Thermostat

Replace the engine thermostat to restore proper engine operating temperature on the 390ci V8, including draining coolant and refilling the cooling system.

Warnings

Tools required

Parts

• Thermostat (180°F or 195°F) × 1 — Use OEM specification for AMC 390ci V8
• Thermostat housing gasket × 1 — Use OEM specification
• Coolant/antifreeze × 1 — Ethylene glycol based coolant

Fluids

• Engine coolant (ethylene glycol 50/50 mix) — 4 qt

Preparation

1. Ensure engine is completely cold to the touch
2. Park vehicle on level surface and engage parking brake
3. Place drain pan underneath radiator drain petcock (driver side bottom of radiator)
4. Remove radiator cap slowly to release any residual pressure

Procedure

1. 1

   Drain cooling system
   Open the radiator drain petcock at the bottom of the radiator by turning counterclockwise. Allow coolant to drain into the pan. If equipped, also open the engine block drain plug on the passenger side of the block to drain additional coolant. Collect and properly dispose of old coolant.
2. 2

   Disconnect upper radiator hose
   Locate the upper radiator hose connection at the thermostat housing on the front of the intake manifold. Loosen the hose clamp using a flathead screwdriver or pliers, then twist and pull the hose off the thermostat housing neck. Be prepared for residual coolant to drain out.
3. 3

   Remove thermostat housing bolts
   Remove the two bolts securing the thermostat housing to the intake manifold using the appropriate socket or wrench (typically 1/2" or 9/16"). Keep these bolts as they will be reused.
4. 4

   Remove thermostat housing and old thermostat
   Carefully pull the thermostat housing away from the intake manifold. The thermostat will either stay in the housing or remain seated in the intake manifold recess. Remove the old thermostat and note the orientation of the spring-side (spring faces toward the engine block).
5. 5

   Clean gasket surfaces
   Use a wire brush, gasket scraper, or razor blade to thoroughly remove all old gasket material from both the intake manifold mating surface and the thermostat housing mating surface. Ensure surfaces are completely clean and smooth with no debris or old gasket remnants that could cause leaks.
6. 6

   Install new thermostat
   Place the new thermostat into the recess in the intake manifold with the spring side facing DOWN (toward the engine). The thermostat should seat flat in the recess. Verify the temperature rating matches your requirements (typically 180°F for performance or 195°F for standard operation).
7. 7

   Install new gasket
   Position the new thermostat housing gasket over the thermostat, aligning it with the bolt holes and the recess in the intake manifold. Some gaskets have a rubber seal that fits into a groove; ensure proper seating.
8. 8

   Reinstall thermostat housing
   Place the thermostat housing over the gasket and align the bolt holes. Thread both bolts in by hand first to ensure proper alignment. Tighten the bolts in an alternating pattern evenly to avoid warping the housing. Snug firmly but do not overtighten (approximately 15-20 ft-lbs is typical, though no official spec is provided).
9. 9

   Reconnect upper radiator hose
   Slide the upper radiator hose back onto the thermostat housing neck, ensuring it is fully seated. Position the hose clamp over the hose on the neck and tighten securely with a flathead screwdriver.
10. 10

    Close drain points and refill cooling system
    Close the radiator drain petcock and any engine block drain plugs that were opened. Using a funnel, slowly pour fresh 50/50 coolant mixture into the radiator until full. Fill the overflow reservoir to the appropriate level mark.
11. 11

    Bleed air from cooling system
    Start the engine with the radiator cap off and allow it to idle. As the thermostat opens (typically after 5-10 minutes), coolant level will drop. Continue adding coolant as needed to keep the radiator full. Rev the engine slightly to help purge air bubbles. When coolant level stabilizes and no more bubbles emerge, install the radiator cap.

Reassembly

1. Verify all hose clamps are tight and properly positioned
2. Check that no tools or parts were left in the engine compartment
3. Inspect the area around the thermostat housing for any coolant leaks while engine is running

Verification

• Run engine until it reaches full operating temperature and verify the upper radiator hose becomes hot, indicating the thermostat has opened
• Check temperature gauge reads in the normal range (typically 180-195°F depending on thermostat rating)
• Inspect thermostat housing and hose connection for any signs of coolant leaks after engine reaches operating temperature
• Allow engine to cool completely, then recheck coolant level in radiator and overflow reservoir, topping off as necessary
• Test drive vehicle and monitor temperature gauge to ensure stable operation under load

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