Question, can having the wrong thermostat or faulty thermostat cause an engine not to get to max rpm. one weekend boat is running great saw 5000 rpm wt 4 adults and fuel. the next weekend 4500 max both engines with 2 people. Changed fuel filter and cleaned flame arrestors off engine. noticed that both temp gauges reading 155 wide open and when slowing down to idle temp drops to 100 then works it way back up to around 140-150. Aren't 350 mag mpi engines suppose to have 160 thermostat.

Thermostat sticking open could cause the engine to run cool and the EFI to dump more fuel to it causing it to run rich. Although it would be odd for them to both to start sticking at the same time.

I THINK you're correct they do run 160 thermostats.