Product Description
Cure rear-cylinder hot spots on your 1994–1998 5.9L 12V Cummins with the FLEECE FPE-CLNTBYPS-CUMMINS-12V 12V COOLANT BYPASS KIT from CPP Diesel. This kit adds a dedicated, thermostat-controlled bypass circuit at the rear of the head to keep coolant moving, stabilize temperatures, and protect critical freeze plugs when you tow hard, add power, or spend long days on the road.
Why this Fleece Coolant Bypass Kit belongs on every 12V
- Addresses the well-known rear-cylinder coolant flow issue on 5.9L 12V Cummins engines
- Secondary thermostat (approx. 180°F) manages flow by temperature — not pressure spikes
- Helps prevent external and under–valve cover freeze plug failures under high load
- Complete kit with AN O-ring fittings and braided hose for a clean, leak-free installation
Fitment
Engine: 5.9L Cummins 12-valve (inline-6 diesel)
Model Years: 1994, 1995, 1996, 1997, 1998
Vehicles: Dodge Ram 2500 & 3500 4x2 and 4x4 equipped with 5.9L 12V Cummins
Use Case: Ideal for towing, performance builds, heavy work trucks, and any 12V that regularly sees high load or added horsepower.
How the Fleece 12V Coolant Bypass works
From the factory, the rear cylinders on a 5.9L Cummins see the hottest coolant and the least flow. That combination raises local temperatures and system pressure at the back of the block, which can push out freeze plugs or stress head gaskets. The Fleece bypass adds a secondary thermostat at the rear of the head that opens based on coolant temperature, not excessive pressure. Once the coolant back there reaches its set point, the bypass routes additional flow away from the hot spot, evening out temperatures across all six cylinders and reducing pressure spikes before they become a problem.
Cross-Reference / Related Fleece Products on CPP Diesel
- FLEECE FPE-CLNTBYPS-CUMMINS-12V-SS – 12V Coolant Bypass Kit 94–98 w/ Stainless Steel Braided Line
- FLEECE FPE-CLNTBYPS-HS-12V-SS – 39.50" 12V Cummins Coolant Bypass Hose (Stainless Steel Braided)
- FLEECE FPE-CLNTBYPS-HS-12V-BLK – 39.50" 12V Cummins Coolant Bypass Hose (Black Nylon Braided)
- FLEECE FPE-CLNTBYPS-CUMMINS-VP – VP44 Coolant Bypass Kit 1998.5–2002
- FLEECE FPE-CLNTBYPS-CUMMINS-MAN – 2003–2007 Cummins Coolant Bypass Kit (Manual)
Installation Notes
- Bolt-on design – no cylinder head removal required
- Uses AN O-ring fittings and braided hose for a clean, leak-resistant install
- Works with stock or modified cooling systems and is compatible with common supporting mods (radiators, thermostats, etc.).
Why do I need a coolant bypass on my 12V Cummins?
The rear cylinders see the highest coolant temperature and the least flow. Over time – especially with added power or heavy towing – that can lead to localized overheating, pressure spikes, and blown freeze plugs. A temperature-based coolant bypass levels out temps across all six cylinders and reduces those spikes before they cause failures.
Does this kit require head removal or machining?
No. The Fleece 12V coolant bypass is a bolt-on kit. It uses existing ports and hardware locations, so you do not need to pull the head or machine the block for installation.
What thermostat temperature does the bypass use?
The kit uses a secondary thermostat around 180°F (approximate) so the bypass opens based on coolant temperature at the rear of the head, not system over-pressure. That keeps coolant moving where it’s needed most without over-cooling under light load.
Is this only for race trucks or big horsepower builds?
No. While it’s highly recommended for high-horsepower setups, a bypass kit is also one of the best reliability mods you can do on a work, tow, or daily-driven 12V. It simply helps the factory cooling system do its job better and protects against known weak points.
Can I upgrade to stainless braided or replacement hoses later?
Yes. CPP Diesel stocks both full stainless-braided 12V coolant bypass kits and dedicated replacement hoses in stainless or black nylon so you can match the look and service the system down the road.
Does this kit have emissions or Prop 65 restrictions?
The kit carries an E-code 3 and Executive Order D-834-9, and includes a California Proposition 65 warning for cancer and reproductive harm. Always verify local regulations before installation.
Specifications for FLEECE FPE-CLNTBYPS-CUMMINS-12V
| Attribute | Value |
|---|---|
| Brand | Fleece Performance Engineering |
| Part Number | FPE-CLNTBYPS-CUMMINS-12V |
| UPC | Not specified by manufacturer or major distributors (UPC not published). |
| GTIN | Not specified (GTIN not listed in available product data). |
| Engine Compatibility | 5.9L Cummins 12V, 1994–1998 Dodge Ram 2500/3500 |
| Configuration | Temperature-controlled secondary thermostat bypass with braided hose and AN O-ring fittings |
| Cross Reference | FPE-CLNTBYPS-CUMMINS-12V-SS; FPE-CLNTBYPS-HS-12V-SS; FPE-CLNTBYPS-HS-12V-BLK; FPE-CLNTBYPS-CUMMINS-VP; FPE-CLNTBYPS-CUMMINS-MAN |
| E-codes / EO | E-code 3, CARB EO D-834-9 (as listed for this kit family) |
| Shipping Method | Standard ground, small parcel (typical coolant bypass kit packaging) |
| Approx. Shipping Weight | ~15 lb (shipping weight as listed by a major distributor). |
| Approx. Package Dimensions | ~4 in x 9 in x 12 in (shipping carton; may vary by retailer). |
| Country of Origin | Not explicitly stated; Fleece kits are generally manufactured in the USA. |
| Prop 65 Warning | WARNING: Cancer and Reproductive Harm – www.P65Warnings.ca.gov |
