I think its pretty rare for a water pump impeller to go bad and not pump enough volume but it has happened.

If you have a spare T stat and want to try something, drill a small hole in the round part that covers the by pass. when the main part opens this round plate covers the by pass. Theres a big long explanitation of why this works, and it worked for me, but if you have no luck give it a try and see.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"