Thermostat Testing

WARNING: This page is about a different car, the 1988 Pontiac 6000, 1988 Oldsmobile Cutlass Cruiser, 1988 Oldsmobile Cutlass Ciera, 1988 Chevrolet Celebrity, and 1988 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.

Visually inspect thermostat for corrosion and proper sealing of valve and seat. If satisfactory, suspend thermostat and a thermometer in a container with a 50/50 mixture of anti-freeze and water. See Fig 1.
Do not allow either thermostat or thermometer to touch bottom of container as this concentration of heat could cause an incorrect reading. Heat water until thermostat just begins to open.

Fig 1: Testing Thermostat in Anti-Freeze/Water Solution
Courtesy of GENERAL MOTORS CORP.

NOTE: Support thermometer so it does not touch bottom of container.
Read temperature on thermometer. This is the initial opening temperature and should be within specifications. Continue heating water until thermostat is fully open and note temperature. This is the fully opened temperature. If either reading is outside of specifications, replace thermostat, as it is not adjustable.
NOTE: General Motors Corp. recommends hanging thermostat in 33% glycol solution at 25°F (4°C) above temperature stamped on thermostat. Valve should open. Remove thermostat from solution and place in similar solution at 10°F (-12°C) below stamped temperature. Valve should close.