Caterpillar 953 LPG 20Z Lift Cylinder Weakness After Pump Cartridge Replacement

[MikePhua]

The 953 LPG and Caterpillar’s Track Loader Innovation
The Caterpillar 953 LPG track loader was part of Caterpillar’s effort to offer alternative fuel machines for urban and enclosed environments. Built on the proven 953 platform, the LPG variant used a liquid propane gas engine to reduce emissions and noise, making it suitable for indoor demolition, waste handling, and municipal work. The 20Z serial prefix identifies a specific production series, often equipped with a closed-center hydraulic system and a mechanical joystick control layout.
Caterpillar introduced the 953 in the mid-1980s, and it quickly became one of the most popular track loaders in the world. With over 20,000 units sold globally, the 953 series evolved through multiple engine and hydraulic configurations, including the LPG variant, which remains a niche but valuable tool in specialized fleets.
Hydraulic System Overview and Pump Cartridge Role
The 953 LPG uses a gear-type hydraulic pump mounted directly to the engine. The pump cartridge is the internal assembly responsible for generating flow and pressure to operate the lift, tilt, and auxiliary circuits. Replacing the pump cartridge is a common service procedure when flow drops or pressure becomes inconsistent.
Key terminology:

• Pump Cartridge: Internal gear or vane assembly that generates hydraulic flow.
  
• Lift Cylinders: Dual hydraulic actuators that raise the loader arms.
  
• Relief Valve: Limits system pressure to prevent damage.
  
• Priority Valve: Directs flow to critical functions like steering or lift.
  
• Closed-Center System: Hydraulic system where flow is blocked until demand is sensed.
  

In one rebuild in Alberta, a technician replaced the pump cartridge on a 953 LPG after noticing slow lift speed. However, even with the new cartridge installed, the lift cylinders remained weak, prompting further investigation.
Symptoms of Weak Lift and Diagnostic Path
Operators may observe:

• Loader arms rise slowly or stall under load
  
• Tilt function works normally but lift is sluggish
  
• No external leaks or visible cylinder damage
  
• Engine does not bog down during lift attempt
  
• Hydraulic fluid level and filter condition are normal
  

Diagnostic steps:

• Measure system pressure at lift circuit (target: ~2,500 psi)
  
• Inspect relief valve for sticking or incorrect setting
  
• Check priority valve for blockage or misalignment
  
• Verify cylinder seals and piston integrity
  
• Test flow rate from pump using flow meter
  
• Inspect control valve spool for wear or scoring
  

In one fleet in Texas, a 953 LPG showed weak lift despite a new pump cartridge. Pressure testing revealed only 1,200 psi at the lift port. The technician discovered a stuck relief valve spring, which was replaced to restore full pressure.
Cylinder Wear and Internal Leakage
Lift cylinders may appear intact externally but suffer from internal leakage due to worn piston seals or scored barrel walls. Symptoms include:

• Lift stalls under load but works when empty
  
• Fluid bypasses internally, reducing effective pressure
  
• Cylinder rods extend unevenly or retract slowly
  
• No visible leaks at rod seals
  

Recommended actions:

• Remove cylinder and inspect piston seal condition
  
• Measure barrel ID and piston OD for wear tolerance
  
• Replace seals with OEM or high-pressure aftermarket kits
  
• Hone barrel if scoring is present
  
• Pressure test cylinder before reinstallation
  

In one rebuild in France, a contractor rebuilt both lift cylinders using a seal kit from Hercules Sealing Products. After reassembly and testing, the loader regained full lift capacity and operated reliably for over 1,500 hours.
Control Valve and Flow Restriction Issues
The lift control valve may restrict flow due to:

• Scored spool or bore
  
• Contaminated fluid causing sticking
  
• Incorrect detent or spring tension
  
• Internal leakage between ports
  

Solutions:

• Disassemble and inspect valve body
  
• Clean with solvent and compressed air
  
• Replace worn spools or seals
  
• Verify detent spring preload
  
• Test valve response with manual override
  

In one municipal fleet in Maine, a technician found metal shavings in the lift valve after a pump failure. Cleaning the valve and replacing the spool restored normal lift function.
Preventive Maintenance and System Optimization
To maintain hydraulic performance:

• Change hydraulic filters every 500 hours
  
• Use ISO VG 46 fluid with anti-wear additives
  
• Inspect relief valve setting annually
  
• Flush system after pump or valve replacement
  
• Monitor lift speed and pressure trends
  

Recommended upgrades:

• Install pressure gauge at lift port for real-time monitoring
  
• Add magnetic drain plug to capture metal debris
  
• Use synthetic seals in high-temperature environments
  
• Label hydraulic ports and maintain service log
  

In one grading operation in Chile, adding a pressure gauge helped detect early pump wear, reducing downtime and improving lift consistency.
Conclusion
Weak lift cylinders in a Caterpillar 953 LPG after pump cartridge replacement often point to deeper hydraulic issues—relief valve malfunction, internal cylinder leakage, or control valve restriction. With methodical diagnostics and targeted repairs, operators can restore full lifting power and extend the life of this versatile track loader. Whether clearing debris or loading trucks, the 953 LPG performs best when its hydraulic system is clean, balanced, and pressure-verified.