I just installed a used oil cooler in my 300 SD, When I start it up I have no leaks, Are the oil cooler hoses pressurized right away. Basically the way I drive the oil cooler never comes into use but I dont want to take any chances.

Thanks for your responses.

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1984 Euro 300SDC, (4spd standard)
1986 Toyota Landcruiser Diesel HJ60 5spd X2

Gone but not forgotten (some sold, some stripped)
1983 300 SD, 1985 300 SD, 1983 240D, 1986 300 SDL, 1985 300 SDL, 1983 300 D, 1984 300 D, 1985 300SD, 1987 300 SDL, 1983 300 SD, 1985 300 TD Euro, 1983 380SEC, 1990 300 D, 1987 300D, 1982 300D, 1982 300D, 1994 E420, 1987 300 TD, 1987 300 D, 1984 300 D

I am familiar with the 240D and 300D oil flow paths. On these engines there is only "high pressure" on the supply line when the oil temperature reaches a point where the oil needs to be cooled, and at that point some percentage of the total output of the oil pump will flow to the oil cooler. The return line is at a slightly lower pressure (so oil will flow), and it is most probable that until the thermostat on the oil filter housing opens and you get actual oil flow through the lines, the cooler itself will be at least partially filled with air, compressed to the oil pressure at the OD of the filter, but filled with some substantial volume of air. I don't believe any of this is a problem, so I wouldn't worry if I was you, especially if you don't see any oil leaks. If someone else knows this vehicle's flow path is different, please speak up. Good luck,

Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Too late now that it's installed but I would have cut a bicycle inner tube, fit the ends over the cooler lines, and used a hand pump or compressor to pressure test.

Sixto
93 300SD