How to replace a thermostat in a cooling system?

Check the coolant level, drive belt tension, and temperature gauge before blaming the thermostat for cooling issues. If the engine warms up slowly or runs hot without a hot upper radiator hose, the thermostat may be stuck open or closed and need replacing. Don't drive without a thermostat as it affects emissions and fuel economy. To replace, disconnect the negative battery terminal, drain the cooling system, and remove the alternator harness bracket or engine trim cover. Find the thermostat housing by tracing the upper radiator hose to the engine, loosen the hose clamp, and remove the deteriorated hose if needed. Replace the thermostat housing cover if damaged. Remove the fasteners, detach the thermostat cover, remove the thermostat, clean off old gasket residue, fit a new gasket, and secure the thermostat housing cover, then refill the cooling system and start the engine. Let it reach normal temperature and check for leaks and proper functioning.

Genuine Mercury Mountaineer Thermostat

Engine Coolant Thermostat

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6 Thermostats found

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Mercury Mountaineer Thermostat Assembly
Part Number: 7L3Z-8575-B
- Other Name: Thermostat Assy
- Replaced by: 7L3Z-8575-E
$17.04 MSRP: $24.18
You Save: $7.14 (30%)

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Fits the following Vehicles:
2002-2010 Mercury Mountaineer | 8 Cyl 4.6L, 8 Cyl 4.6L Romeo SOHC EFI | 5-Speed Automatic Transmission 5R55S, 5-Speed Automatic Transmission 5R55W, 5-Speed Manual Transmission (M5R4), 6-Speed Automatic Transmission (6R80), 6-Speed Automatic Transmission 6R60, Four-Wheel Drive (Full Time) (Left Hand Drive), Four-Wheel Drive (Part Time) (Left Hand Drive), Four-Wheel Drive (Part Time) (Right Hand Drive), Left Hand Drive, Rear-Wheel Drive (Left Hand Drive)
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Mercury Mountaineer Thermostat Assembly
Part Number: 2L2Z-8575-AA
- Other Name: Thermostat Assy; Thermostat
- Replaces: 1L2Z-8575-AA
$17.67 MSRP: $28.18
You Save: $10.51 (38%)

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Fits the following Vehicles:
2002-2010 Mercury Mountaineer | 6 Cyl 4.0L SOHC EFI | 5-Speed Automatic Transmission 5R55S, 5-Speed Automatic Transmission 5R55W, 5-Speed Manual Transmission (M5R4), 6-Speed Automatic Transmission (6R80), 6-Speed Automatic Transmission 6R60, Four-Wheel Drive (Full Time) (Left Hand Drive), Four-Wheel Drive (Part Time) (Left Hand Drive), Four-Wheel Drive (Part Time) (Right Hand Drive), Left Hand Drive, Rear-Wheel Drive (Left Hand Drive)
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Mercury Mountaineer Thermostat Assembly
Part Number: F7TZ-8575-A
- Other Name: Thermostat Assy
- Manufacturer Note: Thermostat, SOHC
- Replaced by: YU3Z-8575-AA
$25.42 MSRP: $40.55
You Save: $15.13 (38%)

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Fits the following Vehicles:
1998-2001 Mercury Mountaineer | 6 Cyl 4.0L | 4AT 4R70W, 5AT 5R55E
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Mercury Mountaineer Thermostat Assembly
Part Number: 1L2Z-8575-AA
- Other Name: Thermostat Assy
- Replaced by: 2L2Z-8575-AA
$17.67 MSRP: $28.18
You Save: $10.51 (38%)

Add to Cart
More Info
Fits the following Vehicles:
2002 Mercury Mountaineer | 6 Cyl 4.0L SOHC EFI | 5-Speed Automatic Transmission 5R55S, 5-Speed Automatic Transmission 5R55W, 5-Speed Manual Transmission (M5R4), 6-Speed Automatic Transmission (6R80), 6-Speed Automatic Transmission 6R60, Four-Wheel Drive (Full Time) (Left Hand Drive), Four-Wheel Drive (Part Time) (Left Hand Drive), Four-Wheel Drive (Part Time) (Right Hand Drive), Left Hand Drive, Rear-Wheel Drive (Left Hand Drive)
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Mercury Mountaineer Thermostat Assembly
Part Number: XL2Z-8575-AB
- Other Name: Thermostat Assy
- Manufacturer Note: #XL2E 8575-AB (same as XL2Z 8575-AA, except new and improved jiggle pin), FROM 8/9/99
- Replaced by: XR3Z-8575-BA
$7.09 MSRP: $11.31
You Save: $4.22 (38%)

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Fits the following Vehicles:
1999-2001 Mercury Mountaineer | 8 Cyl 5.0L | 4AT 4R70W, 5AT 5R55E
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Mercury Mountaineer Thermostat Assembly
Part Number: AL2Z-8575-A
- Other Name: Thermostat Assy
- Replaced by: AL2Z-8575-B
More Info
Fits the following Vehicles:
2006-2010 Mercury Mountaineer | 6 Cyl 4.0L SOHC EFI | 5-Speed Automatic Transmission 5R55S, 5-Speed Automatic Transmission 5R55W, 5-Speed Manual Transmission (M5R4), 6-Speed Automatic Transmission (6R80), 6-Speed Automatic Transmission 6R60, Four-Wheel Drive (Full Time) (Left Hand Drive), Four-Wheel Drive (Part Time) (Left Hand Drive), Four-Wheel Drive (Part Time) (Right Hand Drive), Left Hand Drive, Rear-Wheel Drive (Left Hand Drive)
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Mercury Mountaineer Thermostat Parts Questions & Answers

Thermostat Assembly
1L2Z-8575-AA
Q: How to replace a thermostat in a cooling system?
A: Check the coolant level, drive belt tension, and temperature gauge before blaming the thermostat for cooling issues. If the engine warms up slowly or runs hot without a hot upper radiator hose, the thermostat may be stuck open or closed and need replacing. Don't drive without a thermostat as it affects emissions and fuel economy. To replace, disconnect the negative battery terminal, drain the cooling system, and remove the alternator harness bracket or engine trim cover. Find the thermostat housing by tracing the upper radiator hose to the engine, loosen the hose clamp, and remove the deteriorated hose if needed. Replace the thermostat housing cover if damaged. Remove the fasteners, detach the thermostat cover, remove the thermostat, clean off old gasket residue, fit a new gasket, and secure the thermostat housing cover, then refill the cooling system and start the engine. Let it reach normal temperature and check for leaks and proper functioning.