10-15-2025, 12:40 PM
The F-750 and Its Mid-Duty Diesel Platform
The Ford F-750 is a Class 7 medium-duty truck designed for vocational applications such as utility service, construction, and municipal fleets. When equipped with the Cummins ISB6.7 engine—a 6.7-liter inline-six turbo diesel—the platform offers a balance of torque, fuel economy, and emissions compliance. The ISB6.7, part of Cummins’ B-series lineage, features high-pressure common rail fuel injection, variable geometry turbocharging (VGT), and electronic control modules (ECMs) that interface with transmission and emissions systems. Since its introduction, the ISB6.7 has powered over 1 million vehicles globally, making it one of the most widely deployed mid-duty diesel engines.
Intermittent Power Loss and Shift Hesitation
A recurring issue reported by operators involves intermittent loss of power during gear shifts, particularly when transitioning from first to second gear. The truck accelerates normally in first, but upon shifting, it bogs down, downshifts, and struggles to regain momentum unless the throttle is released. This behavior mimics fuel starvation or turbo lag but occurs without triggering diagnostic trouble codes (DTCs), making it difficult to isolate.
Potential Causes and Diagnostic Pathways
Several factors may contribute to this issue:
In one service case, the truck was towed to a dealer, and the brake position sensor was replaced. Initially, the operator was skeptical, especially given the $500 invoice and lack of clear explanation. However, post-repair testing confirmed that applying the brake during acceleration replicated the derating behavior, validating the diagnosis. The truck resumed normal operation, and the issue did not recur.
Recommendations for Fleet Maintenance
Power loss in the Ford F-750 with Cummins ISB6.7 is often rooted in sensor miscommunication rather than mechanical failure. Brake position sensors, EGR differential pressure sensors, and wiring harness integrity play critical roles in throttle and shift behavior. With careful diagnostics and attention to electronic subsystems, operators can resolve these issues and restore full performance without unnecessary component replacement. The ISB6.7 remains a robust engine, but its electronic complexity demands precision and patience in troubleshooting.
The Ford F-750 is a Class 7 medium-duty truck designed for vocational applications such as utility service, construction, and municipal fleets. When equipped with the Cummins ISB6.7 engine—a 6.7-liter inline-six turbo diesel—the platform offers a balance of torque, fuel economy, and emissions compliance. The ISB6.7, part of Cummins’ B-series lineage, features high-pressure common rail fuel injection, variable geometry turbocharging (VGT), and electronic control modules (ECMs) that interface with transmission and emissions systems. Since its introduction, the ISB6.7 has powered over 1 million vehicles globally, making it one of the most widely deployed mid-duty diesel engines.
Intermittent Power Loss and Shift Hesitation
A recurring issue reported by operators involves intermittent loss of power during gear shifts, particularly when transitioning from first to second gear. The truck accelerates normally in first, but upon shifting, it bogs down, downshifts, and struggles to regain momentum unless the throttle is released. This behavior mimics fuel starvation or turbo lag but occurs without triggering diagnostic trouble codes (DTCs), making it difficult to isolate.
Potential Causes and Diagnostic Pathways
Several factors may contribute to this issue:
- Brake Position Sensor Fault: The brake position sensor communicates with the ECM to manage throttle derating during braking. If the sensor malfunctions or sends false signals, the ECM may interpret that the brake is applied and restrict fuel delivery. This safety feature is designed to prevent throttle override during braking but can misfire under faulty conditions. Replacing the sensor resolved the issue in one documented case.
- EGR Differential Pressure Sensor Contamination: The exhaust gas recirculation (EGR) system uses differential pressure readings to regulate flow. If soot accumulates in the sensor tube, it can give false readings, causing the ECM to reduce boost pressure and alter shift timing. This can result in sluggish acceleration and poor gear transitions. Cleaning or replacing the sensor tube often restores normal function.
- Wiring Harness Damage: A damaged or “pregnant” wire—bulging due to internal corrosion—can intermittently disrupt power to the ECM or sensors. In one case, a red 14-gauge wire under the air cleaner caused failed upshifts and throttle loss. Inspecting and repairing harnesses near the valve cover and intake manifold is recommended.
- Turbo VGT Control Issues: The ISB6.7’s VGT adjusts exhaust flow to optimize boost. If the actuator or control signal is compromised, boost may drop during shifts, causing power loss. While this typically triggers a fault code, early-stage failures may not.
In one service case, the truck was towed to a dealer, and the brake position sensor was replaced. Initially, the operator was skeptical, especially given the $500 invoice and lack of clear explanation. However, post-repair testing confirmed that applying the brake during acceleration replicated the derating behavior, validating the diagnosis. The truck resumed normal operation, and the issue did not recur.
Recommendations for Fleet Maintenance
- Log sensor readings during operation using a diagnostic laptop and Cummins Insite or equivalent software
- Inspect harnesses visually and with continuity testing, especially in high-heat zones
- Clean EGR sensor tubes every 50,000 miles to prevent soot buildup
- Verify brake sensor calibration during annual service intervals
- Document all intermittent faults, even without codes, to assist future diagnostics
Power loss in the Ford F-750 with Cummins ISB6.7 is often rooted in sensor miscommunication rather than mechanical failure. Brake position sensors, EGR differential pressure sensors, and wiring harness integrity play critical roles in throttle and shift behavior. With careful diagnostics and attention to electronic subsystems, operators can resolve these issues and restore full performance without unnecessary component replacement. The ISB6.7 remains a robust engine, but its electronic complexity demands precision and patience in troubleshooting.
