Why Does the CAT 320CL Swing Cause Engine RPMs to Drop

[MikePhua]

The CAT 320CL and Its Global Workhorse Reputation
The Caterpillar 320CL hydraulic excavator, introduced in the early 2000s, was part of Caterpillar’s C-series lineup designed to meet Tier II emissions standards while improving fuel efficiency and hydraulic responsiveness. With an operating weight of approximately 21 metric tons and a 138 hp engine, the 320CL became a staple in infrastructure, demolition, and utility work across North America, Asia, and the Middle East. Caterpillar Inc., founded in 1925, has sold tens of thousands of 320-series excavators globally, with the CL variant known for its balance between mechanical simplicity and electronic control.
The 320CL features a load-sensing hydraulic system, electronically controlled fuel injection, and a swing priority valve that allows simultaneous movement of boom, stick, and swing functions. These systems are designed to optimize flow and pressure based on operator input and load demand.
Symptoms of the Swing-Induced RPM Drop
A recurring issue in older 320CL units involves a sudden drop in engine RPM when the swing function is engaged. The machine may run normally for hours, but when the operator attempts to swing left or right, the engine bogs down, nearly stalling. Boom, stick, and bucket functions remain unaffected, and travel performance is normal. No fault codes appear on the monitor, and fluid levels are within range.
This behavior suggests a localized issue affecting either the swing motor’s hydraulic circuit or the fuel delivery system under dynamic load.
Terminology Annotation
- Swing Motor: A hydraulic motor that rotates the upper structure of the excavator.
- Load-Sensing System: A hydraulic control method that adjusts pump output based on demand from actuators.
- Fuel Sump: The lowest point in the fuel tank where water and sediment collect.
- Fuel Screen: A mesh filter located near the tank outlet that traps debris before it enters the fuel line.
- Suction Line: The hose that draws fuel from the tank to the engine’s injection system.
Fuel Contamination and Sump Blockage
One of the most overlooked causes of swing-induced bogging is water or sediment buildup in the fuel sump. The fuel tank on the 320CL has a depression at its base where contaminants settle. When the swing motor activates, vibrations and hydraulic surge can stir this debris, causing it to be drawn into the suction line. This results in restricted fuel flow, starving the engine momentarily and causing RPMs to drop.
Operators have reported draining the sump only to find thick sludge and water after 600 hours of operation. If the drain valve is plugged, suction may be compromised even at moderate fuel levels. Machines running below 25% fuel capacity are especially vulnerable, as the suction line may be exposed to air pockets or sediment.
Accessing and Cleaning the Fuel System
To access the sump and fuel screen:

• Swing the upper structure so the tracks are perpendicular to the cab
  
• Remove the bottom plate under the fuel tank
  
• Locate the 90-degree elbow fitting at the tank’s lowest point
  
• Disconnect the drain hose and use a vacuum extractor or siphon to remove contaminated fuel
  
• Inspect the inline fuel screen and replace if clogged
  

Some technicians use a 1/16" stranded cable attached to a cordless drill to auger through blocked lines. Others have adapted shop vacs to pull suction through the drain hose, minimizing mess and exposure to diesel.
Why Only the Swing Function Triggers the Problem
The swing motor draws high flow at low pressure, unlike boom or stick functions which demand high pressure. This unique hydraulic profile may cause greater fuel agitation, especially if the swing priority valve opens fully. Additionally, the swing circuit is often the first to respond when multiple functions are engaged, making it a sensitive indicator of fuel delivery issues.
In one case, a CAT backhoe exhibited gear dropouts due to overvoltage from a failing alternator. The lesson: simple electrical or fuel issues can manifest as complex hydraulic symptoms. Always eliminate basic faults before assuming component failure.
Recommended Solutions and Preventative Measures
To prevent swing-induced RPM drops:

• Drain the fuel sump every 250–300 operating hours
  
• Keep fuel levels above 30% to avoid suction line exposure
  
• Replace fuel screens annually or when flow restriction is suspected
  
• Install a transparent inline filter for visual inspection
  
• Monitor engine voltage and alternator output to rule out electrical anomalies
  

Technicians should also consider installing pressure gauges on the swing circuit to compare pump output under load. A drop in pressure during swing but not during boom or stick movement may confirm a hydraulic imbalance or fuel starvation.
Field Anecdotes and Operator Wisdom
One operator in Georgia reported that his 320CL ran flawlessly until the swing function was used after three hours of operation. After draining the sump and replacing the fuel screen, the issue disappeared. He now includes sump draining in every 500-hour service interval.
Another mechanic in New Hampshire shared that he routinely finds water and sludge in machines that haven’t had their sump drained in over a year. He recommends using a clear water bottle to collect samples and inspect for contamination before refueling.
Conclusion
The CAT 320CL’s swing-induced RPM drop is often a symptom of fuel system neglect rather than hydraulic failure. By understanding the interaction between fuel delivery, hydraulic demand, and engine response, operators can diagnose and resolve the issue efficiently. With regular sump maintenance, clean fuel screens, and attention to suction line integrity, the 320CL can continue to perform reliably—even when the swing demands full power. In the world of excavation, sometimes the smallest blockage can stall the biggest machine—but with the right approach, it’s nothing a good drain and a clean filter can’t fix.