Engine protection mode Volvo Penta 5,7GiE

[Ingeinan]

Have a problem with my engine, Volvo Penta 5.7 iE. The engine warned that it was warm because I forgot tighten the cap to the impeller. The cap came loose during operation and the engine did not get adequate cooling. I have switched to a new impeller. After this event, I have had a problem with the engine continues to run in the engine protection mode. Can this be an error because of what happened when the engine that ran warm? If I'm running the engine over 3000RPM will it returns to 2500RPM after about 20 seconds and then down to 1000-1500RPM. Have checked the heat sensor at different temperatures and the resulting service book ohmic value. Once the engine has been turned off, I have measured the oil pressure sensor and it has no contact to ground. Have driven without oil pressure sensor and the error occurs anyway. The oil pressure on the gauge is not too low, it is measured from an independent sensor. Exhaust gas sensors have no contact with the ground when I measure them in normal temperature. Is there any danger to try to disconnect them during one minute when engine is warm and running above 3000RPM to see if there are those who put the engine in to engine protection mode? I assume that the sensors are correct. Have not heard engine buzzer sound the last times. It says it should be measured to ground. I couldn't measure it because it does not seem right with the workshop manual connection pins. Is it possible that the buzzer can get the engine into engine protection mode? Have been out for more than an hour on several occasions without any problems if the engine does not exceed 2000RPM. The temperature sensor stays at 80grader celsius which is the normal operating temperature. The oil pressure is also stable. Would like to have advice how I check that the engine has sufficient cooling water. Is there any good way to check this? Anyone that have any good idea how to flush the cooling system whiteout disassemble the engine and the gear? How can I know that there is nothing that prevent the exhaust? Grateful for all advice.