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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

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.

The Mercedes Benz 450SL is considered by many to be one of the most attractive cars Mercedes produced. It is a stylized combination of power, grace, and comfort and was sold under the model designation R107. At over 3,600 pounds and designed to meet (and actually exceed) strict safety regulations, the 450SL was nicknamed "der Panzerwagen", which means "the armored car", by the engineers who designed it. The 450SL was produced from 1973 through 1980, after which the R107 became known as the 280, 380 and 500 SL.

The SL variant was a 2-seat convertible/roadster with a standard soft top and optional hardtop and optional folding seats for the rear bench. The designation SL derives from the German Sport Leicht, or Sport Lightweight and was first applied to the infamous gullwing 300SL. The 450SL was the third generation SL. The SLC (C107) derivative was a two-door hardtop coupe with normal rear seats. The SLC is commonly referred to as a 'SL coupe'. This was the first time that Mercedes-Benz had based a coupe on an SL roadster platform rather than on a sedan, replacing the 280 and 300 SE coupé.

The robust, V-8 powered coupe is a joy to drive and many drivers test the power of the 450SL. While the speed off the line is questionable, the top end performs smooth and responsive giving the driver a sense of security. The 450SL cruises comfortably at speeds in excess of 75 mph. Of course that means that the engine will heat up. That heat has to be managed and that is where the cooling system comes in.

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.

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. 

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