MTU 4000s. Do the coolant thermostats fail in the open or the closed position? I know Cat 35 series thermostats will fail in the open position so you will not over heat but actually over cool the engine. I got a call from an engineer friend of mine as his MTU oil temps are alarming high (90-93C) on both main engines at high load. Coolant temps are good on both engines as the coolant also cools the oil cooler. The oil cooler was already serviced and not the issue. Techs now want to change thermostats on both engines after servicing oil cooler that was not the problem. Any help is appreciated.