3512 engine vent problem

Hello friends, need your urgent help..our marine engines 3512C are heating up frequently during loads. as we were studying the theory,we found that there is vent line which is (or should be) connected to the expansion tank which can be vented out so as to allow trapped air or gas to escape. however we are unable to find that line or any vent valve.Cant locate in parts manual also. Pls help in 1. finding the vent line and valve along with photo /picture/drawing showing the same. 2. at what frequency we have to do venting? 3. there are many plugs shown on top of thermostat housing and lines connecting to thermostat housing? what is the purpose of those plugs? prompt reply will be highly appreciated