The Cummins Westport ISX12 G natural gas engine is a larger-displacement natural gas engine suitable for a variety of heavy-duty vehicles, including regional-haul truck/tractor, vocational, and refuse applications.  With a displacement of 11.9 liters and up to 400 hp and 1450 lb-ft of torque, the ISX12 G is the natural choice when considering alternative fuel for demanding applications.  The ISX12 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).

Featuring the same proven Stoichiometric cooled Exhaust Gas Recirculation combustion technology, spark ignition and simple Three-Way Catalyst aftertreatment as the Cummins Westport ISL G engine, the ISX12 G delivers strong pulling power and heavy-duty durability and performance that customers can depend on.

For best reliability and durability, the recommended maximum GCVW for linehaul applications is 80,000 lb (36,287 kg). Recommended gearing to optimize fuel economy is 1400 to 1475 rpm at cruise speed for linehaul applications, and 1450 - 1600 rpm for vocational applications.

The ISX12 G 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 fuel consumption regulations.

Note: The ISX12 G is no longer available. It has been replaced by the ISX12N—learn more here.

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Advertised Horsepower 400 hp 298 kW
Peak Torque 1450 lb-ft 1966 N•m
Governed Speed 2,100 rpm
Clutch Engagement Torque 700 lb-ft 949 N•m
Type 4-cycle, spark-ignited, inline 6-cylinder, turbocharged, CAC
Displacement 726.2 cu. in. 11.9 L
Bore and Stroke 5.11 in x 5.91 in 130 mm x 150 mm
Operating Cycles
Oil System Capacity 12 U.S. gal 45.4 L
Coolant Capacity 26.5 U.S. qts 25.1 L
Net Weight w/ Std. Accessories, Dry 2,650 lbs 1.202 kg
Aftertreatment Three-Way Catalyst 
Fuel Types CNG / LNG / RNG
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
Peak Torque 
lb-ft (N•m) @ rpm
Linehaul Truck      
ISX12 G 400 400 (298) 1450 (1966) @ 1200 2100 rpm
ISX12 G 385  385 (287) 1350 (1830) @ 1200 2100 rpm
ISX12 G 350 350 (261) 1450 (1966) @ 1200 2100 rpm
ISX12 G 330 330 (246) 1250 (1695) @ 1200 2100 rpm
ISX12 G 320 320 (239) 1150 (1559) @ 1200 2100 rpm
Refuse Truck      
ISX12 G 350R 350 (261) 1450 (1966) @ 1200 2100 rpm
ISX12 G 350R 350 (261) 1350 (1830) @1200 2100 rpm
ISX12 G 330R 330 (246) 1250 (1695) @ 1200 2100 rpm
ISX12 G 320R 320 (239) 1150 (1559) @ 1200 2100 rpm


Overall, maintenance for the ISX12 G is similar to that of diesel engines, but there are a few key differences: 

ISX12 G Maintenance Intervals - Line Haul Truck
< 80,000 lb (36,287 kg) GCVW

Maintenance Item^ Hours Distance Months
Spin-on Fuel Filter Daily Check
Oil And Filter* 500 25,000 mi (40,000 km) 6
Coolant Filter 1,500 75,000 mi (120,000 km) 12
Spin-on Fuel Filter 1,000 50,000 mi (80,000 km) 9
Spark Plugs 1,000 50,000 mi (80,000 km) 12
Overhead Set Adjustment** 3,000 150,000 mi (240,000 km) 24
Engine Brake Adjustment 6,000 300,000 mi (480,000 km) 24
Cooling System Flush 6,000 300,000 mi (480,000 km) 24
Ignition Coil Extension 10,000 500,000 mi (800,000 km) 5 yrs
Air cleaner / element Follow vehicle manufacturers published recommendations

^Assuming normal line haul duty cycle based on 50 mph / 80 kph average speed.  Maintenance intervals must be reduced for slower average speed applications
**Initial overhead set adjustment at 1,000 hours.
*Requires Natural Gas Engine Oil. For customers using CES 20092 natural gas engine oils only, the recommended interval is 40,000 mi (64,000 km) for vehicles with average road speeds greater than 25 mph (40 kph).

ISX12 G Maintenance Intervals - Urban Truck / Refuse
< 80,000 lb (36,287 kg) GCVW

Maintenance Item^ Hours Distance Months
Oil and Filter* 500 7,500 mi (12,500 km) 6
Fuel Filter 1,000 15,000 mi (25,000 km) 9
Coolant Filter 1,500 22,500 mi (37,500 km) 12
Spark Plugs 1,000 15,000 mi (25,000 km) 12
Cooling System Flush 6,000 90,000 mi (150,000 km) 24
Overhead Set  Adjustment** 3,000 45,000 mi (75,000 km) 24
Engine Brake Adjustment 6,000 90,000 mi (150,000 km) 24
Ignition Coil Extension 10,000 150,000 mi (240,000 km) 5 yrs
Air Cleaner / Element Follow vehicle manufacturer's published recommendations

^Based on normal duty cycle of 15 mph / 25 kph average speed.  Distance intervals will increase or decrease based on average speed.
**Initial overhead set adjustment at 1,000 hours.
* Requires Natural Gas Engine Oil. 

Refer to QuickServe Online or Owners Manual for complete details on maintenance intervals.

Watch a video overview of ISX12 G engine maintenance.

  • Factory-Built, Dedicated Natural Gas Engine – Manufactured on the same assembly line as Cummins ISX diesels, the ISX12 G shares many components and parts with Cummins ISX12 Series diesels.
  • Cooled-EGR Combustion Technology – Takes a measured quantity of exhaust gas and passes it through a cooler to reduce the temperature before mixing it with fuel and the incoming air charge to the cylinder. The use of cooled EGR in combination with stoichiometric combustion improves power density and fuel economy versus lean-burn or traditional stoichiometric natural gas engines.
  • Three-Way Catalyst Aftertreatment – Three-Way Catalysts (TWCs) are effective, simple devices, packaged as part of the muffler, that provide consistent emissions control performance and are maintenance-free.  No Diesel Particulate Filter (DPF) or Selective Catalytic Reduction (SCR) systems are required.
  • Advanced Air Handling – Electronic control for precise air handling. Wastegate turbocharger with water-cooled bearing housing for durability.
  • Engine braking, and manual, automatic, and automated manual transmission capability.
  • Fuel Capability – Natural gas can be stored on the vehicle in either compressed (CNG) or liquefied (LNG) form. The ISX12 G can also operate on renewable natural gas or biomethane that meets fuel-quality specifications.