'03 W203 C270cdi - how to change the water pump?

Greetings - for the past few days, my 107,000 mi 03 C270cdi ( South African assembled) has for the past 24 hours been emitting alarming puffs of water vapour from under the hood at standstill, when fully
warmed up after either highway or city driving. It is also losing coolant very slowly, and not pressurising the cooling system much. My main suspect is the water pump - all hoses seem ok, no symptoms of head gasket failure, radiator is dry, some wetness around the front of the engine bay that looks like it has been flung around by the pump pulley. It may have been failing slowly for some time - maybe twice in the past 35,000 miles the warning for low coolant has come on, a small top- up has dealt with it each time. I think the puffs of vapour are caused by drops of coolant landing on hot things when the car is moving - the puffs stop after a few minutes ticking over. Temp gauge goes no higher than 85 degrees. There is a squeally sound from the front of the engine on tick-over which I think might be the pump bearings giving up.
My request for help and advice from the group is:
(a) does my diagnosis seem reasonable - are these pumps known to fail at this age/milage?
(b) what is the right way to get at the pump, how much of the radiator assembly will have to come out? There seems to be an awful lot of plumbing to be disturbed if the radiator has to come out to gain access. There is not much free space around my engine - 5-cylinder.
(c) How much coolant does the system hold - just so I buy enough with the pump from the MB dealer.
Regards, & thanks in advance
John
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Diagnosis is not adequate the way you describe. You need to get a coolant pressure tester and pressurize your cooling system when cold... that way you can look for any seeping... from hoses, radiator, waterpump, heater aux. pump, and heater hoses.
Water pump is easily diagnosed... you grab the waterpump pulley and if you can wobble it, then the bearing is shot... also when pressurized, you will see leaking from the weaping hole.
It is most likely to be leaking from your radiator before anything else. Radiator in my experience last about 100,000 miles or a bit more, but not that much more.
All cars takes about 1.5 gallons to 2 galllons to fill up... provided you did not drain engine coolant.
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Thanks Tiger - your advice is spot-on as usual. I borrowed a pressure tester, and at 15 lbs a small drip showed from the water pump pulley. With the engine running, it would have landed on the front pulley / harmonic balancer and it would have been flung around, leading to steam from no particular source. The pulley on close examination seems slightly out of paralell which would also indicate that its bearings are shot. I'll change the waterpump today, and tips on getting in there?
John

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My advice is get a service manual for your car... that way, no surprise when you are working on it. Always follow the specification for tightening those bolts.
As for gasket, put gasket dressing on one side of the gasket and adhere it to the waterpump. Otherside to engine is clean.
That's pretty much it.
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OK - I eventually got the pump out, took a while to work out how the fan assembly comes free. Comparing my car with a friend's '00 E200, there is far more clear room in front of his engine than mine so I have a lot more dismantling to do for this project on my model. It turns out that the new pump I was sold by the MB dealer, ordered with the chassis number, is wrong. The original pump has a permanently- fixed smooth pulley. the new one has a flange for a bolt-on pulley (or fan), and the pump sticks out considerably more. I'm going back to the dealership today to try to sort it out.
John
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