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2000 S430, 2003 S430, 2003 S600 TT, and 2005 E320 CDI
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After replacing all coolant hoses that M-B still makes and pressure-flushing the system with new coolant (pressure flusher was a 2-gallon Motive tank designed for this purpose ), I discovered a leak.

After searching all throughout the car, none of the hoses were leaking. All hoses are Genuine-MB. Turns out it was at the water pump. So, I ordered a remanufactured water pump from MBOemParts.com, and I replaced it. Also did the two belt idler pulleys and the belt tensioner pulley assembly, since I was in there anyway and the bearings were just beginning to make the tell-tale noise.

To replace the water pump, you have to remove the fan assembly and the serpentine belt. The fan assembly is so that you have room to actually do the work and see what you're doing. Fortunately, it's not difficult to do. You don't absolutely have to remove the lower or upper radiator hoses to get it out, but it does make it easier, and since it's a water pump job, you might as well flush the system anyway. So, no harm done in pulling those two hoses.

Also, it helps a lot to pull the plastic black piece, shaped and sized about like a Slinky, that connects the air filter to the turbo's input. It takes a bit of oomph and manoeuvering to get it out, but it's definitely doable. Don't forget about that little 1" diameter hose on the Slinky's side that goes to the top of the valve cover! Remember to disconnect that as well; fortunately, that's pretty easy. Removing this "Slinky" is also needed to replace two of the coolant hoses hiding under there, by the way, which I had already done a few weeks ago.

Once you get the fan and the Slinky out, the water pump is accessible. One of the idler pulleys mounts on there, so go ahead and remove it. You'll need to, in order to get to one of the water pump bolts.

Then, just start removing bolts. They are Torx bolts, and there are two sizes, E10 and E12. All the E10's are the same length, as are all the E12's, though I still kept the same order since it was my first time doing it.

There's a metal gasket for the water pump; it comes with the pump if you buy it from M-B. That gasket was showing rust after 15 years, and it is there, I believe, where the small leak was happening. So, I took some Scotch-Brite and cleaned up any rust stains from the mating surfaces on the engine's timing cover, where the pump mounts. Worked like a charm. Don't use sandpaper, since the mating surface is aluminum and it needs to stay fairly smooth and even. Scotch-Brite is quite sufficient, I can assure you.

Mount your new pump w/ metal gasket and torque to manufacturer specifications. The torque specs are as follows.

Little bolts (E10): 14 N-m
Big bolts (E12): 20 N-m

Reinstall your idler pulley(s). Note that the new pump is not threaded for the idler pulley bolt. The idea is that you cut the threads yourself by screwing the bolt in. This appears to be common with Mercedes-Benz water pumps; the S500's new water pump required this as well. Don't worry; the water pump housing is aluminum, and the idler pulley bolt is steel, so it works out fine. A dab of any grade of motor oil can help with this. It takes a little oomph the first time you screw it in, since you're cutting new threads, but it's not like you need The Incredible Hulk's strength to do it. Hardly.

Then, reinstall your serpentine belt. My owner's manual has the path that it's got to follow, so I just used that.

Put in some new coolant, either the yellow G05 (the original spec) or the blue G48 (the newer spec), mixed for your environment. I use a 50/50 mix with distilled water. Either G05 or G48 is fine for the OM648 engine, and both are fully compatible with each other. This time, I happened to use G48 since I'd bought some on sale and thus had it handy. On these engines, it doesn't seem to require a whole lot of "burping" of the system. I just kept filling it until the level stopped going down. Take your car for a drive, with I'd say a gallon of 50/50 coolant mix, and check it as your engine comes to temperature; that part's the same as when you do a regular ol' coolant flush. Check your fluid level over the next week or so, and top off as necessary until burping's done. It shouldn't take very long, again, just like with doing normal coolant flushes.

Check for leaks! Make sure you haven't messed up something. Get in there with a flashlight and have a look. See if anything's still wet.

Did I mention to check for leaks?

And that should do it.
 
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