08-12-2025, 10:06 AM
Introduction: When a Legendary Engine Starts to Falter
The Cummins Big Cam 350 is a workhorse of the heavy-duty diesel world, known for its durability and torque. But even legends stumble. This article unpacks a persistent and baffling issue affecting a 1985 International S2200 equipped with a Big Cam 350: sluggish performance, low turbo boost, and intermittent power loss. Through field diagnostics, component replacements, and creative workarounds, we explore the anatomy of the problem and offer insights for mechanics and operators alike.
Terminology Note: Key Components in Focus
- PT Pump (Pressure-Time Fuel Pump): A mechanical fuel injection system used in older Cummins engines.
- AFC (Air-Fuel Control) Diaphragm: Regulates fuel delivery based on boost pressure to prevent over-fueling under low load.
- Intercooler (Aftercooler): Cools compressed air from the turbo before it enters the engine, improving efficiency and power.
- Aneroid Valve: A boost-sensing device that controls fuel flow in response to intake pressure.
The Initial Symptoms: Sluggish Throttle and No Boost
The truck began losing power while hauling an excavator uphill. Throttle response was poor, and turbo boost was nearly nonexistent. Fuel filters were replaced with no improvement. A used turbo from an older Cummins 290 temporarily restored performance, but the issue returned within weeks. A rebuilt original turbo offered another short-lived fix.
Diagnostic Journey: From Turbo to Fuel System
After exhausting all options, the operator installed a gravity-fed auxiliary fuel tank in the cab. No change. On a final trip to the mechanic, the truck lost power after 5 miles. Shutting off the engine for a few minutes restored normal performance temporarily. This on-off cycling allowed the truck to limp to the yard—a frustrating but revealing symptom.
Suspected Culprits and Confirmed Findings
The Big Cam series, introduced in the 1970s, revolutionized diesel performance with variable timing and PT fuel systems. The 350 model became a staple in vocational trucks, praised for its torque and reliability. However, its mechanical complexity means that age-related issues—especially in fuel and air systems—can be difficult to diagnose without deep experience.
Conclusion: Persistence Pays Off in Diesel Diagnostics
The Cummins Big Cam 350’s intermittent power loss was ultimately traced to a combination of intake restrictions, fuel delivery inconsistencies, and aging components. While no single fix solved the issue permanently, a layered approach—combining diagnostics, component replacement, and creative rerouting—allowed the operator to restore functionality. For mechanics working on legacy engines, this case underscores the importance of understanding system interdependencies and trusting field intuition. Sometimes, the best diagnostic tool is a clear hose and a long hill.
The Cummins Big Cam 350 is a workhorse of the heavy-duty diesel world, known for its durability and torque. But even legends stumble. This article unpacks a persistent and baffling issue affecting a 1985 International S2200 equipped with a Big Cam 350: sluggish performance, low turbo boost, and intermittent power loss. Through field diagnostics, component replacements, and creative workarounds, we explore the anatomy of the problem and offer insights for mechanics and operators alike.
Terminology Note: Key Components in Focus
- PT Pump (Pressure-Time Fuel Pump): A mechanical fuel injection system used in older Cummins engines.
- AFC (Air-Fuel Control) Diaphragm: Regulates fuel delivery based on boost pressure to prevent over-fueling under low load.
- Intercooler (Aftercooler): Cools compressed air from the turbo before it enters the engine, improving efficiency and power.
- Aneroid Valve: A boost-sensing device that controls fuel flow in response to intake pressure.
The Initial Symptoms: Sluggish Throttle and No Boost
The truck began losing power while hauling an excavator uphill. Throttle response was poor, and turbo boost was nearly nonexistent. Fuel filters were replaced with no improvement. A used turbo from an older Cummins 290 temporarily restored performance, but the issue returned within weeks. A rebuilt original turbo offered another short-lived fix.
Diagnostic Journey: From Turbo to Fuel System
- Turbo replaced twice, each time yielding temporary improvement
- Intercooler removed and inspected—found clean
- Air filter replaced
- Boost pressure measured: only 1 psi at 2400 RPM under no load
- Suspected vacuum or restriction in intake system
- PT pump suspected; replaced with new unit—initially resolved issue
- Problem returned weeks later
- Fuel pickup tube and lines inspected
- Clear tubing installed in suction line to detect air bubbles
- No visible aeration, but intermittent power loss persisted
After exhausting all options, the operator installed a gravity-fed auxiliary fuel tank in the cab. No change. On a final trip to the mechanic, the truck lost power after 5 miles. Shutting off the engine for a few minutes restored normal performance temporarily. This on-off cycling allowed the truck to limp to the yard—a frustrating but revealing symptom.
Suspected Culprits and Confirmed Findings
- Aneroid Valve Starved of Boost Pressure
- The aneroid valve receives boost pressure from the intake manifold downstream of the intercooler. A restriction in the intercooler reduced pressure, preventing the valve from signaling the pump to deliver more fuel. A jumper line bypassing the intercooler temporarily restored power.
- Intercooler Pressure Drop
- A 5 psi drop across the intercooler was measured—far above the acceptable 1–1.5 psi range. This suggested internal restriction or gauge error. Later inspection revealed the intercooler was clean, and the pressure drop was due to mismatched gauges.
- Silicone Boot Failure
- The boot connecting the turbo to the intake was found to be deteriorated. Replacing it improved performance briefly before symptoms returned.
- Fuel Line Deterioration
- The aneroid valve receives boost pressure from the intake manifold downstream of the intercooler. A restriction in the intercooler reduced pressure, preventing the valve from signaling the pump to deliver more fuel. A jumper line bypassing the intercooler temporarily restored power.
- Old rubber fuel lines can develop internal flaps that act like check valves, restricting flow under load. Replacing all fuel lines is recommended for aging trucks.
- Boost pressure: Should exceed 10–15 psi under full load
- Fuel pressure to injectors: Check via ⅛" pipe plug on front head
- Aneroid activation: Requires ~30 psi from intake manifold
- Intercooler pressure drop: <1.5 psi ideal; >4 psi unacceptable
- Valve lash: Should be checked every 100,000 miles or annually
- Bypass intercooler for aneroid signal if restriction suspected
- Replace all fuel lines, including pickup tube
- Use clear tubing to detect suction-side air leaks
- Install matched pressure gauges before and after intercooler
- Perform full tune-up including valve lash adjustment
- Replace silicone boots and clamps in intake system
- Consider upgrading to a newer turbo with better flow characteristics
The Big Cam series, introduced in the 1970s, revolutionized diesel performance with variable timing and PT fuel systems. The 350 model became a staple in vocational trucks, praised for its torque and reliability. However, its mechanical complexity means that age-related issues—especially in fuel and air systems—can be difficult to diagnose without deep experience.
Conclusion: Persistence Pays Off in Diesel Diagnostics
The Cummins Big Cam 350’s intermittent power loss was ultimately traced to a combination of intake restrictions, fuel delivery inconsistencies, and aging components. While no single fix solved the issue permanently, a layered approach—combining diagnostics, component replacement, and creative rerouting—allowed the operator to restore functionality. For mechanics working on legacy engines, this case underscores the importance of understanding system interdependencies and trusting field intuition. Sometimes, the best diagnostic tool is a clear hose and a long hill.
