You can remove the old thermostat, place it in a pot of water with a thermometer and bring the water to a boil to see at what temp the thermostat is fully opened. But if it was me, if I'm going through all the work of removing the thermostat I would just replace the thermostat with a new Whaler thermostat. My dad on the other hand will just test it before replacing it since he doesn't like to buy new parts, but his time is free.

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2006 Infiniti G35
2001 BMW 3.0I E53 X5 Build date 08/2000 SOLD
Lotus Europa 1970 Destroyed by fire
Lotus Europa 1970 S2 Renault Powered
Lotus Type 52 1970 Twincam Webers Powered
PORSCHE 911 Targa 1982 The Garage Queen
Audi Avant donated to Kars for Kids
BMW 525IT Sold
Audi 4000CS Quattro Sold
Jensen Healey Lotus Powered Sold
Opel 1900 Sold
Triumph Spitfire 1971 Sold
Triumph Spitfire 1968 Sold
Plymouth "Cuda" 340 Six pack SOLD