Mishimoto MMTS-RAM-99H Hi-Temp Thermostat 1999-2013 Dodge Ram 2500/3500 Cummins

Vehicle Fitment

• 1999-2007 Dodge Ram 2500 (ST, SLT, Laramie, 5.9L Cummins Turbo Diesel 24V, 2WD/4WD)
• 1999-2007 Dodge Ram 3500 (ST, SLT, Laramie, Sport, 5.9L Cummins Turbo Diesel 24V, 2WD/4WD, SRW/DRW)
• 2007.5-2010 Dodge Ram 2500 (ST, SLT, Laramie, SXT, 6.7L Cummins Turbo Diesel, 2WD/4WD)
• 2007.5-2010 Dodge Ram 3500 (ST, SLT, Laramie, SXT, Sport, 6.7L Cummins Turbo Diesel, 2WD/4WD, SRW/DRW)
• 2008-2010 Dodge Ram 4500 (ST, SLT, Laramie, 6.7L Cummins Turbo Diesel)
• 2008-2010 Dodge Ram 5500 (ST, SLT, Laramie, 6.7L Cummins Turbo Diesel)
• 2011-2013 Ram 2500 (ST, SLT, Laramie, Longhorn, Power Wagon, 6.7L Cummins Turbo Diesel, 2WD/4WD)
• 2011-2013 Ram 3500 (ST, SLT, Laramie, Longhorn, 6.7L Cummins Turbo Diesel, 2WD/4WD, SRW/DRW)

Fitment Notes

• Fits the 3rd Generation 5.9L Cummins 24-valve turbocharged inline-six diesel (1999-2007) and all 6.7L Cummins turbocharged inline-six diesel variants (2007.5-2013).
• OEM thermostat activation temperature varies by year: 190°F for 1999-2010 and 2013 model year trucks; 200°F for 2010.5-2012 model year trucks. The Mishimoto MMTS-RAM-99H activates at 205°F across all covered years.
• This is a high-temperature thermostat - it raises the activation point above OEM for faster warm-up. For a lower activation temperature, use Mishimoto MMTS-RAM-99L instead.
• Does not fit 1994-1998 Dodge Ram 5.9L Cummins (2nd Gen) - use Mishimoto MMTS-RAM-94H for those applications.
• Designed to fit the stock thermostat housing - no housing replacement required.

Features

• Higher activation temperature of 205°F (96°C) versus OEM activation at 190°F (88°C) for 1999-2010/2013 trucks and 200°F (93°C) for 2010.5-2012 trucks - restricts coolant flow longer for faster, more complete engine warm-up.
• Ideal for cold-climate operation where quick engine warm-up reduces engine wear, improves fuel efficiency, and maximizes cabin heater output during winter driving.
• Designed for Cummins owners who live in colder regions and find their engine takes too long to reach full operating temperature under stock settings.
• Direct-fit OEM replacement engineered for installation into the factory thermostat housing with no modifications required.
• Helps maintain proper operating temperature range, supporting optimal fuel combustion efficiency and diesel emission system performance in cooler conditions.
• Compatible with stock and upgraded cooling systems including Mishimoto's aluminum performance radiators for the 5.9L and 6.7L Cummins platforms.
• O-ring included for a secure, leak-free seal upon installation.
• Backed by Mishimoto's Lifetime Warranty on all components for long-term reliability and peace of mind.

Specifications

• Part Number: MMTS-RAM-99H
• Mishimoto Activation Temperature: 205°F (96°C)
• OEM Activation Temperature: 190°F (88°C) - 1999-2010 and 2013 model year trucks
• OEM Activation Temperature: 200°F (93°C) - 2010.5-2012 model year trucks
• Engine Application: Dodge/Ram 5.9L Cummins 24V Turbo Diesel (1999-2007) and 6.7L Cummins Turbo Diesel (2007.5-2013)
• Fitment Type: Direct OEM Replacement
• Installation Position: Engine coolant circuit, fits stock thermostat housing
• Package Dimensions: 2.55" x 2.55" x 2.55"
• Shipping Weight: 1.0 lb

Included Components

• 1x Mishimoto High-Temperature Thermostat (MMTS-RAM-99H)
• 1x O-Ring

Product Description

The Mishimoto MMTS-RAM-99H is a high-temperature thermostat engineered as a direct-fit replacement for 1999-2013 Dodge and Ram 2500, 3500, 4500, and 5500 trucks equipped with the 5.9L or 6.7L Cummins turbocharged diesel engine. Unlike a low-temperature thermostat designed to reduce engine heat under load, the MMTS-RAM-99H raises the activation point to 205°F - well above the OEM threshold of 190°F on most covered years - restricting coolant flow for a longer period and allowing the engine to reach full operating temperature more quickly. This makes it the preferred upgrade for Cummins owners in cold climates where factory thermostat settings result in sluggish warm-up times, reduced cabin heat, and inefficient combustion during short trips or winter operation.

The Cummins diesel's performance and longevity are directly tied to operating at the correct temperature range. Running too cool - particularly in cold weather - can lead to increased cylinder wear, elevated fuel dilution in the oil, and incomplete diesel combustion that shortens engine life. The MMTS-RAM-99H corrects this by getting the engine up to operating temperature faster and holding it there more efficiently. It is a direct bolt-in installation using the factory thermostat housing; no modifications, adapters, or specialty tools are required. The included O-ring ensures a clean, leak-free seal.

An important spec distinction: the OEM thermostat activation temperature changed mid-production on the 6.7L Cummins platform - trucks from approximately 2010.5-2012 used a 200°F factory thermostat rather than the 190°F unit found in other years. The MMTS-RAM-99H's 205°F activation point remains an upgrade above OEM in all covered years. This thermostat is fully backed by Mishimoto's Lifetime Warranty and is ideal for Cummins owners running in cold-weather environments who want a simple, proven cooling system upgrade.