08-01-2025, 09:32 PM
Overview: 6068 Engine Behavior After Turbo Replacement
The John Deere 6068 diesel (6.8 L) is robust and commonly used in Magnum 235 skid-steers, loaders, and generators. Issues arise when the original turbo is replaced: the engine initially runs well, only to develop rough running and eventually fail to start. The likely root is fuel or sensor-related—not mechanical timing.
Main Symptoms Experienced Post‑Turbo Swap
Possible Root Causes
Diagnostic & Repair Checklist
Terminology Explained
Real-World Operator Examples
Summary Troubleshooting Flow
Conclusion
Post-turbo replacement rough running and eventual no-start on a JD 6068 engine are usually due to fuel flow interruption or RPM sensing faults—not turbo choice. Begin with fuel system integrity checks—lift pump, air leaks, check valves—and verify rpm sensor output. Carefully bleeding the system and restoring positive fuel feed typically resolves startup issues. If these steps are followed, engine reliability often returns without full injection system overhaul.
The John Deere 6068 diesel (6.8 L) is robust and commonly used in Magnum 235 skid-steers, loaders, and generators. Issues arise when the original turbo is replaced: the engine initially runs well, only to develop rough running and eventually fail to start. The likely root is fuel or sensor-related—not mechanical timing.
Main Symptoms Experienced Post‑Turbo Swap
- Operated well immediately after turbo install, then gradually developed rough idle or misfire under load.
- Eventually cranks but does not start, despite apparent fuel and glow system function.
Possible Root Causes
- Air Intrusion in Fuel System
- Air leaks in supply lines, return loops, or cracked fittings cause loss of prime. Fuel bleeds off when stood idle, leading to starting failure. Similar systems have us install check valves or lift/return line loops to trap fuel—preventing backflow.
- Air leaks in supply lines, return loops, or cracked fittings cause loss of prime. Fuel bleeds off when stood idle, leading to starting failure. Similar systems have us install check valves or lift/return line loops to trap fuel—preventing backflow.
- Lift/Transfer Fuel Pump or Check Valve Fault
- A diaphragm or inlet check failure stops consistent fuel pressure to the injection pump. In generator-powered applications with the same engine, replacing the lift pump resolved intermittent starts.
- A diaphragm or inlet check failure stops consistent fuel pressure to the injection pump. In generator-powered applications with the same engine, replacing the lift pump resolved intermittent starts.
- RPM/Sensor Malfunction Causing Safety Shutdown
- Many machines (especially with MAGNUM auto-start or generator control modules) require a functioning magnetic speed pickup on the flywheel. If no signal is detected during startup or operation, the system shuts down—even if the engine fires. A bad sensor or wiring fault often stops fuel injection immediately.
- Many machines (especially with MAGNUM auto-start or generator control modules) require a functioning magnetic speed pickup on the flywheel. If no signal is detected during startup or operation, the system shuts down—even if the engine fires. A bad sensor or wiring fault often stops fuel injection immediately.
- Faulty Injection Pump or Nozzle Timing
- Though less common immediately after turbo installation, improper timing or worn pump internals can cause rough operation once fuel delivery becomes marginal or temperature changes.
- Though less common immediately after turbo installation, improper timing or worn pump internals can cause rough operation once fuel delivery becomes marginal or temperature changes.
Diagnostic & Repair Checklist
- Confirm Fuel Flow Under Crank and Post-Shutdown
- Loosen supply line to injection pump and crank; note whether fuel continues flowing after extended idle. If flow stops, suspect air leak or check-valve failure.
- Loosen supply line to injection pump and crank; note whether fuel continues flowing after extended idle. If flow stops, suspect air leak or check-valve failure.
- Install Inline Fuel Check Valve and Return Loop
- Use a clear vertical return line loop above the tank to trap fuel and let you watch for bubble movement and backflow.
- Use a clear vertical return line loop above the tank to trap fuel and let you watch for bubble movement and backflow.
- Verify Lift Pump Function and Replace If Leaking
- Replace aging diaphragm pumps; ensure check valves open consistently to avoid pressure loss overnight.
- Replace aging diaphragm pumps; ensure check valves open consistently to avoid pressure loss overnight.
- Test RPM Sensor Function
- Use a diagnostic tool or voltmeter to verify 2–15 VAC from the flywheel pickup during crank and running. If no voltage, clean or replace the sensor and wiring.
- Use a diagnostic tool or voltmeter to verify 2–15 VAC from the flywheel pickup during crank and running. If no voltage, clean or replace the sensor and wiring.
- Bleed Entire Fuel System Thoroughly After Assembly
- Especially after turbo or pump replacement; follow proper bleeding protocols to eliminate air and ensure consistent injection.
- Especially after turbo or pump replacement; follow proper bleeding protocols to eliminate air and ensure consistent injection.
Terminology Explained
- Lift (Supply) Pump: Draws fuel from tank to injection pump; often a diaphragm type with check valves.
- Check Valve: Prevents reverse flow when fuel system pressure drops or engine shuts off.
- Magnetic Pickup Sensor: Mounted near flywheel; detects engine RPM for auto-start or safety shutdown control.
- Prime/Bleed Procedure: Releasing trapped air in fuel lines to maintain full-pressure fuel feed.
Real-World Operator Examples
- A generator user with a 6068T reported the engine would start only briefly if not primed; replacement of the lift pump and repair of check valves resolved no‑start issues.
- Another owner recounted how the engine would shut off immediately due to no RPM signal detected—even when cranking. Cleaning/replacing the flywheel sensor restored normal operation.
Summary Troubleshooting Flow
- Inspect and secure all fuel lines; look for bubbles or leaks under crank.
- Install inline check valve and return feed loop if missing.
- Replace leaking or old lift pump (diaphragm type).
- Test RPM sensor during crank for proper voltage output.
- Bleed fuel system completely post-maintenance.
- Evaluate injector pump if rough running continues after fuel system verified.
Conclusion
Post-turbo replacement rough running and eventual no-start on a JD 6068 engine are usually due to fuel flow interruption or RPM sensing faults—not turbo choice. Begin with fuel system integrity checks—lift pump, air leaks, check valves—and verify rpm sensor output. Carefully bleeding the system and restoring positive fuel feed typically resolves startup issues. If these steps are followed, engine reliability often returns without full injection system overhaul.
