3-second answer: OBS trucks use a 192-195°F thermostat (195 stock). Swap: drain below the housing, unbolt, replace with the jiggle valve UP, torque the bolts lightly, refill and bleed (see [cooling flush](/problems/cooling-flush/)). The 5.0/5.8 housing is plastic and twists easily — use a 6-point socket and don't reef on it.
Test before replacing
- Stuck closed = overheating at speed. Confirm with the upper-hose test: it should get hot once the engine warms — if it stays cold, the T-stat isn't opening.
- Stuck open = runs cold, poor heat, worse mileage.
- The classic trap: a 'thermostat problem' that's actually air in the system (see bleeding) or a water pump with a shed impeller (the 5.0/5.8 composite impellers are notorious).
The job
- Cool engine, drain below the housing.
- Unbolt the housing (keep the 2 bolts — they strip into plastic housings; helicoil if you do).
- New gasket + T-stat (jiggle valve up or aligned with the bleed notch).
- Torque the housing bolts gently — these housings crack/warp if over-tightened.
- Refill + bleed per the cooling procedure.
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Threads
- FTE search: thermostat · overheating