Cummins L9
8.90L Inline-6 turbo diesel • Cummins
About the Cummins L9
The Cummins L9 (formerly ISL9 and before that ISL) is a 8.9-liter inline-6 turbo diesel used in Class 6-8 medium-duty and vocational applications. It's the engine of choice for trucks too small for a 15L X15 but too big for a 6.7L B6.7 — primarily Class 7-8 vocational (dump, concrete, refuse) and Class 7 medium-duty. The L9 is the successor to the legendary Cummins ISL9 and shares architectural heritage going back to the Cummins L-series 8.3L of the 1990s. Common applications include the Freightliner M2, Peterbilt 337/567, Kenworth T370/T880, Mack Granite, International HV/MV.
Overview
The Cummins L9 (formerly ISL9 and before that ISL) is a 8.9-liter inline-6 turbo diesel used in Class 6-8 medium-duty and vocational applications. It's the engine of choice for trucks too small for a 15L X15 but too big for a 6.7L B6.7 — primarily Class 7-8 vocational (dump, concrete, refuse) and Class 7 medium-duty. The L9 is the successor to the legendary Cummins ISL9 and shares architectural heritage going back to the Cummins L-series 8.3L of the 1990s. Common applications include the Freightliner M2, Peterbilt 337/567, Kenworth T370/T880, Mack Granite, International HV/MV.
Specifications
- Manufacturer
- Cummins
- Displacement
- 8.90 liters
- Configuration
- Inline-6 turbo diesel
- Fuel type
- Diesel
- Horsepower
- 260–380 hp
- Torque
- 720–1250 lb-ft
- Production years
- 2017–present
- Truck classes
- Class 6-8
- Applications
- Medium-duty, vocational, refuse, transit bus, severe duty
Design and engineering
8.9L inline-6 with wet-sleeve block (like the larger X15). Variable geometry turbocharger. Common rail fuel system. EPA 2017-compliant SCR aftertreatment with DEF. The L9 architecture is similar to the X15 but at smaller scale — same family DNA.
Generations and emissions eras
Reliability and longevity
L9 is one of the most reliable medium-duty diesels available. The wet-sleeve design enables in-frame rebuilds at 700K-1M miles. Vocational fleets (refuse, concrete) report excellent durability in stop-and-go work where many other engines struggle.
Common problems
EGR cooler failures at 300K-500K miles ($1,500-3,000). DPF clogging in low-load applications (stop-and-go vocational, lots of idle time). DEF dosing valve issues. Some 2017-2018 L9s had injector cup wear that Cummins addressed with TSB-mandated replacement.
Maintenance schedule
Engine oil every 40,000 miles (vocational) or 50,000 miles (highway). Fuel filters every 20,000 miles. Coolant flush every 500,000 miles. DPF cleaning every 300,000-400,000 miles.
Pros and cons
✓ Pros
- • Strong vocational durability
- • In-frame rebuild capability
- • Cummins parts availability
- • Smaller package fits medium-duty trucks
- • Adequate power up to 380 hp
⚠ Cons
- • DPF stresses in stop-and-go vocational work
- • DEF system complexity
- • Higher cost than alternatives like PACCAR PX-9
Resale value impact
Cummins L9-equipped vocational trucks command premium pricing — preferred over PACCAR or Detroit options for refuse, concrete, and dump applications. L9 in a Mack Granite is the gold-standard vocational spec.