Its a chronic problem with raw water cooling and cast iron engines. A flake of rust (could be sand too) gets stuck in there and it won't close. I have done this to see if I can free it: run it up on plane, slow to idle, before it cools off too much tap around the thermo housing with a small hammer. That might dislodge whatever is keeping it from closing.
_________________ 88 Four Winns 200 Horizon 4.3 OMC Cobra-4bbl 2002 Walker Bay 10/2012 Suzuki 2.5 2008 Walker Bay 8
1998 Jeep Grand Cherokee 4.0/Selectrac 2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II
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