The 8.9 litre ISL G natural gas engine is North America’s natural choice for shuttle and school bus, urban transit, vocational and medium-duty truck and tractor natural gas applications. It meets current U.S. Environmental Protection Agency (EPA) and California Air Resource Board (ARB) emission standards and EPA greenhouse gas (GHG) and U.S. Department of Transportation (DOT) fuel consumption regulations.

The ISL G will save valuable fleet dollars through:

  • Lower fuel costs
  • Improved fuel economy
  • Lower total operating costs

First introduced in June 2007, the ISL G operates on 100% natural gas which can be carried on the vehicle in either compressed (CNG) or liquefied (LNG) form.  The ISL G can also run on renewable natural gas (RNG).

For best reliability and durability, the recommended maximum GCVW for the ISL G in linehaul tractor applications is 66,000 lb (29,943 kg). Recommended gearing to optimize fuel economy is 1700 - 1800 rpm for linehaul applications, and 1750-1900 rpm for vocational applications.

The ISL G is #1 in OEM availability - it's a factory built option from all leading truck and bus builders in North America. To find out more about how using natural gas can improve your bottom line, contact your regional Cummins Westport sales representative

See the ISL G walk-around video here.

Download Brochures:
ISL G Brochure
Product Information Bulletin - Intended Use Guidelines
Product Information Bulletin - Altitude Specifications
Natural Gas Driver Tips Card

Advertised Horsepower 320 hp 239 kW
Peak Torque 1,000 lb-ft 1,356 N•m
Governed Speed 2,200 rpm
Clutch Engagement Torque 550 lb-ft 746 N•m
Type 4-cycle, spark-ignited, inline 6-cylinder, turbocharged, CAC
Displacement 540 cu. in. 8.9 L
Bore and Stroke 4.49 in x 5.69 in 114 mm x 145 mm
Operating Cycles 4
Oil System Capacity 7.3 U.S. gal 27.6 L
Coolant Capacity 13.1 U.S. qts 12.4 L
System Voltage 12 V
Net Weight w/ Std. Accessories, Dry 1,625 lbs 737 kg
Aftertreatment Three-Way Catalyst (TWC)
Fuel Types CNG / LNG / Biomethane
Methane number 75 or greater
Availability Click here for OEM availability

Cummins Westport engines can accept a wide range of natural gas fuel compositions.
Use our Fuel Performance Specification Calculator to ensure compatibility.

Engine Model Advertised Horsepower
(kW) @ rpm
Peak Torque 
lb-ft (N•m) @ rpm
ISL G 320 320 (239) @ 2000 1000 (1356) @ 1300 2200 rpm
ISL G 300  300 (224) @ 2100 860 (1166) @ 1300 2200 rpm
ISL G 280 280 (209) @ 2000 900 (1220) @ 1300 2200 rpm
ISL G 260 260 (194) @ 2200 660 (895) @ 1300 2200 rpm
ISL G 250  250 (186) @ 2200 730 (990) @ 1300 2200 rpm

Overall, maintenance for the ISL G is similar to that of diesel engines, but there are a few key differences:

  • Motor oil specifically formulated for natural gas engines is required. Do not use diesel engine oil in a natural gas engine.  If diesel engine oil is used, valve torching, piston scuffing and reduction in spark-plug life will occur.  Cummins has recently reviewed natural gas engine oil requirements and issued a new natural gas engine oil specification - CES 20085.  For more information, see the links below or visit Cummins QuickServe Online:
  • An initial valve adjustment must be performed after 1,000 hours (15,000 miles/24,140 km) of operation, and is an important step in achieving longer engine life.

ISL G Truck < 66,000 lbs (29,937 kg) GCVW

Maintenance Item Distance Hours Months
Check CAC cooler, piping, and Air cleaner 7,500 mi (12,000 km) 250 3
Oil and Filter* 15,000 mi (24,000 km) 500 6
Fuel Filter 30,000 mi (48,000 km) 1,000 12
Coolant Filter 15,000 mi (24,000 km) 500 6
Spark Plugs 45,000 mi (72,000 km) 1,500 18
Coolant Change 60,000 mi (96,000 km) 2,000 24
Valve Adjustment** 60,000 mi (96,000 km) 2,000 24
Air Cleaner / Element Follow manufacturer's published recommendations
* Requires Natural Gas Engine Oil  - click here for more information.
**Initial valve adjustment at 1,000 hours.

ISL G Urban Transit Bus and Refuse Truck

Maintenance Item^ Distance Hours Months
Oil and Filter* 7,500 mi (12,000 km) 500 6
Fuel Filter 15,000 mi (24,000 km) 1,000 12
Coolant Filter 7,500 mi (12,000 km) 500 6
Spark Plugs^ 22,500 mi (36,000 km) 1,500 18
Coolant Change 30,000 mi (48,000 km) 2,000 24
Valve Adjustment** 30,000 mi (48,000 km) 2,000 24
Air Cleaner / Element Follow manufacturer's published recommendations

* Requires Natural Gas Engine Oil  - click here for more information.
**Initial valve adjustment at 1,000 hours.
^Based on normal duty cycle of 15 mph average speed.  Distance intervals will increase or decrease based on average speed. 

  • Default interval is the hours stated. Interval is whichever comes first – hours, miles or time.
  • Refer to QuickServe® Online or Owners Manual for complete details on maintenance intervals.



Factory-Built, Dedicated Natural Gas Engine – Manufactured on the same assembly line as Cummins diesels, the ISL G shares many components and parts with Cummins L Series diesels, inheriting their renowned simplicity.

  • Electronic Control Module: Provides full monitoring and control of engine sensors, fuel system and ignition system, with full interface capability to Cummins INSITE and diagnostic service tools.
  • Three-Way Catalyst: Simple passive device, highly effective hydrocarbon, CO and NOx control with no maintenance costs. No Diesel Particulate Filter (DPF) or Selective Catalytic Reduction (SCR) systems are required.
  • Low installed weight: Highest power:weight ratio in its class.
  • High-Energy Ignition System – Improved Ignition Control Module (ICM) provides better performance and improved spark plug and coil durability, plus self-diagnostics.
  • Fuel Capability - Natural gas can be stored on the vehicle in either compressed (CNG) or liquefied (LNG) form. The ISL G can also operate on renewable natural gas or biomethane that meets fuel-quality specifications.