Temp at 140 while cruising and 165 while pulling

[mf01]

I think my thermostat may be going bad. In the past it has always run around 160. This weekend it would run about 165-170 while pulling someone (18-22mph). Later while cruising and not under load, it would run about 140. Should I just replace the thermostat? I guess it could be stuck partially open.

[TX-Foilhead]

There's a whole list of things that could cause that, impeller going bad, debris in the filter, loose clamps letting air in, pieces of an old impeller blocking the system, and the thermostat. Best thing I've found is the bucket test, search for it here or over at CCF. It's simple, and let's you pinpoint the problem without having to spend a bunch of time guessing.