I have a 1990 Mariah and just repaired a heating problem. I replaced the impeller and the thermostat and remove parts of the impeller from on top of the thermostat .Went to the lake an the temp was back to normal. After about a hour the temp when over 200 deg. When back to the house and took all the hoses loose and ran water threw each hose and transmission cooler no blockage. Took the raw water pump back out and the impeller was good.But the inlet to the raw water pump hosing was full of old impeller.You might check there. I still don't know how it got there since it is not a closed loop system. Hope this helps.