Water Pump and Thermostat Replacement

An article written and kindly provided by Brett Allison

My water pump locked up on me while driving home a couple weeks ago.  Fortunately, I was less than a mile from home and could make it the rest of the way before overheating set in.  I was particularly bummed since I had just had the radiator out to replace the front main seal a couple months ago.  Now, it’s all going to have to come apart again.  At least this time, it took only 45 min. or so to remove the radiator to get started on the water pump.  I was informed that one could do the pump replacement w/o removing the radiator, but I decided I wanted to have the extra room to work, so I removed it.  I decided also to replace the thermostat and to replace all the O-ring seals (5) associated with coolant circulation at the front of the engine.   My car is a 1997 model S600 sedan.  Other model years may be different.

My pump’s failure mode.  Here are pics of the locked up pump.

It turns out the outer bearing had rusted out allowing the shaft to wobble off center.  When this happened, the impeller contacted the pump housing internally causing the pump to lock.  The inner seals were OK so no coolant ever leaked out.  My car spent its earlier life in New Hampshire.  After several New England winters and 115,000 miles, I guess it is about time for this to happen.  I assume the water pump is original to the car.

1.  Drain the coolant.  If you’re going to reuse it, drain into clean containers.  Put a tube (12 mm ID, I think) on the stopcock  outlet at the bottom left of the radiator.  The small pop out panel in the bumper on the left side allows easy operation of the stopcock  with a screwdriver without having to crawl underneath.  Pop out the panel and open the stopcock to drain the radiator and reservoir.  Have enough containers ready.  About 2.5 gallons of coolant will drain out.

2.  Remove the fan and fan clutch and then remove the radiator and hoses.  Follow Pete’s directions here to remove the fan clutch:  http://v12uberalles.com/fan_clutch.htm .  Removing the radiator now involves just disconnecting all hose connections to the radiator (water hoses and transmission cooler lines) and lifting the radiator out of the car.  When disconnecting the transmission cooler line banjo fittings on the right side of the radiator, a small amount of transmission fluid will spill out and make a mess.  You can’t avoid this as far as I can tell.  You’ll also have to move the intake cone for the left-side air box out of the way of  a plastic tab on the radiator in order to lift the radiator out.  Either pull the cone forward or just take the air box out of the car.

3.  Protect the AC condenser from damage.  Cut a piece of cardboard big enough to cover the AC condenser and tape it in place.  You don’t want to have to buy a new condenser.  I dodged a bullet this time.  I initially forgot to protect the condenser, and as I was removing the stuck fan pulley, the pulley released suddenly and struck the condenser causing a significant dent but no leak fortunately.

4.  If the poly-rib belt is still in place, loosen the bolts of the fan pulley and the water pump pulley with the belt holding the pulleys.  Then remove the belt and the pulleys.  To remove the belt, put a 15 mm socket with a breaker bar on the bolt in the center of the tensioner pulley, turn the tensioner clockwise to release tension, and work the belt off.  The pulley mounting shafts are probably rusted making pulley removal a bit difficult.  My pulleys released from the backings OK, but sliding them off was difficult.  A little WD-40 worked well. 

If your belt is no longer in place, use a screwdriver wedged between the bolt heads and the fan pulley flange or the pulley shaft to hold the pulley while you loosen bolts as shown in the pic below.  Note that this pic was taken on reassembly, which explains why everything is so clean.  The bolts should be torqued to only 10 Nm.

5.  Remove the small guide pulley at the top right of the engine.  Pop the small cover off the front of the pulley, and use a 6 mm Allen on the bolt in the center of it.

6.  If you’re going to remove all the coolant pipes to renew the O-rings and thermostat, you’ll need to remove the plastic shrouds that protect some electrical bits at the top left and right of the engine.  Each one has three 5 mm Allen head bolts holding it in place.  The one on the left cylinder head is a particular pain.  It requires disconnecting some vacuum lines and electrical connectors from switches associated with the air injection system. Then you have to battle the power steering reservoir hoses on the lower end of it.  It will come out, but it isn’t easy.  I ended up breaking the shroud at its thinnest point as I pushed it back into place. 

7.  Remove the thermostat and the coolant pipes from the cylinder heads.  Start by removing the 4 bolts (5 mm Allen) holding the thermostat housing.  I thought it would be a piece of cake to remove the thermostat at this point, but the built-in engine lifting eye blocks removal as seen in the pic below.  And, the thermostat blocks removal of the bolts holding the eye in place from what I could tell.  I ended up removing the thermostat housing and the coolant pipes from the cylinder heads as one unit.  Remove the 4 bolts (2 on each side, 6 mm Allen) holding the coolant pipes to the cylinder heads.  Pry the coolant pipes from the cylinder heads.  There was a lot of corrosion and crud built up in the joints so it was difficult to remove the pipes. 

8.  Clean all the joints and mating surfaces where O-rings seal the pipes.  This pic is of the thermostat housing seal.  All of the joints looked like this.

I used a wire wheel bit on a Dremel tool on all the O-ring seats.  It took a little over an hour.  This is where doing your own work becomes satisfying.  No mechanic who wants to be profitable would take all the time necessary to really clean things up good.  Here are the results.  Some pitting from corrosion has occurred, but mostly it’s external to where the O-rings seal the joints.

I’m a little concerned about some larger pits on the underside of the thermostat housing, where it meets the water pump.  Hopefully, the O-ring will seal it OK.

9.  Remove the water pump.  Remove the 10 bolts (nine 13 mm and one 6 mm Allen) holding the water pump to the engine.

10.  Scrape away the old gasket and clean the surface.  I used the Dremel wire wheel again.  Here’s the old gasket.

And, here’s the cleaned up surface.

11.  Fit the new water pump with a new gasket.  Directions on www.alldatadiy.com specifically say not to use any gasket sealant.  It’s difficult, however, to get the new dry gasket to stay in place as you fit the pump.  What I did was dab a very small amount of silicone gasket sealant on the very outer edge of the bolt hole “ears” on the gasket then placed the gasket on the engine block.  That was enough to stick it in place.  Torque bolts to 21 Nm (not very much at all, but it’s enough).

12.  Fit all the new O-rings, install the new thermostat, and reinstall the thermostat housing and coolant pipes on the cylinder heads.  I wet the new O-rings with distilled water as a lubricant to help with installation.  I had to knock the thermostat housing slightly with a hammer to get the thermostat bolt holes to line up with the water pump again.  The power steering pump reservoir hoses have to get squished a little for the thermostat housing to line up correctly.

13.  Reinstall the pulleys.  Torque bolts to 10 Nm.  I had one issue reinstalling my water pump pulley.  The water pump has a drain or vent pipe exiting just behind the pump pulley.  I suppose this allows the air in the cavity between the inner and outer shaft bearings to escape as the engine heats up in order to prevent the seals from being pushed out of place from gas pressure.  It also could be a coolant drain in the event of failure of the inner seal. 

Well, this tube comes pretty close to the back side of the pulley, which has a strange lip all the way around it.  My guess is that the tube sort of fits inside this lip, which helps to prevent water, salt, and any other debris from entering the cavity and corroding the pump bearings from the inside out.

The pump I bought is made by a German company called LASO.  Apparently it’s not OEM since the pulley came into contact with this tube and couldn’t be installed.  I used my trusty Dremel with a cut-off wheel to hack the drain pipe a bit to make clearance for the pulley.

14.  Reinstall everything else in reverse order of removal.  The small guide pulley gets 30 Nm.

15.  Refill with coolant (50/50 mix of distilled water and MB factory coolant).  Start and run until warm with the heater running to fully circulate the coolant and clear out air bubbles.  Top off the coolant as necessary, and check for leaks.

16.  It’s probably a good idea to replenish the lost transmission fluid.  Not much comes out, but it’s worth making sure the transmission is filled properly.  Unfortunately, you have to break the red plastic clip on the dipstick tube cover to be able to add fluid, and you have to buy a dipstick to measure the fluid level properly at 80 degrees C fluid temp.  Thank you, Mercedes-Benz, for making simple maintenance difficult.

Brett

Thanks go to Brett for this fine write up!

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