If you've spent any time around 5.9L and 6.7L Cummins engines, you've probably heard about the rear cylinder cooling issue. Maybe you've seen forum posts about blown freeze plugs. Maybe you've experienced mysterious coolant loss with no visible leak. Or maybe you're just proactive and want to understand what makes these engines tick before problems start.
I've been running a 2006 5.9L for years, and more recently helped a friend with his 2012 6.7L work through some cooling system concerns. That's when I really started digging into the rear cylinder cooling problem that affects these engines.
The issue is real, and it's been documented by Cummins enthusiasts for years. The rear cylinders on these engines simply don't get the same coolant flow as the front. That leads to higher temperatures, pressure buildup, and in worst-case scenarios, popped freeze plugs or head gasket issues.
I recently installed the TruckTok coolant bypass kit on my friend's 6.7L, and after seeing the difference, I wanted to share what I learned about why this modification matters and what the kit actually does.
The problem: The rear cylinders on these engines suffer from reduced coolant flow. The cooling system is designed such that coolant naturally flows more readily to the front of the engine, leaving the rear cylinders with less circulation.
The consequence: With less flow, the rear cylinders run hotter than the front. Under load, especially when towing or climbing grades, this temperature differential becomes more pronounced. Hotter cylinders mean higher thermal stress and, eventually, pressure buildup in the cooling system.
The failure mode: When pressure builds excessively, it has to go somewhere. On some engines, it blows out freeze plugs—either the external ones or the smaller diameter plugs under the valve cover. Either way, you've got a coolant leak and a major repair on your hands. In other cases, the pressure contributes to head gasket failure, which is an even bigger job.
The temperature factor: The product description notes that rear cylinders experience the highest coolant temperatures. This isn't speculation—it's been measured and documented by multiple sources in the Cummins community.
5.9L applications: For 2003-2007 5.9L trucks, the kit only works with manual transmissions. This is important to note—if you have an automatic 5.9L, this specific kit won't fit your application.
6.7L applications: For 2007.5-2018 6.7L trucks, the kit works with both manual and automatic transmissions. The cooling system architecture on the 6.7L is different enough to accommodate both.
Why the difference: The transmission type affects the bellhousing and rear engine configuration, which changes how the coolant bypass can be routed. The kit is engineered specifically for each application, so paying attention to fitment matters.
Coolant bypass hose: The braided hose with AN O-ring fittings routes coolant to the rear of the engine. AN fittings are the gold standard for leak-free connections—they seal properly and won't leak over time.
Coolant bypass thermostat housing: This housing contains a secondary thermostat that opens based on temperature, not pressure. This is a key design difference from some other kits.
Thermostat riser block: This component positions the secondary thermostat correctly and ensures proper flow through the system.
O-rings: Quality O-rings ensure leak-free sealing at all connection points.
Mounting brackets: Two brackets secure the hose in place, preventing it from rubbing against other components or moving around.
Hardware: All necessary bolts are included—socket head cap screws, flange head bolts in various sizes. No chasing down missing hardware.
Scouring pad: Included for cleaning the freeze plug hole surface before installation. A small but thoughtful inclusion.
How pressure-based systems work: Some coolant bypass kits rely on pressure buildup to open a valve or bypass. By the time pressure is high enough to activate them, the problem is already happening. Pressure has already built, temperatures have already risen, and the engine is already under stress.
How temperature-based systems work: This kit adds a secondary thermostat that opens based on coolant temperature, not pressure. It ensures consistent temperatures across all cylinders by managing flow proactively rather than reactively.
Why this matters: Solving the problem before pressure becomes an issue is always better than dealing with the consequences after the fact. Consistent temperatures mean less thermal stress, less pressure buildup, and longer engine life.
Lower rear cylinder temperatures: This is the primary benefit. With improved coolant flow to the rear, cylinder temperatures even out across the engine. No more hot spots, no more excessive thermal stress.
Reduced cooling system pressure: When temperatures are consistent, pressure buildup is minimized. The cooling system operates within its design parameters rather than being pushed to the limit.
Elimination of freeze plug failures: One of the most common failure modes on these engines is blown freeze plugs, especially at the rear. By addressing the root cause—excessive pressure from uneven heating—this kit helps prevent that failure.
Improved head gasket reliability: Head gaskets fail when they're subjected to uneven thermal expansion and excessive pressure. Consistent temperatures across all cylinders reduce the stress on head gaskets, potentially extending their life.
Peace of mind: For anyone who plans to keep their Cummins-powered truck long-term, knowing that a known weak point has been addressed is valuable. It's one less thing to worry about.
For detailed steps, please refer to the installation instructions.
The whole process takes a few hours for someone who's done similar work before. Having the right tools and a clean workspace makes it easier.
Aluminum components: The thermostat housing and riser block are made from quality aluminum, machined to precise specifications.
Braided hose with AN fittings: This isn't cheap rubber hose. The braided construction resists abrasion and pressure, while the AN fittings provide leak-free connections that can be disconnected and reconnected if needed.
Quality hardware: All bolts are the correct grade and size for the application. No soft metal, no questionable fasteners.
Included cleaning supplies: The scouring pad is a small detail, but it shows attention to the installation process.
The TruckTok Coolant Bypass Kit addresses this issue directly. It adds a secondary temperature-based thermostat that ensures consistent coolant flow to the rear cylinders, preventing the temperature differential that leads to pressure buildup and failures.
Key benefits:
For Cummins owners who plan to keep their trucks long-term, this kit represents smart preventative maintenance. It addresses a known weak point before it becomes a problem, improving reliability and extending engine life.
If you've installed a coolant bypass on your Cummins, what was your experience? Drop your thoughts below.
I've been running a 2006 5.9L for years, and more recently helped a friend with his 2012 6.7L work through some cooling system concerns. That's when I really started digging into the rear cylinder cooling problem that affects these engines.
The issue is real, and it's been documented by Cummins enthusiasts for years. The rear cylinders on these engines simply don't get the same coolant flow as the front. That leads to higher temperatures, pressure buildup, and in worst-case scenarios, popped freeze plugs or head gasket issues.
I recently installed the TruckTok coolant bypass kit on my friend's 6.7L, and after seeing the difference, I wanted to share what I learned about why this modification matters and what the kit actually does.
Part 1: Understanding the Cooling Issue on Cummins Engines
The 5.9L and 6.7L Cummins engines are legendary for their durability. They power everything from pickups to industrial equipment, and they've earned a reputation for running hundreds of thousands of miles with proper maintenance. But no engine is perfect, and these have a known weak point in the cooling system.The problem: The rear cylinders on these engines suffer from reduced coolant flow. The cooling system is designed such that coolant naturally flows more readily to the front of the engine, leaving the rear cylinders with less circulation.
The consequence: With less flow, the rear cylinders run hotter than the front. Under load, especially when towing or climbing grades, this temperature differential becomes more pronounced. Hotter cylinders mean higher thermal stress and, eventually, pressure buildup in the cooling system.
The failure mode: When pressure builds excessively, it has to go somewhere. On some engines, it blows out freeze plugs—either the external ones or the smaller diameter plugs under the valve cover. Either way, you've got a coolant leak and a major repair on your hands. In other cases, the pressure contributes to head gasket failure, which is an even bigger job.
The temperature factor: The product description notes that rear cylinders experience the highest coolant temperatures. This isn't speculation—it's been measured and documented by multiple sources in the Cummins community.
Part 2: Why the 5.9L and 6.7L Are Different
The kit fits both 5.9L and 6.7L engines, but there are some important distinctions.5.9L applications: For 2003-2007 5.9L trucks, the kit only works with manual transmissions. This is important to note—if you have an automatic 5.9L, this specific kit won't fit your application.
6.7L applications: For 2007.5-2018 6.7L trucks, the kit works with both manual and automatic transmissions. The cooling system architecture on the 6.7L is different enough to accommodate both.
Why the difference: The transmission type affects the bellhousing and rear engine configuration, which changes how the coolant bypass can be routed. The kit is engineered specifically for each application, so paying attention to fitment matters.
Part 3: What the TruckTok Coolant Bypass Kit Actually Does
The TruckTok kit addresses the rear cylinder cooling issue directly. Here's what's included and why each component matters.Coolant bypass hose: The braided hose with AN O-ring fittings routes coolant to the rear of the engine. AN fittings are the gold standard for leak-free connections—they seal properly and won't leak over time.
Coolant bypass thermostat housing: This housing contains a secondary thermostat that opens based on temperature, not pressure. This is a key design difference from some other kits.
Thermostat riser block: This component positions the secondary thermostat correctly and ensures proper flow through the system.
O-rings: Quality O-rings ensure leak-free sealing at all connection points.
Mounting brackets: Two brackets secure the hose in place, preventing it from rubbing against other components or moving around.
Hardware: All necessary bolts are included—socket head cap screws, flange head bolts in various sizes. No chasing down missing hardware.
Scouring pad: Included for cleaning the freeze plug hole surface before installation. A small but thoughtful inclusion.
Part 4: Temperature-Based vs. Pressure-Based Design
One of the key features of this kit is worth emphasizing: it uses a temperature-based secondary thermostat rather than a pressure-based design.How pressure-based systems work: Some coolant bypass kits rely on pressure buildup to open a valve or bypass. By the time pressure is high enough to activate them, the problem is already happening. Pressure has already built, temperatures have already risen, and the engine is already under stress.
How temperature-based systems work: This kit adds a secondary thermostat that opens based on coolant temperature, not pressure. It ensures consistent temperatures across all cylinders by managing flow proactively rather than reactively.
Why this matters: Solving the problem before pressure becomes an issue is always better than dealing with the consequences after the fact. Consistent temperatures mean less thermal stress, less pressure buildup, and longer engine life.
Part 5: What You Gain by Installing a Coolant Bypass
After installing this kit on my friend's 6.7L, several benefits became apparent.Lower rear cylinder temperatures: This is the primary benefit. With improved coolant flow to the rear, cylinder temperatures even out across the engine. No more hot spots, no more excessive thermal stress.
Reduced cooling system pressure: When temperatures are consistent, pressure buildup is minimized. The cooling system operates within its design parameters rather than being pushed to the limit.
Elimination of freeze plug failures: One of the most common failure modes on these engines is blown freeze plugs, especially at the rear. By addressing the root cause—excessive pressure from uneven heating—this kit helps prevent that failure.
Improved head gasket reliability: Head gaskets fail when they're subjected to uneven thermal expansion and excessive pressure. Consistent temperatures across all cylinders reduce the stress on head gaskets, potentially extending their life.
Peace of mind: For anyone who plans to keep their Cummins-powered truck long-term, knowing that a known weak point has been addressed is valuable. It's one less thing to worry about.
Part 6: The Installation Process
The kit includes detailed installation steps, and the process is manageable for someone with intermediate mechanical skills.For detailed steps, please refer to the installation instructions.
The whole process takes a few hours for someone who's done similar work before. Having the right tools and a clean workspace makes it easier.
Part 7: The Material Quality
The kit uses high-quality materials throughout, which matters for long-term reliability.Aluminum components: The thermostat housing and riser block are made from quality aluminum, machined to precise specifications.
Braided hose with AN fittings: This isn't cheap rubber hose. The braided construction resists abrasion and pressure, while the AN fittings provide leak-free connections that can be disconnected and reconnected if needed.
Quality hardware: All bolts are the correct grade and size for the application. No soft metal, no questionable fasteners.
Included cleaning supplies: The scouring pad is a small detail, but it shows attention to the installation process.
Part 8: The Verdict
The 5.9L and 6.7L Cummins engines are legendary for a reason. They're robust, powerful, and capable of immense mileage when properly maintained. But they're not perfect, and the rear cylinder cooling issue is a known weak point that can lead to expensive failures.The TruckTok Coolant Bypass Kit addresses this issue directly. It adds a secondary temperature-based thermostat that ensures consistent coolant flow to the rear cylinders, preventing the temperature differential that leads to pressure buildup and failures.
Key benefits:
- Lower rear cylinder temperatures
- Consistent temperatures across all cylinders
- Reduced cooling system pressure
- Prevention of freeze plug failures
- Improved head gasket reliability
- Temperature-based design (proactive, not reactive)
For Cummins owners who plan to keep their trucks long-term, this kit represents smart preventative maintenance. It addresses a known weak point before it becomes a problem, improving reliability and extending engine life.
If you've installed a coolant bypass on your Cummins, what was your experience? Drop your thoughts below.
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