I just bought a 2005 Nissan UD 245 series truck. It runs well, no smoke, no water in engine but when I open the radiator capp I found a fair bit of oil in the water. I flushed the radiator and filled it up with new coolant and ran it for 20 minutes only to find loads of oil in the radiator again? Could I have a problem with my oil cooler, water pump or do i have a blown head gasket or a cracked engine...I need some basic advice before i rush off to mechanic and open my wallet. you help is much needed.
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Obviously you have an oil leak. Where it's at you won't know until you (or your trusted mechanic) tears into it to figure it out. You could test your oil cooler theory by bypassing the cooler (I'm assuming your cooler is inside of the radiator?). Your problem, like you said, could be the cooler, a head gasket leak, or a crack somewhere ... I would put them in this order of probability, though. It would not be the water pump as in most every engine I've seen there isn't any oil passages going to it, just coolant.
As far as water not being in the oil, just remember that oil pressure is usually 20+ psi, while a radiator cap only keeps about 16 psi. If there is a leak, the first direction it's going to go is from the oil into the water.