The next morning after my 930 mile one day holiday drive I find the classic coolant puddle dripping off the oil pan. Most likely the water pump from what I can find in the archives here.

However, the goofy thing doesn't seem to drip noticeably while it's running, only after it stops. I let it sit for the past 6 days and it lost the overflow tank contents, took about 1/2 gallon to fill it back up.

Is this normal for a water pump to only leak while sitting, or should I be looking for something else like a thermostat gasket?

I have no idea how many miles are on the pump, I bought the 300 SEL about 12k miles ago and it now has 162k.

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(2) 1988 300 SEL W126

You probably need a new pump. When the engine is running (and warmed up) the cooling system is pressurized, so any leaks will be flowing more than a cold engine. It could be by design that your pump leaks less when the engine is running, but don't quote me on that. You need to be sure of exactly where the leak is coming from. Pumps have a "weep hole" that coolant will come out of only after the pump is at the end of its' life.

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Fourings

84 Audi Coupe GT with Turbo engine, EFI (buh-bye CIS), Jamex seats, Koni's/H&R's, e-code lights. 210k and counting.
1992 Anthracite 400E, 161k, "Grey Ghost".
1986 VW Quantum Syncro, 192k, gone to the crusher in the sky.
1989 300TE 172k. (Was mine, now also Mom's car)
72 280SE 4.5 (Mom's car)

I spent the day talking to some real MB mechanics in the Dallas/Ft. Worth area - they said the leaking water pump only when shut off was normal and it was driveable until you heard the bearings start to make noise... that is the sign of impending doom.

The bearings are still solid when I grab onto the water pump pulley and try to feel for play, so it should be ok as far as traveling to a shop.

I agree with you that somehow the pump must be designed to leak less when the system is pressurized... which is real lifesaver when you're on the road.

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(2) 1988 300 SEL W126

that.

rather i would think that when running the air from the fan spreads it around so much that you dont notice it in any one place.

but you are headed the right way... to get a new one.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.