I've noticed lately that my engine runs hot on muffs with the hose, about 180 degrees, but ok in water, about 140 degrees. Seemed like this started this summer one time when i noticed her getting hot on the hose and saw there wasn't much water coming out of the exhaust, then she made a big spurt of water and steam out the exhaust but continued to run hot. I had the impellor off earlier this year before this happened and it was fine. Could it be anything other then the impeller? The thermostat is also new last year.