Deere 690B Excavator Fuel Tank Contamination and Field Remediation

[MikePhua]

The 690B and Deere’s Hydraulic Excavator Expansion
The John Deere 690B excavator was introduced in the late 1970s as part of Deere’s push into the hydraulic excavator market. Built during a time when Deere was transitioning from cable-operated machines to fully hydraulic systems, the 690B featured a robust steel frame, mechanical simplicity, and a six-cylinder diesel engine delivering around 120 horsepower. With an operating weight near 45,000 pounds and a digging depth exceeding 20 feet, the 690B became a staple in roadbuilding, utility trenching, and site clearing across North America.
Despite its mechanical reliability, the 690B—like many older machines—faces challenges with fuel system contamination, particularly from aging tanks, degraded diesel, and poor filtration. Dirty fuel tanks can lead to injector damage, pump wear, and hard starting, especially in machines stored for long periods or operated in dusty environments.
Fuel Tank Contamination and Its Consequences
Fuel tanks in older excavators are typically made of mild steel and lack internal coatings. Over time, condensation, microbial growth, and sediment accumulation degrade fuel quality. Common symptoms include:

• Engine hesitation or stalling under load
  
• Clogged fuel filters within hours of replacement
  
• Black sludge or rust particles in the fuel bowl
  
• Injector knock or misfire during acceleration
  
• Difficulty starting after overnight rest
  

Key terminology:

• Diesel algae: Microbial growth in fuel caused by water contamination, forming sludge and acids.
  
• Fuel pickup tube: A pipe inside the tank that draws fuel into the system, often clogged by debris.
  
• Return line: A hose that sends unused fuel back to the tank, potentially stirring up sediment.
  
• Water separator: A filter component that removes water from diesel before it reaches the engine.
  

One operator in Saskatchewan reported that his 690B would run for 30 minutes before sputtering and dying. After draining the tank, he found a layer of black sludge and rust flakes coating the bottom. The fuel pickup tube was nearly sealed shut.
Tank Cleaning Procedure and Best Practices
Cleaning a contaminated fuel tank requires more than just draining the diesel. The goal is to remove sediment, microbial growth, and rust without damaging the tank or leaving residue.
Recommended steps:

• Drain the tank completely using the lowest drain point
  
• Remove the fuel pickup tube and inspect for blockage
  
• Flush the tank with clean diesel or kerosene to loosen debris
  
• Use a long-handled brush or magnetic wand to remove rust flakes
  
• Vacuum out sludge with a fluid extractor or wet/dry vacuum
  
• Rinse with biocide-treated diesel to kill microbial residue
  
• Replace all filters and bleed the fuel system before restart
  

Tips:

• Avoid using water unless the tank can be fully dried and treated
  
• Use nitrile gloves and eye protection when handling biocides
  
• Inspect the tank interior with a borescope if access is limited
  
• Replace rubber hoses if they show signs of swelling or cracking
  

One technician in Georgia fabricated a custom wand from copper tubing to reach the rear corners of the tank. After three flushes and a biocide soak, the machine ran clean for over 500 hours.
Fuel System Upgrades and Preventive Measures
To prevent future contamination:

• Install a high-capacity water separator with a clear bowl
  
• Use diesel stabilizer additives during seasonal storage
  
• Keep the tank full to reduce condensation
  
• Replace the fuel cap gasket to prevent moisture ingress
  
• Add a magnetic drain plug to monitor for metal particles
  

Modern diesel contains up to 5% biodiesel, which absorbs water more readily than traditional fuel. This makes older tanks more vulnerable to microbial growth and corrosion.
One fleet in Alberta began using tank desiccant breathers—filters that remove moisture from air entering the tank during cooling cycles. This reduced water contamination and extended filter life.
Injector and Pump Protection After Contamination
If contaminated fuel has reached the injection pump or injectors, further steps may be needed:

• Flush the fuel lines with clean diesel
  
• Replace injector tips if scoring or pitting is found
  
• Inspect the injection pump for varnish or wear
  
• Use fuel system cleaner additives for mild contamination
  
• Bench test injectors for spray pattern and pressure
  

One operator in Maine rebuilt his injector pump after sludge from the tank caused internal scoring. After cleaning the tank and replacing the filters, he installed a secondary inline filter rated at 10 microns to protect the pump.
Conclusion
Fuel tank contamination in older machines like the John Deere 690B is a silent threat that can cripple performance and lead to costly repairs. With proactive cleaning, upgraded filtration, and moisture control, operators can restore fuel system integrity and extend the life of their excavators. Whether trenching in clay or clearing brush, clean diesel is the lifeblood of reliable operation. In the field, a clean tank isn’t just maintenance—it’s insurance against downtime.