HKS Low Temp Oil Cooler Thermostat for Mitsubishi Evo I–X — Part #15999-AM001

The HKS 15999-AM001 Low Temp Oil Cooler Thermostat is a high-performance replacement thermostat designed to control oil temperature in the Mitsubishi Lancer Evolution oil-cooling system. HKS lists this thermostat for Evolution I through Evolution X, making it a versatile upgrade for both classic and modern Evo platforms.

Product Details

• Brand: HKS
• Product: Low Temperature Oil Cooler Thermostat
• HKS Part Number: 15999-AM001
• Application: Mitsubishi Lancer Evolution I–X
• Engines: 4G63 / 4B11 depending on Evo generation
• Type: Low-temperature oil thermostat
• Thermostat Opening Temperature: 75°C
• OEM Thermostat Opening Temperature: Approximately 100°C
• Function: Controls oil flow to the oil cooler based on temperature
• Installation: Designed as a replacement for the factory oil thermostat
• Weight: Approximately 0.1 kg
• HKS Origin: Japan

HKS specifies the 15999-AM001 as a low-temperature thermostat for its oil-cooler systems and lists it across the Evolution range, including Evo VII, VIII, VIII MR, IX, IX MR and X.

How It Works

The HKS low-temperature thermostat helps regulate engine-oil temperature by controlling when oil is routed through the oil cooler. With its 75°C opening temperature, it begins opening significantly earlier than the approximately 100°C stock thermostat, allowing oil to reach the cooler sooner under increasing temperature conditions.

This can be particularly beneficial for track driving, spirited driving, turbocharged applications and high-load conditions, where controlling oil temperature is important for maintaining consistent lubrication performance.

Performance Benefits

• Helps reduce excessive engine-oil temperatures
• Allows oil-cooler operation at a lower temperature
• Helps maintain more consistent oil temperatures during hard driving
• Useful for track and motorsport applications
• Can help reduce heat transfer from excessively hot oil into the cooling system
• Provides an easy upgrade for compatible HKS oil-cooler configurations

HKS specifically notes that lowering oil temperature can also help prevent engine coolant temperature from rising, making the thermostat useful as part of an overall engine thermal-management setup.

Evo Fitment

HKS officially lists 15999-AM001 for:

• Evo I — CD9A — 4G63
• Evo II — CE9A — 4G63
• Evo III — CE9A — 4G63
• Evo IV — CN9A — 4G63
• Evo V — CP9A — 4G63
• Evo VI — CP9A — 4G63
• Evo VII — CT9A — 4G63
• Evo VIII — CT9A — 4G63
• Evo VIII MR — CT9A — 4G63
• Evo IX — CT9A — 4G63
• Evo IX MR — CT9A — 4G63
• Evo X — CZ4A — 4B11
• Evolution Wagon — CT9W — 4G93

HKS therefore describes the thermostat as a broad Evolution-family application rather than limiting it to a single generation.

HKS Oil Cooler Compatibility

The 15999-AM001 is also specified by HKS as the separate low-temperature thermostat used with its oil-cooler kits. For the Evo IX, HKS states that using the low-temperature thermostat with its oil-cooler kit provides improved performance and helps control oil and water temperatures.

OEM Replacement / Upgrade

The HKS 15999-AM001 is an excellent choice for Evo owners looking to upgrade the factory oil-temperature control system without replacing the entire oil-cooler assembly. It is especially suited to vehicles that see high RPM, track use, racing, spirited driving or increased engine power.

Brand: HKS
Part Number: 15999-AM001
Product: Low Temperature Oil Cooler Thermostat
Opening Temperature: 75°C
Stock Thermostat: Approximately 100°C
Application: Mitsubishi Lancer Evolution I–X
Engine Applications: 4G63 / 4B11 / 4G93 depending on model
Weight: Approximately 0.1 kg
Type: HKS Performance Oil Cooler Thermostat