What Causes a Cummins Engine to Stall While Throttling?

When a Cummins engine gradually stalls while the throttle remains engaged, the underlying issue is often related to insufficient or disrupted fuel supply. This guide provides a clear explanation of potential causes, diagnostic methods, and solutions to help restore proper engine performance.

Common Causes of Gradual Stalling

1. Insufficient Diesel Fuel

A low fuel level in the tank can interrupt the supply to the injection system, causing the engine to lose power.

2. Blocked Vent Valve in the Fuel Tank Cap

A clogged vent valve restricts airflow, creating a vacuum in the tank and hampering fuel delivery.

3. Clogged Fuel Filter or Water Separator

Accumulated debris or water in the filter or separator can reduce the efficiency of the fuel supply.

4. Damaged or Leaky Fuel Lines

Fuel lines with cracks, air ingress, or breaks lead to unstable fuel delivery and engine performance issues.

5. Faulty Fuel Pump

A malfunctioning fuel pump fails to provide adequate fuel pressure, leading to stalling.

6. Water Contamination in the Fuel Tank

Water in the fuel can block injectors and compromise combustion, causing the engine to lose power gradually.

Step-by-Step Diagnosis and Solutions

Step 1: Perform Initial Checks

1. Inspect the fuel level in the tank. Refill if necessary.
2. Examine the fuel tank cap vent valve to ensure it is clear and functional.

Step 2: Check for Air in the Fuel System

1. Loosen the air release screw on the fuel injection pump.
2. Use the hand pump to expel fuel through the air release screw.

Observations:

• No fuel flow: Indicates a blockage in the fuel lines. Inspect and clear blockages section by section.
• Steady diesel spray: Confirms that the fuel line is clear.
• Bubbles in fuel flow: Suggests air ingress. Tighten all fuel line connections and check for leaks, cracks, or wear.

Step 3: Diagnose the Fuel Pump

1. Once the air release procedure confirms no bubbles, secure the hand pump.
2. Use the starter motor to rotate the engine and observe fuel flow.

Results:

• No fuel flow: Points to a malfunctioning fuel pump. Replace or repair as needed.

Key Recommendations for Preventive Maintenance

• Regularly Inspect the Fuel System: Clean the filters and separators periodically to ensure smooth operation.
• Check Fuel Quality: Use high-quality diesel and remove water or contaminants from the fuel tank.
• Maintain the Fuel Pump: Test and service the fuel pump during scheduled maintenance.
• Examine Connections: Ensure all fuel line connections are secure and free from wear or damage.

By following these steps, you can quickly diagnose and resolve fuel supply issues that cause a Cummins engine to stall under throttle. With consistent maintenance, you can minimize disruptions and keep your engine running efficiently.