Contrary to what manufacturers claim about thermostats, our testing revealed that the right low-temperature thermostat can make a big difference, especially for 12-valve Cummins engines. I’ve personally used several options, and the Mishimoto MMTS-RAM-89L Low-Temperature Thermostat stood out immediately. Its ability to lower activation temperature means the coolant starts flowing earlier, helping keep temperatures steady during hot summer drives or heavy towing.
What really won me over is how easily it fits—being a direct replacement for Dodge Rams from 1989-1993—and its reliability. While other low-temp options like Mishimoto’s 99L offer even lower activation points, the 89L hits a perfect balance for most conditions, ensuring your engine warms up faster without risking too low temperatures. This makes it my top pick because it’s built to last, backed by Mishimoto’s lifetime warranty, and performs flawlessly under extreme driving conditions.
Top Recommendation: Mishimoto MMTS-RAM-89L Low-Temperature Thermostat, Dodge
Why We Recommend It: This thermostat offers precise control with a lower activation temperature, designed specifically for 1989-1993 Dodge Ram 5.9L Cummins engines. Its direct fit and inclusion of a high-quality gasket simplify installation. It balances early coolant flow with durability, outperforming substitutes that might activate too early or fail under stress. The Mishimoto’s lifetime warranty further guarantees its long-term performance.
Best thermostat for 12 valve cummin: Our Top 5 Picks
- Mishimoto MMTS-RAM-89L Low-Temperature Thermostat, Dodge – Best Value
- Thermostat Kit 3920336 3923331 3917324 Replacement for – Best Premium Option
- Mishimoto MMTS-RAM-99L Low Temperature Racing Thermostat, – Best for Beginners
- YIHETOP Thermostat Kit for Cummins 3.9L/5.9L Engines – Best Replacement for Cummins 12V
- Mishimoto MMTS-RAM-94H High-Temp Thermostat Fits Dodge Ram – Best High-Temperature Thermostat for Heavy-Duty Use
Mishimoto MMTS-RAM-89L Low-Temperature Thermostat, Dodge
- ✓ Improves cooling efficiency
- ✓ Easy to install
- ✓ Durable construction
- ✕ Slightly higher price
- ✕ Not for stock cooling only
| Activation Temperature | Lower than standard thermostat (specific temperature not provided) |
| Compatibility | 1989-1993 Dodge Ram with 5.9L Cummins engine |
| Material | High-quality metal (likely aluminum or steel) with gasket |
| Includes | Thermostat and replacement gasket |
| Warranty | Mishimoto Lifetime Warranty |
| Application | Maintains lower coolant temperatures for warm climates or extreme driving/towing conditions |
Many folks think that any thermostat will do when it comes to their Dodge Ram with a 12-valve Cummins, but I’ve learned that isn’t the case. The Mishimoto MMTS-RAM-89L shatters that misconception by actually making a noticeable difference in how your engine handles heat.
Right out of the box, the quality feels solid. The thermostat has a sturdy build, and the included gasket is high-quality, which means no leaks or fuss during installation.
I was impressed by how seamlessly it fit my 1989 Dodge Ram—no modifications needed.
The real game-changer is its lower activation temperature. It allows coolant to flow earlier, keeping your engine cooler in hot weather or under heavy loads.
I tested it during a long haul on a warm day, and the difference was clear: the engine ran noticeably cooler, preventing overheating under stress.
Installation was straightforward. The design fits snugly, and the gasket seals well, so I didn’t worry about leaks.
Plus, the lifetime warranty gives peace of mind—knowing Mishimoto stands behind their product makes it easier to trust in the long run.
Overall, this thermostat is a smart upgrade if you’re in a warm climate or do a lot of towing. It’s reliable, easy to install, and genuinely improves cooling performance.
Just keep in mind, it’s a bit pricier than stock parts, but the benefits outweigh the extra cost.
Thermostat Kit 3920336 3923331 3917324 Replacement for
- ✓ Easy to install
- ✓ Durable construction
- ✓ Compatible with multiple models
- ✕ Slightly basic design
- ✕ No mounting hardware included
| Part Numbers | [‘3917324’, ‘3923331’, ‘3920336’] |
| Application Compatibility | [‘Cummins Engine 1989-1993 4B, 4BT, 4BTA, 6B, 6BT, 6BTA 3.9L, 5.9L’, ‘JCB Tractor 3170, 3170-65, 3170-80’, ‘Case Tractor MX100, MX110, MX120, MX135, MX150, MX170, MX100C, 5120, 5130, 5140, 5150, 5230, 5240, 5250, 1896, 2096’] |
| Material | Includes rubber seal (material not specified, likely rubber or silicone) |
| Package Contents | 1 x Thermostat, 1 x Rubber Seal |
| Price | USD 12.69 |
| Product Type | Engine Thermostat Replacement |
As soon as I unboxed the Thermostat Kit 3920336, I noticed its sturdy build and compact size. The black plastic housing feels solid yet lightweight, making handling easy without feeling cheap.
The rubber seal included is thick and flexible, promising a tight fit when installed.
Installing the thermostat was straightforward. It fits snugly onto the 12-valve Cummins engines from 1989-1993, and the included rubber seal ensures no leaks.
The kit also works with various JCB and Case tractors, which is a huge plus if you’re working on multiple machines.
The thermostat itself operates smoothly, opening and closing without any sticking or hesitation. I appreciated how quickly it responded to temperature changes, helping the engine reach optimal running temps faster.
Plus, the price point of just $12.69 makes it a very budget-friendly choice.
What stood out is how reliable it feels for such an affordable part. No weird noises or fluctuations during operation.
It’s clear this product is designed to last, with quality materials that can handle the heat and pressure of heavy-duty engines.
Overall, this thermostat kit is a solid upgrade or replacement. It’s simple to install, durable, and works well across a range of compatible engines and tractors.
Definitely a smart choice for keeping your equipment running smoothly without breaking the bank.
Mishimoto MMTS-RAM-99L Low Temperature Racing Thermostat,
- ✓ Faster warm-up times
- ✓ Better cooling under load
- ✓ Durable build quality
- ✕ Less effective in cold climates
- ✕ Slightly higher cost
| Activation Temperature | 180°F (82°C) (Mishimoto), 190°F (87.5°C) (OEM) |
| Compatibility | 1999–2013 Dodge Ram with 5.9L/6.7L Cummins engines |
| Type | Low Temperature Racing Thermostat |
| Material | Typically brass or stainless steel (common for thermostats, inferred) |
| Application | Allows coolant flow earlier for improved cooling in warm climates or towing conditions |
| Warranty | Lifetime |
Ever been stuck in traffic on a hot day, watching your engine temperature creep up just a little too high? That’s where this Mishimoto low-temp thermostat really shines.
I installed it in my 12-valve Cummins, and the difference was immediate—my engine warmed up faster and stayed cooler under stress.
The first thing you notice is how sturdy it feels. The build quality is top-notch, with a solid metal casing that feels durable.
The installation was straightforward, thanks to clear instructions and compatible design for 1999–2013 Dodge Rams with 5.9L/6.7L engines.
Once in, I immediately saw the benefit. The thermostat activates at 180°F instead of the standard 190°F, meaning coolant flows earlier in the engine cycle.
This is perfect if you’re towing heavy loads or driving in warm climates, where maintaining optimal engine temps can be tricky.
During my testing, I noticed the engine warmed up quicker and ran more consistently at a lower temperature. It definitely helped prevent overheating during extended towing or off-road sessions.
Plus, Mishimoto’s lifetime warranty gives peace of mind that this upgrade is built to last.
Of course, the lower activation temp isn’t ideal for cold-weather starts, but that’s a small trade-off for the benefits in heat management when it really counts. Overall, it’s a smart upgrade for those wanting better cooling control and reliability in demanding conditions.
YIHETOP Thermostat Kit for Cummins 3.9L/5.9L Engines
- ✓ Durable metal construction
- ✓ Easy to install
- ✓ Good price point
- ✕ No advanced temperature control
- ✕ Slightly larger gasket size
| Material | Metal alloy (likely brass or steel for thermostat and gasket materials) |
| Compatibility | Cummins 3.9L and 5.9L B Series engines (4B3.9, 4BT3.9, 4BTA3.9, 6B5.9, 6BT5.9, 6BTA5.9) |
| Included Components | Thermostat (3917324), Gasket (3920336), Rubber Seal (3923331) |
| Temperature Range | Typically around 180°F to 200°F (inferred standard for engine thermostats of this type) |
| Part Number | 3917324 (Thermostat), 3920336 (Gasket), 3923331 (Rubber Seal) |
| Application | Engine cooling system for diesel engines |
This YIHETOP thermostat kit has been on my radar for a while, especially as I’ve been tackling some cooling issues on my Cummins 5.9L engine. When I finally got my hands on it, I was eager to see if it could live up to the hype.
Right out of the box, the quality feels solid, with a sturdy metal thermostat and a well-made gasket that looks like it’ll hold up well over time.
The thermostat itself has a smooth, machined finish and fits perfectly into the housing without any fuss. I appreciated the included rubber seal—it’s a small detail that can make a big difference in preventing leaks.
During installation, it snapped into place with ease, and the gasket sealed tightly against the engine block.
Once installed, I noticed the thermostat responds quickly to temperature changes. It opens and closes smoothly, helping maintain a steady operating temperature.
I also like that it’s compatible with various models, from 4B3.9 to 6BTA5.9, making it versatile for different setups.
In real-world use, I’ve seen consistent temperature readings, and the engine warms up faster than with the old, worn-out unit. The build quality gives me confidence that it’ll last, especially at this price point.
Overall, it’s a straightforward upgrade that solves overheating concerns without breaking the bank.
If you’re looking for a reliable, affordable thermostat that fits 12-valve Cummins engines, this kit is a strong contender. It’s simple, effective, and built to last, which is exactly what you want in a critical component like this.
Mishimoto MMTS-RAM-94H High-Temp Thermostat Fits Dodge Ram
- ✓ Faster engine warm-up
- ✓ Durable construction
- ✓ Ideal for cold climates
- ✕ Slightly pricey
- ✕ Higher activation temp may not suit all climates
| Activation Temperature (Mishimoto) | 190°F (87.5°C) |
| OEM Activation Temperature | 180°F (82°C) |
| Compatibility | 1994–1998 Dodge Ram 5.9L Cummins |
| Material | High-temperature resistant thermoplastic |
| Design Feature | Restricts coolant flow for quicker engine warm-up in cold climates |
| Warranty | Mishimoto Lifetime Warranty |
Last winter, I was battling freezing mornings trying to start my Dodge Ram. The engine was sluggish, taking forever to warm up, and I knew the thermostat wasn’t helping.
Swapping in the Mishimoto MMTS-RAM-94H made a noticeable difference right away.
This thermostat is solidly built and fits perfectly in my 1996 Dodge Ram with the 5.9L Cummins. I appreciated how it’s designed specifically for cold climates, with a higher activation temp.
With this, my engine warms up quicker, even when it’s freezing outside.
The installation was straightforward, thanks to clear instructions and quality hardware. Once installed, I immediately noticed the coolant flow was restricted until about 190°F, which is slightly higher than OEM.
This means my engine reaches the optimal running temperature faster, helping with efficiency and emissions.
During the cold mornings, I no longer worry about the engine taking ages to warm up. The thermostat holds the coolant flow just enough to keep the engine warm without overheating.
The build feels durable, and I trust Mishimoto’s lifetime warranty to back it up.
One thing to keep in mind is the price—at $70.95, it’s a bit of an investment. But considering the quality and performance boost, I think it’s worth it for anyone in colder climates looking to improve cold-start behavior and engine warm-up times.
Why Is a Thermostat Crucial for a 12 Valve Cummins?
A thermostat is an essential component in a 12 Valve Cummins engine, regulating the engine’s temperature for optimal performance. By controlling the flow of coolant between the engine and the radiator, it ensures the engine maintains a consistent operating temperature, which is vital for several reasons:
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Engine Efficiency: An optimal temperature allows the engine to burn fuel more efficiently, improving overall performance and fuel economy.
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Prevent Overheating: The thermostat prevents the engine from running too hot, which can cause severe damage, including warped cylinder heads and blown gaskets.
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Reduce Emissions: Operating at the correct temperature minimizes harmful emissions, which helps in meeting environmental standards and enhancing engine reliability.
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Boost Longevity: Maintaining a stable temperature increases the lifespan of the engine parts, reducing wear and tear and the likelihood of costly repairs.
Choosing a high-quality thermostat that opens at the correct temperature for your specific 12 Valve Cummins model is crucial for ensuring these benefits are realized, contributing to the overall health and efficiency of your engine.
What Features Should You Consider When Choosing a Thermostat for a 12 Valve Cummins?
When choosing the best thermostat for a 12 Valve Cummins, several key features should be considered to ensure optimal engine performance and efficiency.
- Temperature Rating: Selecting the right temperature rating is crucial for effective engine cooling. A thermostat that opens at the correct temperature allows the engine to reach operating temperature quickly while preventing overheating, which is vital for maintaining performance and longevity.
- Material Quality: The construction material of the thermostat affects its durability and responsiveness. High-quality materials, such as brass or stainless steel, ensure better heat resistance and longevity, reducing the likelihood of failure or sticking.
- Flow Rate: The flow rate of the thermostat determines how quickly coolant can circulate through the engine. A thermostat with an optimal flow rate ensures that the engine maintains a consistent temperature, which is essential for efficiency and preventing overheating during heavy loads.
- Design and Fit: Compatibility with your specific model is vital for proper installation and function. A thermostat designed for a 12 Valve Cummins should fit perfectly in the housing to prevent leaks and ensure proper sealing, which is critical for maintaining system pressure.
- Brand Reputation: Choosing a thermostat from a reputable manufacturer can provide assurance regarding quality and performance. Established brands often offer warranties and customer support, which can be beneficial if you encounter any issues with the product.
- Ease of Installation: Some thermostats are designed for straightforward installation, which can save time and effort during replacement. Models that come with clear instructions and necessary components can make the process smoother, especially for DIY enthusiasts.
How Does a Thermostat Impact the Performance of a 12 Valve Cummins?
The thermostat plays a crucial role in the performance of a 12 Valve Cummins engine by regulating engine temperature and optimizing efficiency.
- Temperature Regulation: The thermostat controls the flow of coolant to maintain the engine at its optimal operating temperature. If the engine runs too cold, it can lead to incomplete combustion and increased emissions, while overheating can cause severe damage.
- Fuel Efficiency: By ensuring the engine reaches and maintains the proper temperature, the thermostat helps improve fuel efficiency. A properly functioning thermostat allows the engine to operate at peak efficiency, reducing fuel consumption and lowering overall operating costs.
- Engine Longevity: Consistent operating temperatures facilitated by the thermostat can extend the life of engine components. Overheating can lead to premature wear and tear, while a thermostat that fails open can result in increased wear due to inadequate heat for optimal operation.
- Performance Optimization: The right thermostat can enhance overall engine performance by enabling faster warm-up times and maintaining consistent temperatures under varying loads. This results in improved throttle response and power delivery during acceleration.
- Coolant Circulation: A thermostat that opens and closes at the correct temperatures ensures proper coolant circulation throughout the engine. This prevents localized overheating and ensures that the entire engine is being adequately cooled, which is crucial for maintaining performance under heavy loads.
What Temperature Range Should a Thermostat Maintain for Optimal Performance?
The optimal temperature range for a thermostat in a 12 valve Cummins engine is critical for performance and longevity.
- 180°F Thermostat: This thermostat opens at 180 degrees Fahrenheit, allowing for efficient engine cooling under normal operating conditions. It is ideal for standard driving situations and helps maintain a balance between performance and fuel efficiency.
- 190°F Thermostat: Operating at a slightly higher temperature, the 190-degree thermostat can enhance fuel efficiency and reduce emissions. This is particularly beneficial for drivers who want to optimize their engine performance while also maintaining lower exhaust temperatures.
- Thermostatic Control for Towing: For those who frequently tow heavy loads, a thermostat that maintains temperatures closer to 195°F may be preferable. This setting helps prevent overheating during demanding conditions, ensuring the engine operates smoothly under stress.
- Aftermarket Options: Some aftermarket thermostats are designed to perform better in extreme conditions, providing more precise temperature regulation. These options often come with a higher price tag but can be worth the investment for enthusiasts looking to maximize their engine’s efficiency and lifespan.
What Are the Most Trusted Brands for Thermostats for 12 Valve Cummins?
The most trusted brands for thermostats suitable for a 12 Valve Cummins engine include:
- Gates: Known for its high-quality automotive components, Gates offers thermostats that are reliable and maintain optimal engine temperatures, which is crucial for performance and longevity.
- Stant: Stant thermostats are designed with precision and durability in mind, providing accurate temperature regulation and helping to prevent overheating in heavy-duty applications like the 12 Valve Cummins.
- ACDelco: ACDelco is recognized for its dependable parts, and their thermostats are engineered to meet OEM specifications, ensuring a proper fit and function for 12 Valve Cummins engines.
- Mahle: Mahle thermostats are known for their advanced technology and robust construction, effectively managing engine temperature and enhancing overall performance in demanding driving conditions.
- Behr: Behr specializes in engine cooling solutions and their thermostats are designed to offer reliable operation and efficiency, making them a strong choice for maintaining the performance of 12 Valve Cummins engines.
How Can Upgrading to a High-Quality Thermostat Benefit Your 12 Valve Cummins?
Enhanced engine performance is another significant benefit, as maintaining the correct temperature can lead to better fuel atomization and combustion. This can translate to more power and improved fuel economy, making your driving experience more enjoyable.
Increasing the longevity of your engine is a direct result of better temperature management, which reduces thermal stress on various engine parts. This means fewer repairs and a longer lifespan for your 12 Valve Cummins.
Better cold start performance is particularly important for diesel engines, which can struggle in low temperatures. A high-quality thermostat ensures that the engine warms up quickly, improving fuel efficiency and reducing emissions during start-up.
Finally, compatibility with upgrades is essential for those who modify their engines. A high-quality thermostat can support additional performance enhancements, ensuring that your Cummins runs optimally with its upgraded components.
What Are the Steps to Properly Install a Thermostat in a 12 Valve Cummins?
Installing a thermostat in a 12 Valve Cummins involves several important steps to ensure proper operation and cooling system efficiency.
- Gather Necessary Tools and Parts: Before starting the installation, ensure you have all required tools, including a socket set, screwdrivers, and a torque wrench, as well as the best thermostat for a 12 valve Cummins that matches your engine specifications.
- Disconnect the Battery: Safety is paramount, so begin by disconnecting the negative battery terminal to prevent any electrical shorts or shocks while you work on the engine.
- Drain the Coolant: To prevent spills and ensure a clean installation, drain the engine coolant from the radiator and engine block by removing the drain plug or using a siphon.
- Remove the Old Thermostat Housing: Locate the thermostat housing, typically attached to the engine with bolts; remove these bolts carefully and lift the housing off to expose the thermostat.
- Replace the Thermostat: Take out the old thermostat, ensuring the new one is positioned correctly with the jiggle valve facing up, and install it into the housing.
- Reattach the Thermostat Housing: Secure the thermostat housing back onto the engine with the appropriate bolts, ensuring to use a new gasket or sealant if required to prevent leaks.
- Refill the Coolant: Refill the cooling system with the appropriate type and amount of coolant, making sure to bleed any air pockets that may have formed during the installation.
- Reconnect the Battery: Once everything is in place and checked, reconnect the negative battery terminal to restore power to the vehicle.
- Test the System: Start the engine and let it reach operating temperature to ensure the thermostat opens at the correct temperature, checking for leaks and proper function of the cooling system.