see if the valve is open or closed. If it's closed, test it to see if it's opening properly. To do this, bring a pot of water to a boil, then put the thermostat in the water. If the thermostat is working properly, the valve will open. If it doesn't open, that means your thermostat is faulty and needs to be replaced. If, on the other hand, the thermostat is open when you remove it, you can test it by dropping it in a bowl of ice water. If the valve doesn't close in the ice water, it's stuck. That means you need a new thermostat.  
  3. Installing the New Thermostat: Clean the surface of the thermostat housing and the mounting area on the engine block with a putty knife. This removes any residual gasket material that could interfere with the necessary seal. Then, wipe everything clean with a damp cloth. Place the thermostat in the housing, then attach the new gasket to the housing with some gasket sealer. Make sure that the thermostat housing is properly lined up so that you can replace the bolts, then press it into place. Insert the bolts and tighten them until you can't tighten them anymore by hand. Then, replace the upper coolant hose on the thermostat housing. Refill the coolant system, then let the engine idle until it warms to its normal operating temperature. Check around the housing to ensure that there are no leaks.

If you aren't comfortable doing this on your own, or the thermostat doesn't appear to be the problem, you should talk with a local bmw auto repair shop to troubleshoot and repair the problem for you.