Summary theory (short)
- The thermostat is a spring-loaded valve with a wax element that expands at its rated temperature (typically ~82 °C / 180 °F on Mazda BP setups). Below that temp it stays closed, forcing coolant through the heater/core/bypass so the engine warms quickly. At or above its opening temp it opens and allows full flow through the radiator to maintain operating temperature.
- Symptoms: stuck-open = long warm-up, poor cabin heat, reduced efficiency. Stuck-closed = rapid overheating, high temp gauge, possible head gasket damage.
- Replacing the thermostat restores the correct temperature-controlled restriction so the engine reaches and holds its designed operating temperature.

Ordered procedure (do this exact sequence)
1. Safety & prep
- Work on a cold engine. Relieve any pressure by removing the radiator cap only when cold.
- Gather 1 new OEM-spec thermostat and gasket/O-ring, coolant, catch pan, basic hand tools, torque wrench, scraper, clean rags, funnel, and optionally jack/stands for access.
- Park level; block wheels.

2. Drain coolant (partial)
- Place drain pan under the radiator/drain petcock. Open petcock and drain enough coolant to drop level below the upper radiator hose/thermostat housing so it won’t spill when you remove the hose/housing. Catch fluid for reuse or proper disposal.

3. Access thermostat
- Locate thermostat housing where the upper radiator hose connects to the engine (on the BP 1.8 this is a small housing on the cylinder head/thermostat island).
- Remove clamps and hose from the housing if necessary to get access.

4. Remove thermostat housing
- Remove the housing bolts and carefully separate the housing from the engine. Expect some coolant spillage.
- Note orientation of the old thermostat as you remove it. Many Mazda thermostats have the spring facing the engine block; there’s normally a small bleed/jiggle hole that must be oriented at the top when installed.

5. Remove old thermostat & gasket; clean surfaces
- Pull out the thermostat. Note any corrosion, stuck valve, or deformed rubber.
- Remove old gasket/seal and thoroughly clean mating surfaces on housing and engine block; scraper and solvent as needed. Do not leave old gasket material.

6. Install new thermostat
- Install new thermostat with the spring side facing the engine and the jiggle/bleed hole at the top of the assembly (this lets trapped air escape). Use the correct orientation and new gasket or O-ring per the part instructions.
- Light sealant only if the gasket requirement specifies it; do not over-apply RTV that will intrude into coolant passages.

7. Reinstall housing and hose
- Refit thermostat housing and tighten bolts evenly to manufacturer torque (tighten in a cross pattern). If you don’t have the exact spec, snug then a final small torque; do not overtighten and distort the housing.
- Reconnect upper radiator hose and clamp.

8. Refill coolant
- Refill with correct coolant mix (typically 50/50 long-life ethylene/glycol for Mazda). Use a funnel and fill to the radiator neck (if accessible) and overflow reservoir to the proper level.

9. Bleed air from the system
- Open any bleed screw if present. Start the engine with the radiator cap off and heater on max (fan and temp to hot) so heater core fills.
- Let engine idle until thermostat opens (watch temperature gauge and feel upper hose become hot). Watch for air bubbles escaping from radiator neck; add coolant to maintain level.
- Once bubbling stops and temperature stabilizes in normal range, top off and install radiator cap.
- Cycle heater on/off, blip revs, and monitor for air / temperature fluctuations. Test drive and recheck levels after cool-down.

10. Final checks
- Inspect for leaks around housing and hose. Verify proper operating temperature (gauge mid-range) and good heater performance.
- Properly dispose of old coolant.

How the repair fixes the fault (mechanical theory)
- Faults: If the thermostat is stuck open, coolant bypasses the radiator constantly and the engine cannot reach designed temp. If stuck closed, coolant cannot pass to the radiator, so heat builds up and the engine overheats.
- Replacement restores a functioning wax-element valve that opens at the correct temperature. This reestablishes controlled restriction so the engine warms quickly (improving efficiency, emissions and cabin heat) and then allows controlled flow through the radiator once the operating temp is reached to prevent overheating.
- Proper installation (orientation, sealed mating surfaces, correct bleed) ensures there is no trapped air or leakage that would prevent the thermostat sensing correct coolant temperature or allow coolant to bypass.

Common pitfalls (brief)
- Wrong orientation of thermostat or blocked jiggle hole → air trapped → false temp readings/overheat.
- Reusing old gasket or over-torquing housing → leaks or warped housing.
- Not bleeding air → overheating or heater voids.
- Using incorrect coolant or wrong thermostat temp rating → poor control of engine temperature.

Done.
rteeqp73

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