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Pelican Technical Article:

Thermostat Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$10 to $40

Talent:

**

Tools:

10mm socket flathead screwdriver

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Thermostat, O-ring, coolant

Hot Tip:

Work on a cool car

Performance Gain:

Proper operating temperature

Complementary Modification:

Flush the radiator

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 and should never be attempted. Also, be sure to wash your hands thoroughly after working around it.

A couple of safety precautions/instructions you may want to observe before beginning:

Allow cooling system to cool down to a coolant temperature of less than 90°C. Open the cap of cooling system slowly; turn a conventional coolant cap as far as the first detent and turn a screwed coolant cap approx. 1/2 turn and release the pressure. Wear protective gloves, protective clothing and eye protection. Always dispose of used coolant in accordance with the regulations in your region; NEVER pour coolant down a drain, on the ground or into a sewer.

Coolants last a much longer time than they did in the past, so if your coolant is still good there is no need to dispose of it. Drain it into a clean container and you can reuse it. If your coolant is old this is a great time to drain and flush the complete system.

Remember your car may have been serviced before and parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve as they grow older. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

While you do not need to fully drain all of the coolant from the system you do need to drain it to the point that it is below the bottom of the thermostat housing.
Figure 1

While you do not need to fully drain all of the coolant from the system you do need to drain it to the point that it is below the bottom of the thermostat housing. Please see our article on coolant drain and flush for additional assistance with that project.

The thermostat housing is located on the front right side of the engine (red arrow).
Figure 2

The thermostat housing is located on the front right side of the engine (red arrow). Since you have already drained the coolant, use a flathead screwdriver and remove the radiator to thermostat upper hose (yellow arrow) as this will give you more room to work.

Move the A/C line out from the bracket on the front of the thermostat housing (red arrow).
Figure 3

Move the A/C line out from the bracket on the front of the thermostat housing (red arrow). Use a short flathead screwdriver and remove the clamp and hose from the bottom of the thermostat housing (yellow arrow). Even though you have drained the coolant be prepared for additional coolant to come out when you remove this hose.

Use a 10mm socket and remove the three bolts holding the front of the thermostat housing in place (red arrows).
Figure 4

Use a 10mm socket and remove the three bolts holding the front of the thermostat housing in place (red arrows). These bolts get corroded over time so make sure the socket is well seated, you do NOT want to strip these bolts or you are going to make an easy job a lot more difficult.

With the bolts removed pull the thermostat and cover off.
Figure 5

With the bolts removed pull the thermostat and cover off.

Check where the cover mounts to the housing and clean it with a wire brush to make sure there is a good clean flat mounting surface when reinstalling the cover (red arrow).
Figure 6

Check where the cover mounts to the housing and clean it with a wire brush to make sure there is a good clean flat mounting surface when reinstalling the cover (red arrow).

If your thermostat has not been changed in a long time the rubber gasket can become brittle and difficult to remove.
Figure 7

If your thermostat has not been changed in a long time the rubber gasket can become brittle and difficult to remove. Make sure you get all of the old gasket out from the cover.

Make sure to clean the inside of the cover as well.
Figure 8

Make sure to clean the inside of the cover as well. If your cover looks like this one it is time to give your system a good flush.

Clean everything up with a wire brush and some cleaner, you do NOT want to gouge the matting surfaces or you run the risks of leaks.
Figure 9

Clean everything up with a wire brush and some cleaner, you do NOT want to gouge the matting surfaces or you run the risks of leaks. Installation is the reverse of removal and do not forget to fill and bleed the coolant.



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