for about a month I had to constantly add water to my overflow tank- looks like it gets too much pressure in there. I have replaced the tank and a cap so far.Please help with any ideas.
The car is '87 300TD with about 140000 miles and a radiator that was serviced in a shop about a year ago. While driving, the car does not overheat, but after being parked it leaks the coolant unless there is only very little left in the bottom(light comes on).

Are you finding puddles of coolant?

your engine is known to have cracked heads. The coolant can be escaping without you noticing it through the combustion chambers

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any coolant on the ground?

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

http://www.peachparts.com/shopforum/showthread.php?t=224942&highlight=cracked+head

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Yeap, coolant on the ground.

Try using a little compressed air to pressurize the system when the car is off. This will force coolant out where it is leaking. And you can poke around looking for the leak, without parts moving. I would put my money on the water pump.

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Where under the car. it is coming out the overflow hose, upper & lower rad hose, waterpump, t-stat housing, head gasket. We need more info.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

My bet is on water pump or thermostat seal.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

X2

also check your connections and any cracks in your hoses. Also look closely at your radiator.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)