Coolant leaks - Replacing Radiator Hoses and Engine Thermostat

About the thermostat.

The thermostat is located between the engine and the radiator coolant duct and acts as a temperature controlled engine coolant valve. If this valve fails and slams shut, so does the cooling system. An overheated engine is often the unfortunate end result. The good news is thermostats are inexpensive and fairly simple to replace. Symptoms of a failed thermostat are an engine that takes forever to warm up, or an engine that's running hot despite every other part of the cooling system being in good shape. So while driving, do make a habit of glancing at the  temperature gauge as well once in a while

The primary function of the thermostat is to allow the engine to warm up quickly by restricting coolant flow to the radiator during the first few minutes after the engine is kicked over. Getting the engine to operating temperature quickly helps it run more efficiently. Sensors peering into engine coolant tell the computer how much fuel to add and at what idle speed to run the engine on cold start. Fuel is added in greater quantities and RPM is kicked up a few to prevent cold stalling. These same sensors tell the computer to give the fuel "rich" condition a rest once the engine is warm. In colder climates getting the engine warm fast also means warm drivers and passengers, as heater operates by way of toasty warm engine coolant. See the figure below to get an idea of what a thermostat looks like :

The secondary function of the thermostat is regulating coolant flow to maintain steady engine operating temperature and cooling system efficiency. Each thermostat has a temperature at which it will fully open. Engines are designed to run most efficiently at a certain temperature and for this reason it's important to always replace a thermostat with another of the correct temperature rating. Installing a 160-degree thermostat into an engine designed to operate at 195 degrees can cause poor fuel economy, as the computer will meter an incorrect amount of fuel.

An engine running too cool can fool the engine computer into running the engine too rich. When a thermostat fails outright one of two things happen. The better thing is when the thermostat gets "stuck" open. The worst thing is when the thermostat gets "stuck" closed, and corks up hot coolant in the engine. Coolant trapped inside a running engine quickly overheats. A bunch of problems can arise from an overheated engine - none of them good for the engine. Engine overheating is the number one cause of cylinder head cracking and failure. Melted and cracked aluminum engine components such as cylinder heads are much more expensive to replace than a thermostat. As draining the coolant or removing the radiator hoses are the first steps in replacing a thermostat this is a good time to check and replace the thermostat.

About the coolant hoses :- These are the hoses running from the radiator to the engine block and back , that carry the coolant. Some times there are also hoses that carry coolant between parts of the engine block. Ageing can damage these hoses. Sometimes temperature sensors plug into these hoses at various points. Some times leaks can also happen at these points dues to a worn out or aged O ring type washer/seal. See example of coolant hoses here :

How to find leaks : Park your car overnight and put a large white piece of paper underneith your car. In the morning check the paper. If the paper is wet with liquid, you could have a coolant leak either from hoses,sensor points or the waterpump itself. If the paper has green,blue or red (depending on your coolants color) fluid on it, there is a good chance your vehicle is leaking coolant (sometimes refered to as "water"). In many cases, when you are leaking coolant it could be coming from the water pump.


Any type of repair done on the cooling system always involves draining the entire coolant and then replacing the damaged part involved and then filling back the coolant. So here is what you need to do the job :-

Thermostat and Gasket


Distilled Water (if coolant is not pre-mixed with water already)

Container to drain coolant into ( I usually use 1 GAL milk or water jugs)


You can buy thermostats and hoses online or the Parts section of the authorized dealer. Sometimes they might need the VIN number to give you the exact part. If your car uses a special type coolant , make sure to buy that as well ahead. Here is an example of the BMW blue coolant :

Thermostats almost always reside in a housing at the engine side end of the upper radiator hose. Drain coolant into a container. Be sure not to pour it on the ground where children and animals can be endangered by it. If you accidentally spill fluid, wipe it up thoroughly before hosing down the area, and then place the rags in a sealed plastic bag before disposing of them. Loosen and remove radiator hose to access thermostat housing. Consult the service manual if the housing cannot be found. Loosen the bolts that hold down the thermostat housing. Remove the housing to reveal thermostat. Note the orientation of thermostat relative to the engine. Take a picture of the orientation with a digital camera. Remove thermostat. That small valve in the thermostat body allows trapped air to pass, and needs to be on one side or the other.

Testing a thermostat is easy. Go to your kitchen and put the thermostat in a saucepan full of cold water. Turn on the heat. Watch the thermostat, and at what temperature it opens. This temperature is usually on the bottom of the thermostat itself. If the thermostat does not open the problem has been found! Clean saucepan carefully. Use a scraper to remove old gasket material from all mounting surfaces. All of it.  The surface should be smooth and clean now when you run your hand.

Place new gasket on housing. Use a small amount of grease to hold it in place during reassembling. Install new thermostat. Spring side down. Make sure new thermostat is oriented properly and reinstall the housing. Look up the torque specs of the bolts on the repair manual and do not over tighten the bolts on the thermostat housing. The aluminum housings are fragile and can easily crack.

Replacing a damaged coolant  hose is easy as well. Basically remove the old one by loosening the clamps on it and wriggle it out. Putting a new hose on  is just the reverse. After all is done Close radiator Drain plug (or petcock). Reconnect all hoses. Either return original coolant to the cooling system or refill the radiator a 50-50 mix of new coolant and distilled water. See here a can of distilled water.

Minerals in tap water can contribute to cooling system corrosion buildup. Fill radiator 3/4 or so full. Start engine. Slowly top off radiator with engine running to burp any trapped air from the cooling system. Replace radiator cap. Check overflow tank level. Add coolant if required. Do this 2-3 times and you should be done

Make sure to check the level again after 2-3 days of driving as well.