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Mercedes Thermostat Replacement
 
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Pelican Technical Article:

Mercedes Thermostat Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$20

Talent:

*

Tools:

E10 Torx driver

Applicable Models:

Mercedes-Benz C320 (2001-04)
Mercedes-Benz CLK320 (1998-04)
Mercedes-Benz E320 (1998-04)
Mercedes-Benz ML320 (1998-03)
Mercedes-Benz S320 (1998-99)
Mercedes-Benz SLK320 (2001-03)

Parts Required:

New thermostat

Hot Tip:

Work on a cool car

Performance Gain:

Proper engine temperature

Complementary Modification:

New hoses and clamps

The thermostat helps control the engines temperature. If your car is running too hot and there is the proper amount of coolant in the car and it is not leaking, or your car is taking a long time to warm up, there is a very good chance your thermostat is bad and needs to be replaced.

If you are going to be working on the thermostat, make sure the car is cool and not under pressure. Working on a hot engine or one under pressure can cause serious harm (coolant is also very toxic) and should never be attempted. Coolant needs to be collected and disposed of in accordance with your local regulations as pouring coolant down a drain or into the street is illegal. Also, be sure to wash your hands thoroughly after working around it.

You will need to remove the two air inlet ducts as well as the front engine cover to get access to the thermostat and give yourself enough room to work.

Remove each duct by compressing them towards the engine and slipping them off the air inlet.

With the ducts off remove the front engine cover. It pulls up and away from the engine. The cover is held on by five clips and will easily come off with hand pressure.

You will need to remove the Poly-V or drive belt from the front of the engine to get the thermostat out. Using an E10 (reverse Torx socket) and driver locate the tensioner and using the Torx driver turn it counter-clockwise 45 degrees. You can now slip the belt off the tensioner. You can also lock the tensioner into the open position if you need both hands to remove or install the belt.

Turn the tensioner all the way counter-clockwise and insert a retaining pin between the rotating part and the tensioner base. If you do not have a retaining pin you can use a 5mm Allen key. Carefully inspect the belt and replace as necessary. I always carry a spare belt in my trunk and if the belt on the car is over a year old it might be a good idea to install the spare and order another one for the trunk.

There is a picture below that shows you the routing of the belt that you can use to reinstall the belt when you are done.

With the car cool, begin by releasing any residual pressure in the system by removing the cap on the coolant reservoir. Note: Never open the cooling system when the car is still warm.

Next, move to the thermostat itself and unscrew the large hose coming out of the unit. Most of the coolant for the car is below this level but there will be some coolant that spills so be ready for it with catch bin and a rag.

Use a flat head screw driver and remove the clamp from the hose attached to the thermostat, then remove the hose. The hose may stick a little and you may have to run a screw driver between the hose and housing to break it loose.

Remove the two E10 Torx bolts connecting the thermostat housing to the engine block.

Gently wiggle and pull the thermostat from the block. Do not try and pry anything between the pump and block.

There is a rubber gasket sealing the thermostat to the block, this must be replaced every time you remove the thermostat, even if you are reusing the same thermostat.

Do not try and separate the thermostat from the hose outlet. Unlike most thermostats this one comes permanently attached into the outlet housing.

If you order a new thermostat it will come with the gasket included.

Installation is the reverse of removal.

You will need to remove the two air inlet ducts (red arrows) as well as the front engine cover (yellow arrow) to get access to the thermostat and give yourself enough room to work.
Figure 1

You will need to remove the two air inlet ducts (red arrows) as well as the front engine cover (yellow arrow) to get access to the thermostat and give yourself enough room to work.

Remove each duct by compressing them towards the engine (red arrow) and slipping them off the air inlet.
Figure 2

Remove each duct by compressing them towards the engine (red arrow) and slipping them off the air inlet.

With the ducts off remove the front engine cover.
Figure 3

With the ducts off remove the front engine cover. It pulls up and away from the engine (yellow arrow).

The cover is held on by five clips (red arrows) and will easily come off with hand pressure.
Figure 4

The cover is held on by five clips (red arrows) and will easily come off with hand pressure.

Locate the tensioner and using the E10 Torx driver turn it counter-clockwise 45 degrees.
Figure 5

Locate the tensioner and using the E10 Torx driver turn it counter-clockwise 45 degrees. You can now slip the belt off the tensioner and install the new one.

You can also lock the tensioner into the open position if you need both hands to remove or install the belt.
Figure 6

You can also lock the tensioner into the open position if you need both hands to remove or install the belt. Turn the tensioner all the way counter-clockwise and insert a retaining pin between the rotating part and the tensioner base. If you do not have a retaining pin you can use a 5mm Allen.

This is a picture that shows you the routing of the V-belt; the tensioner is shown by the red arrow.
Figure 7

This is a picture that shows you the routing of the V-belt; the tensioner is shown by the red arrow. Note: the front of the car has been removed to give you a better view.

With the car cool, release any residual pressure in the system by removing the cap on the coolant reservoir.
Figure 8

With the car cool, release any residual pressure in the system by removing the cap on the coolant reservoir. Note: Never open the cooling system when the car is still warm.

Next, move to the thermostat itself and unscrew the large hose coming out of the unit.
Figure 9

Next, move to the thermostat itself and unscrew the large hose coming out of the unit. Most of the coolant for the car is below this level but there will be some coolant that spills so be ready for it with catch bin and a rag. Use a flat head screw driver and remove the clamp from the hose attached to the thermostat, then remove the hose. The hose may stick a little and you may have to run a screw driver between the hose and housing to break it loose.

Next remove the two E10 Torx bolts connecting the thermostat housing to the engine block.
Figure 10

Next remove the two E10 Torx bolts connecting the thermostat housing to the engine block. Gently wiggle and pull the thermostat from the block. Do not try and pry anything between the pump and block.

There is a rubber gasket sealing the thermostat to the block, this must be replaced every time you remove the thermostat, even if you are reusing the same thermostat.
Figure 11

There is a rubber gasket sealing the thermostat to the block, this must be replaced every time you remove the thermostat, even if you are reusing the same thermostat.

Do not try and separate the thermostat form the hose out let.
Figure 12

Do not try and separate the thermostat form the hose out let. Unlike most thermostats this one comes permanently into the outlet housing.

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Comments and Suggestions:
Caribbeanboy Comments: Hi my 2002 mercedes C240 takes me so long to get warm and its not even enough to fight a 45degrees F

It's getting cold and I do not know what to do and I do not have to much money to spend to get it fixed.
October 27, 2016
  Followup from the Pelican Staff: Sounds like you need to replace the thermostat. - Nick at Pelican Parts  
Chola Patrick Comments: My c200 m111 engine over heating badly help
May 11, 2016
  Followup from the Pelican Staff: Check the thermostat and water pump, also be sure the coolant level is correct. - Nick at Pelican Parts  

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