What Are the Signs of a Failing or Leaking Injector Cup? An injector cup failure is a serious event that can lead to catastrophic engine damage if not caught early. Because the cup separates the fuel system from the cooling system, leaks can go in two directions. Here are the tell-tale signs: Sign #1: Coolant in the Oil (The Milky Oil) This is the most common and devastating sign. If the O-rings or the cup itself fail, coolant can seep past the injector and drain down into the crankcase. Visual: The oil on the dipstick will appear milky, foamy, or chocolate-milk colored. Result: Coolant destroys the lubricating properties of oil, leading to rapid bearing failure. Stop the engine immediately. https://www.cq-longshine.com/cummins-injector-cup