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Pelican Technical Article:

Replacing the Thermostat

Mike Holloway


1 hour1 hr


$100 to $600




13mm wrench

Applicable Models:

Mercedes-Benz R107 (1972-89)

Parts Required:

Thermostat, gasket, MBZ, coolant

Hot Tip:

Make sure engine is cool

Performance Gain:

Better performing engine from thermo-management

Complementary Modification:

Replace hose, fan clutch, water pump

If your car is starting to run hot or it is overheating while sitting at idle, it's possible you may need to replace the thermostat, seals or the water pump. Typically, the seals and bearings are the first items to fail. An easy way to check your water pump is to look underneath the car and check for water seepage out of the pump. This means that water is leaking past the bearing and seals, indicating the pump is on its way out. If the seals have failed there will be coolant leaking behind the drive pulley. You may not need a new pump but new seals are in order. If the bearings have failed you may hear a squealing or there will be too much play when you move the drive pulley back and forth. Another way to check to see if it is pump failure is to squeeze and hold the upper radiator hose for a few seconds (CAUTION: the hose may be hot) while the car is running. If you feel no pressure building up, it is a sign that water is not circulating and the pump may be bad.

When a water pump begins to fail, you'll notice that the car tends to overheat at low engine speed, such as sitting at a stoplight. When you accelerate, the engine temperature will drop. Now, this is not always indicative of a water pump, but a good starting point. You may also want to try squeezing the top radiator hose with the engine warmed up and running. You should feel pressure build up on the back of the hose and surge once it is released. If you feel no pressure, it's a fair bet that the water pump is failing. Sometimes, you can also see a slight drip coming from the water pump housing itself. These weep holes are cast into the pump housing and allow excess coolant to drain out if it has leaked past the various seals in the pump. One is located on top, the other below.

Make sure the car is cool before you begin working on it. You will drain the coolant from the radiator. To drain the coolant, locate the drain plug on the bottom of the lower front passenger side of the radiator. Place your drain bucket under the hole and turn the plug and empty the contents into a proper drain pan. Coolant will begin to come out but will not drain rapidly because of the vacuum caused by the sealed system. To get all the fluid out, go up to the coolant reservoir and undo the cap. This will break the vacuum and allow the coolant to flow freely. Be careful not to spill any coolant on the ground. If you have pets, they may lick it up and die.

Before you replace the water pump, remove the thermostat and put it in very hot water. In fact, fill a saucepan with water, submerge the thermostat in it and heat the pan. As the water gets hot, the thermostat should open. If it doesn't then it needs to be replaced.

Removing the thermostat is straightforward but will require draining of the coolant. After the coolant has been drained, remove the hose leading to the thermostat.

Before you do any work on your car it is important that you wear safety glasses, work gloves and dispose of all fluids in a safe manner. Coolant is poisonous and should be treated with great care. Animals and small children have been known to die from ingesting coolant. If you have to jack up your car, make sure to use jack stands and chock your wheels as well as applying the parking brake.

Using a 13mm socket, loosen and remove the two bolts holding the thermostat to the water pump.
Figure 1

Using a 13mm socket, loosen and remove the two bolts holding the thermostat to the water pump.

The thermostat is not ready to be tested.
Figure 2

The thermostat is not ready to be tested. Installation is the reverse of removal. 

Comments and Suggestions:
mecccatwin Comments: Just to the right of the alternator, Attached to the left side of water pump. Water pump is directly behind a pulley. There is large coolant hose coming out the bottom of the thermostat housing
January 27, 2017
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
mecccatwin Comments: 3 bolt not 2 for the thermostat housing. They are 10mm bolts my car is 1978 450slc same engine as 450sl
January 27, 2017
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
george Comments: where is the thermostat located
November 3, 2016
  Followup from the Pelican Staff: In the water pump. - Nick at Pelican Parts  

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Page last updated: Wed 1/17/2018 02:53:05 AM