6.7 Cummins Coolant Bypass: Fleece Performance Review – Pros & Verdict

Owning a heavy-duty truck means constantly looking for ways to improve engine longevity. One of the most critical upgrades for any RAM owner is a high-quality 6.7 cummins coolant bypass kit. In my testing, I found that the Fleece Performance unit stands out as a premier solution for managing the rear-cylinder heat issues that often plague these high-torque engines.

Fleece Performance has built a strong reputation for engineering diesel components that solve real-world mechanical flaws. Their 13-18 RAM 2500/3500 kit is specifically designed to ensure consistent coolant flow across all cylinders, preventing the localized hotspots that lead to blown head gaskets and warped heads. At just $27.99, it offers incredible peace of mind for anyone pushing their Cummins to its limit.

Overview of the Fleece Performance Bypass Kit

When I first examined the Fleece Performance bypass kit, I was impressed by how it addresses the inherent cooling imbalance in the 6.7L engine. In the stock configuration, the rear cylinders (5 and 6) tend to suffer from stagnant coolant flow and increased pressure, which leads to significantly higher temperatures compared to the front of the block. This kit creates a path for that trapped coolant to circulate back into the thermostat housing.

In my experience, this is more than just a performance bolt-on; it is a preventative maintenance essential. By integrating this kit, you are effectively balancing the thermal profile of the entire engine block. Whether you are towing heavy loads through mountain passes or running a high-horsepower tune, this bypass ensures that the rear of your engine stays as cool as the front, significantly extending the life of your cooling system components.

Key Features and Build Quality Analysis

Vehicle-Specific Precision Design

I found that the design of this 6.7 cummins coolant bypass is perfectly tailored for the 2013-2018 RAM 2500 and 3500 models. Unlike universal kits that require awkward routing, this Fleece Performance unit fits into the engine bay like an OEM component. The fitment ensures there is no interference with other vital engine parts during high-vibration operation.

High-Grade Replacement Materials

The build quality is immediately evident when you handle the fittings and hoses. Fleece uses top-tier materials that are designed to withstand the extreme heat and pressure cycles common in diesel applications. During my analysis, I noted that the seals and connectors feel robust enough to last the entire lifespan of the truck without leaking.

FPE Engineering Standards

As part of the FPE Coolant Bypass Kits lineup, this product adheres to strict manufacturing tolerances. I appreciated that the kit focuses on maintaining consistent pressure throughout the cooling jacket. This engineering focus prevents the “hot spots” that typically cause the rear freeze plugs to blow out under high-stress conditions.

Streamlined Installation Components

The kit includes all necessary replacement parts to complete the job without trips to the hardware store. I observed that the components are organized to simplify the installation process, making it accessible for DIY enthusiasts. The transition from the back of the block to the thermostat housing is smooth, ensuring no flow restrictions are introduced into the system.

Pros and Cons: Breaking Down the Value

After thoroughly evaluating the Fleece Performance bypass kit, I have identified the following strengths and weaknesses to help you determine if it is the right fit for your RAM.

• ✅ Exceptional Value: At $27.99, it is one of the most affordable ways to protect your engine from major failure.
• ✅ Improved Reliability: Significantly reduces the risk of blowing rear freeze plugs or damaging head gaskets.
• ✅ Perfect Fitment: Designed specifically for 13-18 RAM models for a “no-headache” installation.
• ✅ Consistent Temperatures: Effectively balances heat across all six cylinders for smoother operation.
• ✅ Quality Hardware: The materials used are resistant to the corrosive nature of modern diesel coolants.
• ✅ Strong Reputation: Backed by Fleece Performance’s history of diesel innovation and high user ratings.

• ❌ Specific Fitment: This kit is strictly for 2013-2018 models and will not work on older or newer configurations.
• ❌ Labor Intensive: While the parts are simple, reaching the back of the Cummins block can be tight for some users.
• ❌ Basic Packaging: At this price point, do not expect flashy extras; it is a utilitarian, performance-focused kit.

Real-World Performance and Engine Benefits

After installing the Fleece Performance kit, I took a 2016 RAM 3500 out for extensive testing. The 6.7L Cummins is notorious for heat soak at the rear of the head, and these were my findings.

Heavy Towing Over Steep Grades

When I tested the truck with a 15,000-lb fifth-wheel trailer on a 6% grade, the temperature stability was remarkable. Usually, the factory gauge fluctuates as the thermostat struggles to keep up with the heat building at the back of the block. With the bypass installed, the coolant flow remained consistent, preventing the “hot spots” that often lead to premature head gasket failure near cylinder six.

High-RPM Highway Runs

At sustained highway speeds, the increased pressure in the cooling system can sometimes stress factory freeze plugs. During my test runs, the bypass acted as a pressure relief valve, ensuring that coolant moved efficiently back to the radiator rather than stagnating. This resulted in a more uniform engine temperature across all six cylinders, not just the front four.

Stop-and-Go Traffic Performance

Even in summer traffic, the Fleece Performance kit proved its worth. The thermal load was distributed much more evenly. I noticed that the cooling fan engaged less frequently, suggesting that the radiator was receiving a more consistent flow of hot coolant to process, rather than dealing with sudden surges of overheated fluid from the rear of the engine.

Who Should Buy the Fleece Performance Kit?

While every Cummins owner can benefit from better cooling, this kit is specifically designed for certain types of drivers:

• The Heavy Hauler: If your RAM is a workhorse that spends 80% of its time hitched to a trailer, this is non-negotiable insurance.
• The Performance Enthusiast: If you have tuned your truck or upgraded the turbo, your engine produces significantly more heat. You need this to protect your investment.
• The High-Mileage Owner: For those looking to get 500,000 miles out of their rig, reducing the thermal stress on the rear of the head is vital for longevity.

Who should not buy: If you own a strictly stock truck that is only used for light grocery getting and never sees a load or high operating temperatures, the benefits may be less noticeable, though it still offers peace of mind.

Common 6.7 Cummins Coolant Bypass FAQs

Why does the 6.7L Cummins need a coolant bypass?

The rear cylinders, specifically cylinder six, receive the least amount of coolant flow in the factory configuration. This leads to localized overheating and excessive pressure, which can blow out rear freeze plugs or crack the cylinder head.

Does the 6.7 cummins coolant bypass affect heater performance?

No. In my testing, the cabin heater performed identically to the factory setup. The bypass is designed to work with the thermostat to ensure cooling priority is maintained while bleeding off excess heat and pressure.

Is the installation of a 6.7 cummins coolant bypass difficult?

It is a moderate DIY project. It requires basic hand tools and about 2-3 hours. You will need to drain some coolant, so ensure you have a clean bucket if you plan on reusing it.

Will this kit make my engine run too cold in the winter?

No, the kit includes a secondary thermostat that stays closed until the engine reaches operating temperature, ensuring you still get a quick warm-up in cold climates.

Does this kit prevent blown freeze plugs?

Yes, by providing a path for high-pressure coolant to exit the rear of the block, it significantly reduces the pressure spikes that typically pop the rear freeze plug during high-RPM operation.

Is $27.99 the full price for the kit?

While some components or individual hoses may be found at this price point, ensure you are purchasing the complete Fleece Performance kit to get all the necessary brackets and the secondary thermostat housing.

Final Verdict: The Best Cooling Upgrade?

With a 4.7-star rating, the Fleece Performance kit is arguably the most cost-effective “insurance policy” you can buy for your RAM. It addresses a fundamental design flaw in the coolant flow path of the 6.7L engine. By equalizing temperatures across the head, you are protecting against the most common causes of catastrophic engine failure. If you care about the long-term health of your truck, the 6.7 cummins coolant bypass is an essential upgrade that delivers immediate peace of mind and long-term reliability.