Cummins ISL G (natural gas, legacy)
8.90L Inline-6 natural gas • Cummins
About the Cummins ISL G (natural gas, legacy)
The Cummins ISL G was the dominant natural-gas engine in North America for over 15 years — produced 2007-2023 in 8.9L displacement for medium-heavy Class 6-8 applications. Refuse fleets, transit buses, and port drayage operators ran ISL G engines on CNG/LNG/RNG fuel before the engine was replaced by the L9N in 2024. The ISL G achieved similar emissions benefits to the ISX12N but at lighter-duty applications. Production wound down 2023.
Overview
The Cummins ISL G was the dominant natural-gas engine in North America for over 15 years — produced 2007-2023 in 8.9L displacement for medium-heavy Class 6-8 applications. Refuse fleets, transit buses, and port drayage operators ran ISL G engines on CNG/LNG/RNG fuel before the engine was replaced by the L9N in 2024. The ISL G achieved similar emissions benefits to the ISX12N but at lighter-duty applications. Production wound down 2023.
Specifications
- Manufacturer
- Cummins
- Displacement
- 8.90 liters
- Configuration
- Inline-6 natural gas
- Fuel type
- CNG / LNG / RNG
- Horsepower
- 250–320 hp
- Torque
- 660–1000 lb-ft
- Production years
- 2007–2023
- Truck classes
- Class 6-8
- Applications
- Refuse, transit bus, regional, port drayage (legacy)
Design and engineering
8.9L inline-6 natural-gas, spark ignition, three-way catalyst aftertreatment.
Generations and emissions eras
Reliability and longevity
Generally reliable for refuse and bus applications — 500K-800K miles before major service. Spark plug wear is the main service item.
Common problems
Spark plug wear. Three-way catalyst contamination from poor-quality NG. Lower power than diesel L9.
Pros and cons
✓ Pros
- • Proven NG platform — 15+ years of fleet service
- • Simpler aftertreatment than diesel
- • Refuse + transit + port drayage workhorse
⚠ Cons
- • Production ended 2023
- • Replaced by L9N
- • Lower power than L9 diesel