Fuel System Troubleshooting on a 1989 Mack RD686 with EC6-350 Engine

[MikePhua]

The RD686 and Its Mack Diesel Heritage
The Mack RD686 is a heavy-duty vocational truck built for construction, hauling, and aggregate work. Introduced in the 1980s, the RD series was known for its rugged frame, high ground clearance, and straightforward mechanical systems. The EC6-350 engine, a variant of Mack’s E6 series, is a turbocharged inline-six diesel producing approximately 350 horsepower. It features a mechanical fuel injection system and a gear-driven transfer pump, making it reliable but sensitive to fuel delivery issues.
Mack Trucks, founded in 1900, built its reputation on durability and self-contained engineering. The E6 engine family was widely used in dump trucks, mixers, and lowboy tractors throughout the 1980s and 1990s. Many of these trucks remain in service today, especially in rural fleets and restoration projects.
Symptoms of Fuel Delivery Failure
Operators of the RD686 with EC6-350 have reported a recurring issue:

• Engine starts normally but loses power after 1–2 minutes
  
• Throttle response fades even at full pedal depression
  
• Engine stalls as if fuel supply is cut off
  
• Restart attempts may succeed temporarily, then fail again
  
• Fuel filters appear full, but suction noise is heard when removed
  
• Pressure gauge shows adequate supply during failure
  

Terminology Note: “Transfer pump” refers to the low-pressure pump that moves fuel from the tank to the injection pump. “Overflow valve” regulates return flow and maintains pressure in the injection system.
In one case, a technician in Pennsylvania installed a pressure gauge between the secondary filter and injection pump. Despite good pressure readings, the engine still stalled under load, suggesting a deeper issue beyond simple fuel starvation.
Common Causes and Diagnostic Path
Fuel system failures in this configuration often stem from:

• Collapsed or blocked suction lines from the tank
  
• Debris or foreign objects in the tank (e.g., rags, rust flakes)
  
• Weak or failing transfer pump unable to maintain volume
  
• Air intrusion from cracked fittings or loose clamps
  
• Return line restrictions causing pressure imbalance
  
• Internal governor or throttle linkage interference
  

Diagnostic steps:

• Blow back suction line to tank using compressed air
  
• Replace all rubber fuel lines with reinforced hose
  
• Drain and inspect tank for contaminants
  
• Install clear line to monitor air bubbles
  
• Test overflow valve for proper pressure regulation
  
• Inspect throttle linkage for full travel and internal stops
  

A mechanic in Alberta discovered a green twist tie lodged near the throttle arm, limiting travel and causing the governor to prematurely reduce fuel delivery. After removing the obstruction, the engine regained full RPM range.
Bleeding and Priming Challenges
After replacing lines and filters, bleeding the EC6-350 can be difficult due to:

• Long suction path from tank to primary filter
  
• Air pockets in the transfer pump housing
  
• Lack of hand primer or bleeder screw on some models
  
• Return line routing that traps air
  

Solutions:

• Use compressed air to gently pressurize the fuel tank
  
• Install a temporary electric lift pump to assist priming
  
• Crack injector lines during cranking to release trapped air
  
• Fill filters manually before installation
  
• Cycle ignition and throttle to purge governor cavity
  

In 2023, a restorer in Maine used a low-pressure electric pump to push fuel through the system during bleeding. This reduced cranking time and allowed the truck to start cleanly after filter replacement.
Advanced Troubleshooting and Component Testing
If basic steps fail, deeper issues may include:

• Sticky plungers in the injection pump
  
• Worn pump barrels or damaged O-rings
  
• Heat-related pump fade due to internal wear
  
• Governor spring fatigue or misalignment
  
• Return check valve malfunction
  

Recommendations:

• Remove and inspect plungers using a specialized puller tool
  
• Replace O-rings and lap plungers if scoring is found
  
• Test pump under load with infrared temperature monitoring
  
• Pour cool water over pump housing to simulate thermal recovery
  
• Replace check valves in transfer pump and return circuit
  

A technician in Ohio diagnosed hot-start failure and traced it to weak plungers. After rebuilding the injection pump with new barrels and seals, the truck ran reliably even after extended idling.
Preventive Measures and Long-Term Solutions
To prevent future fuel issues:

• Replace fuel lines every 5 years or 100,000 miles
  
• Use biocide additives to prevent microbial growth in diesel
  
• Install inline fuel screens before primary filter
  
• Monitor fuel pressure with permanent gauge
  
• Keep tank at least half full to reduce suction strain
  

Upgrade options:

• Retrofit electric lift pump for priming and cold starts
  
• Add fuel heater for winter operation
  
• Install throttle stop screw to prevent over-travel
  
• Use stainless steel return lines for durability
  

In 2024, a fleet manager in Texas added fuel heaters and upgraded lines on all EC6-powered trucks. The result was improved cold-weather reliability and reduced downtime during seasonal transitions.
Conclusion
Fuel system issues in the 1989 Mack RD686 with EC6-350 engine often stem from a combination of age-related wear, suction line restrictions, and internal pump degradation. With methodical diagnostics, component upgrades, and preventive maintenance, these trucks can continue to serve reliably. In the world of mechanical diesels, fuel delivery is everything—and solving it means listening to every line, valve, and throttle arm. When the articulation works and the fuel flows, the old iron still earns its keep.