So there is thick black oil in my water system on my transit 07 MWB van
Got a new Oil Cooler to replace mine, as it's a high possibility this why there's oil in the water.

But will be needing to flush out all the water system, pipes, rad a lot to clean out the oil.

Should I do with with the old oil cooler on? 
Or fit the new oil cooler then do all the flushing?

 Thanks   

with the old cooler still conne4cted you will still be pumping oil in system. so replace the oil cooler and flush the system, best way to flush is with plenty of liquid clothes detergent and hot water, fill system and run engine for 10 mins, drain from bottom hose and redo, will take lots of flushing, and still you will residue for months after collecting in expansion bottle

I have changed cooler flushed system till clear but headder black and oil in it again??? Not overheating but black as in headder replaced it be for started flush as old one was full with thick oil any ideas anyone new to diy mechanics  got me a hayens manual lol 

Edited August 14, 20223 yr by Jonboy2508
More to add

Ihave grey sludge in my header tank but no sign of water on the oil dipstick is it likley that the head gasket is ok and it might just be an oil cooler issiue /