10-21-2025, 06:12 PM
CAT 977L Background and Fuel System Overview
The Caterpillar 977L track loader was introduced in the 1970s as part of CAT’s heavy-duty crawler loader lineup. Built for excavation, loading, and slope work, the 977L featured a turbocharged diesel engine, mechanical injection system, and a robust undercarriage with single grouser track shoes. With a 10-foot blade and high breakout force, it remains a popular choice for terrace building and slope reclamation. Its fuel system includes a mechanical transfer pump, primary and secondary filters, and a brass mesh screen at the injection pump inlet.
Terminology Note
After approximately 45 minutes of operation, the 977L begins to lose power, especially when pushing uphill on 2:1 slopes. Simultaneously, the fuel pressure gauge drops—though it remains within the green zone. The correlation between power loss and gauge behavior suggests a fuel delivery issue. The machine had previously been rented out, and operators occasionally noted sluggishness, but the problem became more pronounced during recent owner-operated use.
Initial Checks and Findings
A brass mesh-type filter sits at the bottom of the injection pump. It consists of stacked brass washers with perforations, spaced apart by projections to allow fuel flow. This filter is often overlooked but can become clogged with fine debris, restricting flow even when upstream filters are clean.
To clean:
In Iowa, an excavating contractor noted that the machine had sat idle for nearly a year before being reactivated. The previous owner used it sparingly, and the injection system had not been serviced recently. After cleaning the pump screen and inspecting the lines, performance improved noticeably. In Western Australia, a technician observed similar behavior in older CAT loaders and traced it to heat-induced clearance changes in the transfer pump, reducing fuel delivery as the engine warmed.
Preventive Maintenance Recommendations
Power loss and fuel pressure drop in the CAT 977L are often linked to overlooked components like the injection pump screen filter and aging fuel lines. With targeted inspection and cleaning, the machine’s performance can be restored—ensuring reliable operation even under demanding slope conditions. For legacy equipment like the 977L, attention to small details in the fuel system makes a big difference.
The Caterpillar 977L track loader was introduced in the 1970s as part of CAT’s heavy-duty crawler loader lineup. Built for excavation, loading, and slope work, the 977L featured a turbocharged diesel engine, mechanical injection system, and a robust undercarriage with single grouser track shoes. With a 10-foot blade and high breakout force, it remains a popular choice for terrace building and slope reclamation. Its fuel system includes a mechanical transfer pump, primary and secondary filters, and a brass mesh screen at the injection pump inlet.
Terminology Note
- Fuel Transfer Pump: A mechanical pump that draws fuel from the tank and delivers it to the injection pump under pressure.
- Fuel Pressure Gauge: An analog or digital indicator showing fuel delivery pressure to the injection system.
- Brass Mesh Filter: A primary screen located at the injection pump inlet, designed to trap debris before fuel enters the pump.
- Pinholing: Corrosion-induced micro-leaks in steel fuel lines, allowing air ingress or fuel seepage.
After approximately 45 minutes of operation, the 977L begins to lose power, especially when pushing uphill on 2:1 slopes. Simultaneously, the fuel pressure gauge drops—though it remains within the green zone. The correlation between power loss and gauge behavior suggests a fuel delivery issue. The machine had previously been rented out, and operators occasionally noted sluggishness, but the problem became more pronounced during recent owner-operated use.
Initial Checks and Findings
- The fuel tank vent was confirmed to be clear.
- The fuel filter and water separator were replaced.
- Fuel flow to the separator appeared strong, so backflushing was skipped.
- The injection pump was visually inspected, and attention turned to the internal screen filter.
A brass mesh-type filter sits at the bottom of the injection pump. It consists of stacked brass washers with perforations, spaced apart by projections to allow fuel flow. This filter is often overlooked but can become clogged with fine debris, restricting flow even when upstream filters are clean.
To clean:
- Unscrew the retaining nut and remove the clip.
- Slide off the metal bowl carefully.
- Unscrew the threaded shaft holding the brass washers.
- Separate and clean each washer with solvent and compressed air.
- Reassemble and reinstall with care to avoid leaks.
- Inspect steel fuel lines for pinholes, especially near clamps. These can allow air ingress, reducing pump efficiency.
- Monitor fuel pressure during warm-up and under load. A consistent drop after 45 minutes suggests thermal expansion or pump clearance issues.
- Consider replacing the fuel transfer pump if pressure continues to fall despite clean filters and lines.
- Check for floating debris in the tank that may intermittently block the outlet.
In Iowa, an excavating contractor noted that the machine had sat idle for nearly a year before being reactivated. The previous owner used it sparingly, and the injection system had not been serviced recently. After cleaning the pump screen and inspecting the lines, performance improved noticeably. In Western Australia, a technician observed similar behavior in older CAT loaders and traced it to heat-induced clearance changes in the transfer pump, reducing fuel delivery as the engine warmed.
Preventive Maintenance Recommendations
- Clean the injection pump screen annually or every 500 hours.
- Replace fuel lines showing signs of corrosion or wear.
- Use high-quality diesel and keep the tank topped to reduce sediment disturbance.
- Log fuel pressure readings during operation to track performance trends.
Power loss and fuel pressure drop in the CAT 977L are often linked to overlooked components like the injection pump screen filter and aging fuel lines. With targeted inspection and cleaning, the machine’s performance can be restored—ensuring reliable operation even under demanding slope conditions. For legacy equipment like the 977L, attention to small details in the fuel system makes a big difference.
