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Pelican Technical Article:

Water Pump Replacement

Steve Vernon

Time:

3 hours3 hrs

Tab:

$40 to $210

Talent:

**

Tools:

10mm socket, rubber mallet, large bucket

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Water pump

Hot Tip:

Remove the radiator

Performance Gain:

A dry garage or driveway

Complementary Modification:

Check coolant expansion tank

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 bad water pump, but it's 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.

This job is not difficult, just time consuming. To replace the water pump you will need to remove the fan and shroud, drain the coolant and loosen the drive belts. While you have gone to all this trouble I highly recommend removing the radiator; it is very easy to do, gives you lots more room and can save you from a costly mishap. Please see our articles on all of these procedures for additional assistance.

You will need to drain the coolant before you begin to remove the water pump. Modern coolants last a very long time and if you need to drain your coolant you really should consider draining it into a clean container so you can reuse 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.

Over the years your car may have been serviced by multiple people including previous owners and this can lead to parts replaced with different size fasteners and hardware. 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.

Our vehicle may vary slightly from yours as Mercedes redesigned systems and sub-system as the models developed over the years. If something is different on your vehicle please let us know and share your info to help other users. If you have any questions or comments or even a different procedure you would like to share please leave it below and please leave your vehicle information.

The water pump is driven by two belts off the crank pulley.
Figure 1

The water pump is driven by two belts off the crank pulley. While the pump itself is easy to replace you must remove some components to get access to it (red arrow). To replace the water pump you will need to remove the fan and shroud, drain the coolant and loosen the drive belts. While you have gone to all this trouble I highly recommend removing the radiator; it is very easy to do, gives you lots more room and can save you from a costly mishap. Please see our articles on all of these procedures for additional assistance.

With all of the above work completed you need to remove the water pump pulley (red arrow) from the end of the pump (yellow arrow).
Figure 2

With all of the above work completed you need to remove the water pump pulley (red arrow) from the end of the pump (yellow arrow). These usually can be wiggled off by hand but if yours is corroded you may need to use a standard puller to remove it.

Next use a 10mm socket and remove the five bolts holding the pump to the housing.
Figure 3

Next use a 10mm socket and remove the five bolts holding the pump to the housing. Be prepared as even though you have drained the coolant there will still be more that will escape when you remove the bolts and pump, so be prepared to catch it and reuse it if possible.

Try wiggling the pump out from the housing, if that doesn't happen you can try gently tapping the pump with a rubber mallet.
Figure 4

Try wiggling the pump out from the housing, if that doesn't happen you can try gently tapping the pump with a rubber mallet. If the pump has been in long enough to corrode in place or the previous installer loved to use RTV and was of the school more is better, then you may have a difficult time removing the pump. Just stick with it, it will come out eventually.

When you remove the pump be prepared for the remaining coolant to escape.
Figure 5

When you remove the pump be prepared for the remaining coolant to escape.

If your pump is heavily corroded around the flanges be sure to check the corresponding area on the housing.
Figure 6

If your pump is heavily corroded around the flanges be sure to check the corresponding area on the housing.

Use a plastic blade to scrape any old gasket or RTV off the housing and make sure to clean the mount flange and inside with a little Emory cloth to give you a clean flush mounting surface.
Figure 7

Use a plastic blade to scrape any old gasket or RTV off the housing and make sure to clean the mount flange and inside with a little Emory cloth to give you a clean flush mounting surface. If the surface is prepped properly you should not have to use any RTV. Installation is the reverse of removal. Do not forget to refill the coolant and check for leaks.

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Comments and Suggestions:
300d Comments: I have a 10 millimeter bolt that broke off in the water pump housing.Do you have any suggestions on how to remove the broken bolt in the water pump housing.Can you also get a secure fit With just using the four 10 millimeter bolts And leaving the broken off bolt in the housing?
Jeff
November 7, 2016
  Followup from the Pelican Staff: The pump will not seal without all the fasteners.

You will have to drill a small hole in center of bolt, then use a bolt extractor to remove it. - Nick at Pelican Parts
 

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