If the oil cooler was leaking, then you need to carry on cleaning the cooling system, what are you using to clean the cooling system? I use a liquid detergent for washing clothes, two whole bottles per system fill, try not to froth it up too muck when filling water, isolate the filter, you will need to do this about 10 times, then I use Fleetgaurd Restore 3 or 4 times, then add antifreeze and DCA The only other places oil can get into the coolant are cylinder block - cracked, cylinder head - cracked, cylinder liner - porous. Remember any residual oil will float on the top of the header tank once it has been flushed through the system, and will stay there. Hi Paul, The reason you need to test the old cooler is to see if it was leaking, if it was not leaking, then the oil leak issue was not addressed, and we need to look further. If you drain the cooling system from the bottom, the oil will "line" everything out comes in contact with, and there is a lot more to clean out at a later stage. The best way to remove it is to force it out the top, by overflowing the header tank, before draining the cooling system. Oil leaking into the coolant does not go through the same violent mixing that coolant entering the oil does, so does not become emulsified, the oil will float to the top of the coolant, and will be prevalent in the header tank portion of the radiator. It could come from a cylinder head gasket, I have not seen this in a K19. I have seen oil in the coolant from a porous liner, however this engine also had coolant in the oil. There is special tooling required to pressure test the oil cooler, your local Cummins dealer should be able to do it. (USED) Cummins BCIV 88NT Warranty: 180 Day Parts and Labor Assembly: CPL 838 Serial: 43117919 Horsepower: 315 CUMMINS BCIV 88NT, 315HP. Good, Used Fan Clutch (spins good) off of a Cummins ISX 15. This is located on the right hand side of the engine, behind the coolant header plate. 1989 Cummins Big Cam IV Engine for a International S2200. 628 Cummins Fan Clutch/Hub Heavy Truck Parts for sale. Hi Paul, The most likely place the oil has entered into the coolant and the radiator is from the engine lubricating oil cooler.