My 351 PCM starboard engine overheated. Zebra mussels (lake Erie) plugged the oil cooler water inlet causing a water pump impeller failure . I replaced the impeller and cleaned out the damaged impeller piece's. Took the boat out on the lake at cruising speed temp climbed to 180,had to stay under 2000 rpm to keep it at 160. I checked the output of the water pump and it looked good. I took the thermostat out and tried again. The temp stayed at 140 at cruising same as the port engine. Do you need to have the thermostat? Can I continue to run without it? Any ideas on what's going on?