Dons right.. could be thermal expansion. You'll want one if you don't have one for the expanded water caused by engine heating
The relief valves on all waterheaters (in the US anyway) have a T&P (temperature and pressure) relief. So excess pressure will open the relief too.
Two quick options come to mind. one install a 170 degree T stat on the engine.
Could rig a temperature control with a normally closed asco 12V solenoid interlocked with the engine ignition so the solenoid opens when the engine runs and the water temp is below 150-160.
Some of the water heater manufactures also sell a non-powered temp control, but not sure if its sold seperate from the water heaters..