How to change the Thermostat

Thermostat change W123
Reading Time: 5 minutes

Is your engine experiencing climate change?

Too hot or too cold engine will result in power loss and inefficiency in the combustion process. In the worst case total engine failure. The thermostat is actually a wear item that should be changed every 5-8 years. In some cases even between every season if you are living very far north where the winter temperatures varies extremely much between the summer season. I think this applies even for Norway especially in a Diesel where it’s difficult to get the engine warm in the winter.

So how do you change it? Is it difficult? I will tell you all if you hang on with the article.

Preparation

Parts you will need

  • New thermostat. Thermostats have different temperature  ranges where it is fully open, check with the manufacturer/ owners manual what is recommended for different ambient temperatures. For cold weather thermostats that open later are common, i.e. it will open at a higher engine temperature.
  • New upper radiator hose and clamps? Evaluate the condition of the upper radiator hose, change if it’s worn. Since a broken radiator hose might cause catastrophic engine failure. Also the hose clamps might break if they are old and rusted.
  • Engine coolant.

Tools

  • Screwdriver
  • coolant catcher/pan
  • Funnel
  • socket tools
  • Wire brush

Procedure

Start with emptying the radiator coolant. There is a plug on the underside which can simply be open by a screwdriver. When it is fully drained don’t forget to put the drain plug back in. Then go to the next step.

Upper Radiator hose w123 M110 engine
Loosen the Upper radiator hose.

Now you have to remove the upper radiator hose. Each end is attached with hose clamps. Hopefully the hose clamps tightener are easily reachable. They usually can be loosened with a small socket or a screwdriver. Choose the socket if you can since you might just destroy the screw notches if it’s stuck. Be aware that more coolant might come out when you loosen it so be ready to catch it.

After removing the upper radiator hose you have access to the thermostat housing. It is attached with 3 bolts. These on the M110 engine is usually ok to get out, but on some other engines like the OM617 they are terribly constructed. Now you have to take extreme care not to ruin the bolts otherwise your quick fix will turn into a long and unpleasant affair. The aluminium housing of the water pump housing will stick to the steel bolts after some time.

Thermostat housing w123 M110
Unbolt the thermostat housing. Notice the heavy deposits of aluminium oxide on the neck.

After the housing is removed, you have access to the thermostat. Just pull it out with your hands. Also note the placement of the rubber gasket. This have to be replaced by the new gasket which comes with the new thermostat.

Thermostat w123 M110

If you really want you can test the old thermostat by boiling it in a kettle with an accurate thermometer. When the spring opens is when the thermostat is heat rated. Probably your old one has deviated slightly from it’s original heat rating. If you are totally paranoid you can the test the new one as well!

Thermostat housing without thermostat w123 M110
Clean the housing of dirt and oil residue before putting the new one in.

Clean up the housing before putting the new thermostat in the water pump housing. Put the gasket on top of the thermostat as seen below.

New thermostat w123

Before putting it all together you have to clean off the oxidized aluminium from the thermostat housing. It is important to avoid coolant leaks in the future. This is easily done with a simple wire brush.

Oxidized thermostat housing w123
The aluminium oxidizes when in contact with water when air cannot reach the surface, such as here between the neck and the rubber hose.
Thermostat housing w123 M110
The oxide comes off easily with a wire brush. Now it will make a much better seal with the rubber hose.

The bolt on the thermostat housing should be tightened down until both the surface of the metal meets. Do not over tighten the bolts since you will end up stripping the soft threads in the water pump. Then you will be in big trouble. Use only a small ratchet to avoid the large torque. Get your new upper radiator and clamps ready.

New upper radiator hose w123 M110
New upper radiator hose and clamps.

After fitting the new upper radiator hose. You have to refill engine coolant. This is not as simple as just pouring in new coolant and closing the lid. You have to let coolant circulate and get out the air out from inside the engine.

Start by turning the heater on MAX inside the car. This will allow for coolant to circulate the entire system. Ideally you should have a large funnel which can be screwed down into the radiator, but this is a specialized tool which most of us don’t have. So start pouring coolant until you see it forming though the opening, but don’t make it full. Now you have to turn on the engine with the radiator cap open so air can escape from the engine. CHECK for coolant leaks!!! Did you remember to tighten the radiator drain plug? While the engine is running you will probably see the level decrease slightly while the engine burps, but bear in mind that coolant also expands as it heats so a level change might not be visible. When the engine starts to get warm and has burped some, try filling until the minimum mark and close the radiator cap.

Then go for a short spin. Observe the temperature gauge closely. Mine increased suddenly until over 100°C then did a sudden jump back down to 80°C when the radiator popped open. I might have had some air still in the engine and the thermostat might have been slow on the first opening since it was new. It never run above 80°C after that one time. To be on the safe side, stop the car if you see such sudden peaks in the temperature gauge.

When you are back. Let the engine cool down at least one hour, before tempting to open the radiator cap, since most likely coolant will pour out and scolding your hands. After one hour open the radiator cap and refill coolant until it reaches the maximum mark.

Cheers, Robs out!

Author: robs

Passionate about older Mercedes Benz where driving, maintaining and learning new things is what keeps me doing this. Enjoying writing about cars and teaching others on the black arts of car maintenance. Have a major in space technology engineering and currently doing IT consulting for a living.

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