When a oil cooler goes bad, what is it that happens. Is it the water passages get pluged or the oil passages? Or do they start leaking water or oil. How can you tell if it is ok or needs replacement? Thanks.

 

Two things can happen. The water passages can get plugged and cause the EGR cooler to fail and the oil to overheat OR the cooler can spring an internal leak between the oil and water passages and the cooling system gets filled up with engine oil (BIG MESS). Monitoring the difference between the coolant and oil temps can help you know when the water passages are getting plugged but I don't think there's any way to know when an oil passage is going to rupture.