Step-by-Step Guide to Bleeding Air from a Cummins PT Fuel System
Air in the fuel system is a common reason a Cummins diesel engine becomes difficult to start after fuel-system maintenance. A filter change, an empty fuel tank, a disconnected fuel line, or work on the PT pump or injectors can all allow air to enter the system. The result may be hard starting, rough idle, low power, surging, or excessive smoke.
This is particularly relevant to older Cummins engines using the PT fuel system, including the NT855 and K19 families. Cummins identifies the PT system as a Pressure-Time fuel system, with camshaft-actuated injectors and a fuel pump designed to meter fuel according to engine operating conditions.
The good news is that trapped air does not always mean there is a serious fuel-system fault. In many cases, the engine simply needs to be properly primed and the source of the air leak removed.
The procedure below explains the basic approach in a practical way. Because fuel-system layouts vary between engine models and applications, always check the correct Cummins service information before loosening fuel connections or applying a specified torque.
Safety Precautions Before You Start
Diesel fuel is flammable, and fuel-system work should never be treated as a routine adjustment.
Work on a cool engine in a well-ventilated area. Keep fuel away from exhaust components, electrical sparks, open flames, and other ignition sources. Wear safety glasses and fuel-resistant gloves, and keep absorbent rags and a suitable container available for spilled fuel.
Do not put your hands near a fuel leak while the engine is running. Fuel under pressure can cause serious injury if it penetrates the skin. Cummins service documentation also emphasizes that fuel-system bleeding and high-pressure fuel-system work should be performed by qualified personnel.
If the engine is installed in a truck, generator, marine application, or heavy machine, make sure it is securely parked or supported before starting the work.
Tools and Materials You May Need
The exact tools depend on the engine and installation, but most bleeding jobs require only basic equipment:
- Wrenches suitable for the fuel-filter and fuel-line fittings
- Clean rags or absorbent pads
- A container for collecting diesel fuel
- Clean diesel fuel
- A hand primer, if the installation is equipped with one
- A clear temporary fuel line or sight glass for diagnosis, where appropriate
- A battery charger if extended cranking is expected
- The correct Cummins service manual for the engine model
Do not assume that a particular wrench size, bleed-point location, or torque specification applies to every PT engine. NT855, N855, K19 and other Cummins engines can have different configurations.
When Does a Cummins PT Fuel System Need to Be Bled?
You should consider bleeding or priming the fuel system after:
- Replacing a fuel filter
- Running the fuel tank completely empty
- Disconnecting or replacing a fuel supply line
- Removing the PT fuel pump
- Performing work on injectors or other fuel-system components
- Leaving the engine unused for an extended period
- Finding an air leak on the fuel-supply side
- Experiencing hard starting or unstable running after fuel-system maintenance
If the engine was running normally before a filter or fuel-line repair and suddenly becomes difficult to start afterward, trapped air should be one of the first things to check.
Step-by-Step Procedure for Bleeding Air
Step 1: Check the Fuel Supply First
Before opening any fuel connection, make sure there is enough clean diesel in the tank.
A low fuel level can sometimes make the problem harder to diagnose, especially if the machine is parked on an incline. If the system has a fuel shut-off valve, make sure it is fully open.
Then check the fuel lines from the tank to the fuel filter. Look for loose fittings, cracked or damaged hoses, worn sealing rings, or other possible points where air could enter the system.
With a Cummins PT fuel system, an air leak on the suction side may not cause a visible fuel leak. A loose fitting or damaged seal can allow air to enter the fuel system while leaving little or no diesel around the connection. For this reason, check the fuel connections carefully even if there are no obvious signs of fuel leakage.
Step 2: Prime and Bleed the Fuel Filter Side
If the fuel filter has recently been replaced, first check that it is installed correctly and that the seals are seated properly.
If the system is equipped with a hand primer, use it according to the engine service procedure. If the filter housing has a designated vent or bleed point, open it carefully and allow fuel to flow until the air is removed and the fuel comes out steadily without obvious bubbles.
Once you have a solid flow of fuel, close the vent securely.
The exact bleeding procedure can vary between Cummins engines and equipment. Some PT fuel systems use a supply-and-return arrangement, and Cummins service procedures for certain diesel systems call for bleeding the low-pressure side while allowing the system to purge remaining air during operation.
Do not loosen additional fuel fittings or add another bleed point unless the service procedure specifically calls for it. Use the bleeding and priming procedure specified for your particular engine and fuel system.
Step 3: Check for Air Between the Tank, Filter and PT Pump
If the engine still will not start after priming, check whether air is getting back into the fuel system.
Inspect the fuel supply connections from the tank to the PT pump, paying particular attention to the filter housing, fuel lines, fittings, and seals. On an older engine, a hardened O-ring, loose fitting, or deteriorated fuel hose can allow air into the system and cause repeated hard-starting problems.
A temporary section of clear fuel line may also help you see whether air is entering the supply side. If you use one, make sure it is installed safely and in accordance with the applicable service procedure.
If you see a continuous stream of air bubbles in the fuel, do not keep cranking the engine in an attempt to clear them. Find and correct the source of the air leak first.
The NT855 is one example of a Cummins engine that uses a PT fuel system. Its fuel system includes a spin-on fuel filter and drilled fuel supply and return passages through the cylinder heads. Because of this arrangement, its fuel routing and bleeding points may look different from those on newer common-rail diesel engines.
Step 4: Crank the Engine Carefully
Once the fuel supply has been primed and any obvious air leaks have been corrected, try starting the engine.
Do not crank the engine continuously for long periods. Use short cranking cycles and allow the starter and batteries to recover between attempts. If the battery is weak, the engine may crank too slowly, making an existing fuel or air problem more difficult to diagnose.
If the engine starts to fire but does not keep running, repeat the recommended priming procedure before checking the fuel system again. Avoid loosening several fuel connections at once, as this can introduce more air and make the problem harder to trace.
Keep in mind that the correct procedure depends on the specific engine and PT fuel system. A bleeding method intended for a modern common-rail diesel should not automatically be applied to a Cummins PT system. Always use the service procedure specified for the engine you are working on.
Step 5: Check Injector Fuel Delivery Only When Required
If the engine still will not start after the low-pressure side has been properly primed, the next step may be to check fuel delivery at the injectors.
This part of the diagnosis requires more care. PT injectors are part of the fuel metering and injection system, and the Cummins PT system uses camshaft-actuated injectors. They should not be treated like ordinary low-pressure fuel lines for bleeding purposes.
Some older service procedures may call for loosening a fuel connection during troubleshooting. However, the correct connection, test procedure, and tightening torque vary by engine model and injector arrangement. Do not rely on a general torque specification. Check the applicable Cummins service manual for the engine being serviced.
If the injectors are not receiving fuel after the supply side has been confirmed, trapped air may not be the only problem. Further checks may include the fuel shut-off mechanism, PT pump output, fuel filter condition, restrictions in the supply line, and injector condition.
What If the Engine Still Runs Roughly After Bleeding?
If the engine starts after bleeding but continues to surge, smoke, or run unevenly, there may be another fuel-system problem that needs to be checked.
Begin by checking whether air is still entering the fuel supply. If you continue to see air bubbles in the supply line, inspect the tank pickup, fuel hoses, filter seals, and fittings for possible leaks or restrictions.
If the engine is receiving fuel but has poor power or continues to run unevenly, check the fuel supply and confirm that the PT pump is delivering fuel correctly. If there is no clear air leak or supply restriction, the problem may involve the PT pump, injectors, governor, fuel shut-off mechanism, or another engine component.
Avoid replacing parts based only on symptoms. A clear section of fuel line, appropriate fuel pressure or flow tests, and the specifications for the exact engine model can help identify the problem before major components are removed.
For more involved PT fuel-system work, use the service information for the specific engine. Cummins also provides technician training covering PT fuel-system identification, fuel-pump and injector service, troubleshooting, and maintenance procedures on engines such as the K19. If testing points to a problem with the PT pump or injectors, having the system diagnosed and serviced by a qualified technician can help prevent unnecessary repairs.
How to Prevent Air From Entering the PT Fuel System
Good maintenance can prevent many bleeding problems in the first place. When replacing a fuel filter, follow the filter manufacturer’s installation instructions and use clean diesel fuel where pre-filling is appropriate for that particular filter and engine. Keep dirt out of open fuel lines and fittings during maintenance.
Pay particular attention to the suction side of the fuel system. Loose connections, damaged hoses and worn seals can introduce air even when there is no obvious external fuel leak.
It is also worth checking the fuel system after maintenance rather than assuming the job is complete as soon as the engine starts. Let the engine run long enough to confirm stable idle and normal response, then inspect the connections for fuel leaks.
For engines such as the NT855 and K19, the PT fuel system remains a key part of the engine’s operating and maintenance requirements. Cummins continues to list PT fuel systems on these engine families and describes the system as a proven design intended for reliable fuel metering and ease of maintenance.
Final Thoughts
Bleeding air from a Cummins PT fuel system is generally straightforward, but the correct procedure varies with the engine model and fuel-system configuration.
Start by checking the fuel supply, filter, and low-pressure side. Prime the system properly and inspect the suction side for air leaks. If the engine still will not start or continues to run poorly, check the fuel delivery system rather than repeatedly cranking the engine or loosening fuel connections without following a specific procedure.
For engines such as the NT855, K19, and N855, do not rely on generic bleeding instructions or torque specifications. The service information for the specific engine and serial number should be used to confirm fuel-line locations, tightening specifications, and diagnostic procedures.
The same applies when replacing fuel-system components. Using the correct filters, O-rings, injectors, and PT fuel-system parts can help prevent further starting and performance problems. If you need Cummins PT fuel system parts, you can view the available PT fuel system components or contact a qualified supplier Longshine to confirm compatibility with your engine model and application.