Which is better, 5.9 or 6.7 Cummins?
In most scenarios, the 6.7 Cummins generally offers more power, modern emissions technology, and stronger towing capability, making it the better all-round choice for new builds and heavy-use trucks. The 5.9 Cummins remains highly regarded for its simple design, reliability, and lower ongoing maintenance costs, especially for enthusiasts who prioritize ease of repair and budget ownership.
5.9 Cummins: strengths and trade-offs
The 5.9 Cummins is a veteran inline-six known for straightforward mechanical design, easy maintenance, and broad aftermarket support. Here are its key advantages and the trade-offs you should weigh.
- Pros: Rugged and proven reliability with a simple, robust engine architecture that’s easy to service in many shops.
- Pros: Low upfront cost and widespread availability of used units and replacement parts, which keeps ownership affordable.
- Pros: Wide aftermarket support for common upgrades and proven bolt-on power gains without sophisticated electronics.
- Pros: Generally easier and cheaper to diagnose and repair due to fewer modern emission or turbo systems to contend with.
Conclusion: The 5.9’s appeal lies in its durability, simplicity, and affordability, especially for owners who value straightforward maintenance and lower annual costs. It remains a solid option for budget buyers and hobbyists who work on their trucks themselves.
5.9 Cummins: limitations and caveats
Despite its strengths, the 5.9 has drawbacks in today’s market, particularly for buyers seeking peak power, modern emissions compliance, and the latest fuel-management technology.
- Cons: Lower power and torque by modern diesel standards, which can limit heavy towing or high-load applications.
- Cons: Older emissions setups may complicate compliance on newer roads and in certain jurisdictions, and some variants lack aftertreatment found on newer diesels.
- Cons: As the engine ages, parts such as fuel systems and turbochargers may require more frequent maintenance or refurbishment.
- Cons: Fewer factory refinements in area like fuel efficiency and heat management when compared to newer designs.
Conclusion: If you’re prioritizing simplicity, low purchase cost, and ease of maintenance, the 5.9 remains compelling. If you anticipate frequent heavy towing or strict emissions requirements, you’ll likely prefer the newer platform.
6.7 Cummins: strengths and trade-offs
The 6.7 Cummins represents the modern evolution of Cummins’ heavy-duty line, bringing more power, better torque delivery, and contemporary emissions technology. Here are its main strengths and the caveats to consider.
- Pros: Significantly higher power and torque, with improved performance for towing, hauling, and highway acceleration.
- Pros: Modern common-rail fuel system and advanced turbocharging, which translate to better throttle response and efficiency under load.
- Pros: Emissions controls (DPF/DEF systems) and updated electronics provide better compliance with current regulations and improved long-term emissions performance.
- Pros: Stronger factory support for newer trucks, updated parts availability, and a broader ecosystem of diagnostics and service tooling.
Conclusion: For buyers who need maximum towing capability, stronger acceleration, and modern fuel-management technology, the 6.7 is typically the better choice in today’s market.
6.7 Cummins: limitations and caveats
There are trade-offs to the 6.7, particularly around complexity and maintenance costs, which can impact ownership economics over time.
- Cons: Higher initial purchase price for new or newer used trucks, and potentially higher maintenance and parts costs over the life of the engine.
- Cons: Emissions systems add complexity and potential failure points (DPF, DEF, EGR) that can require specialized service and parts.
- Cons: Modern fuel-system components and sensors can be more fragile or expensive to replace if they fail, especially outside of warranty.
- Cons: Some owners report higher preventative maintenance needs to keep the aftertreatment system functioning optimally, which can increase downtime and cost.
Conclusion: The 6.7’s advanced features come with increased complexity and ownership costs. If you value top-tier performance and modern tech and are prepared for potentially higher upkeep, it’s a strong performer; if you’re budget-conscious or prefer simpler maintenance, this can be a drawback.
Choosing between them: which engine fits your needs?
Decision factors largely hinge on how you use the truck, your budget, and your willingness to handle emissions tech and more complex maintenance. Below is a quick guide to help orient your choice.
- Budget and ownership costs: If upfront price and ongoing maintenance costs matter most, the 5.9 often wins.
- Towing and payload needs: For serious towing or heavy loads, the 6.7 typically provides a noticeable advantage in power and torque.
- Maintenance capability and shop access: If you have easy access to skilled diesel shops and prefer simpler diagnostics, the 5.9 is easier to service.
- Emissions and compliance: If you need strong emissions compliance and modern efficiency, the 6.7’s system is preferable, accepting higher maintenance complexity.
Bottom line: For traditionalists or budget-focused buyers, the 5.9 delivers reliable, straightforward performance. For those who demand modern power, better towing capability, and compliant emissions technology, the 6.7 is generally the better bet.
Summary
The 5.9 Cummins stands out for its rugged simplicity, affordability, and ease of maintenance, making it an enduring favorite among enthusiasts and budget-minded owners. The 6.7 Cummins shines with more power, refined performance, and modern emissions technology, suited to heavier duty use and buyers who prioritize current-day efficiency and compliance. The right choice depends on your use case, budget, and tolerance for complexity. Both engines have earned devoted followings, and both can serve well when matched to the right vehicle and ownership plan.
What is the biggest problem with the 5.9 Cummins?
Common 5.9L Cummins Engine Problems
- Gear Case Leaks & Dowel Pin Problems.
- Fuel System Failures.
- Cylinder Head Gasket Leaks.
- Water Pump Failure.
- #53 Engine Block Cracks (Selective Years)
- Heater Grid Bolt Failures.
- Transmission Wear.
- Design Simplicity vs.
Which is better, 5.9 Cummins or 6.7 Cummins?
Compared to the 5.9, the 6.7 Cummins offers more torque at low RPM. This is thanks to the 6.7's increased stroke: 4.88 inches vs the 5.9 Cummins' 4.72 inches.
What is considered the best Cummins engine?
There is a driveability difference between the engines. The Cummins I-6 is generally considered the best for long haul heavy towing and the V8 engines generally rev higher and behave more like the gas engines most are more used to.
What year is 5.9 Cummins to avoid?
You should avoid 5.9 Cummins years with known weaknesses, specifically 1998.5–2002 (due to the unreliable VP44 injection pump and potential lift pump failure) and 1999–2001 (due to the "53 block" casting, which is prone to cracking). While other years may have specific issues, these are the most frequently cited for potential major failures.
Specific years and common problems to avoid:
- 1999–2001: These are the most critical years to avoid due to the "53 block" casting with thin water jacket walls that can lead to cracking and coolant leaks.
- 1998.5–2002: These models use the Bosch VP44 injection pump, which is prone to failure. A failure in the lift pump that feeds the injection pump can cause a chain reaction, leading to the expensive VP44 failing as well.
- 2004.5–2005: These models have injectors that were more prone to premature wear, which can lead to rough starts, smoke, or fuel in the oil.
Other considerations:
- Transmission: The 47RE automatic transmission, found in 1994-2002 trucks, is a weak point due to its older design and smaller components.
- 1998: This year is a bit of a transition year, but can still have the VP44 injection pump issues mentioned above.
Years considered more reliable:
- 1994–1998: These 12-valve engines are highly reliable due to their mechanical P7100 injection pump.
- 2006–2007: The last of the 5.9L common rail engines are considered by some to be the peak of 5.9L performance before the introduction of the 6.7L.
