The spring is there because it allows the thermostat to lift off its seat at higher speeds, for additional cooling. 143 degrees is correct. 15s are great motors with one big problem: Over time, the upper water tube grommet under the power head gets pushed in by corrosion and blocks a great deal of water flow, which will lead to overheating. It is an odd situation because often, you can’t tell this is happening by just looking at the tell tale output. If it were my engine I would spend the time, pull the power head and clean this area up, replacing the grommet. You’ll also need the power head base gasket. Have a propane or Mapp gas torch ready. You may need it on the inner exhaust housing/tuner bolts. Then again, you might not, if you’re lucky.
Long live American manufacturing!
