Rebuilding the Cummins QSB4.5 Engine in the JLG G12-55A Loader

Comprehensive repair of the Cummins QSB4.5 (Tier 4 Interim) engine in the JLG G12-55A telehandler

Introduction

A telehandler JLG G12-55A arrived at our WIBAKO service, used in intensive loading and construction work. The machine is equipped with a modern, high-performance Cummins QSB4.5 (Tier 4 Interim) diesel engine, known for its high work culture, efficiency, and reliability. This compact, 4-cylinder engine is widely used in off-highway machines, characterized by low fuel consumption and compliance with stringent exhaust emission standards.

The telehandler was used daily on construction sites in the Małopolskie Voivodeship, around the city of Tarnów, where our mobile service performed the initial diagnostics. The reported fault concerned a noticeable lack of power and occasional smoking, especially when lifting heavy loads with the full extension of the boom. Based on the initial symptoms, a problem with the crankshaft-piston system or the fuel system was suspected.

After conducting detailed diagnostics, we confirmed the cause: wear of the cylinders and pistons, which resulted in a drop in compression, unstable engine operation, and increased oil consumption. This required a complete rebuild of the power unit.

Preliminary diagnostics in the machine

The WIBAKO mobile service arrived at the work site of the JLG G12-55A telehandler to conduct preliminary diagnostics. Upon starting the machine, characteristic symptoms were observed: hard cold operation, irregular operation at low RPM, and a noticeable drop in power under increased load. The first step was to perform a compression test, which showed significant differences between individual cylinders.

Additionally, an endoscopic inspection was conducted, revealing clear scratches and excessive wear on the working surfaces of the cylinder liners. The Tier 4 Interim version of the Cummins QSB4.5 engine is particularly sensitive to proper lubrication and fuel quality, which further confirmed the diagnosis.

After confirming the damage, a decision was made to dismantle the engine and transport it to our headquarters in Kojszówka.

Engine disassembly from the machine

Dismantling the engine from the JLG G12-55A telehandler requires exceptional caution due to the tight engine compartment layout. Our mechanics began by securing all hoses, disconnecting the electrical system, and draining the operating fluids.

Components such as the intake system, air hoses, cooling hoses, exhaust system elements, and engine mounts were removed. The entire process was carried out using a crane provided by the client, following safety procedures. After disconnecting all components, the engine was carefully removed and prepared for transport.

Engine verification in the workshop

After delivering the Cummins QSB4.5 engine to our workshop, we began a full verification of the unit. The engine was disassembled, thoroughly cleaned, and measured to assess the degree of wear.

The verification confirmed previous suspicions: cylinders number 3 and 4 were significantly worn, causing a drop in compression. The pistons had visible damage and scratches. The crankshaft, despite its mileage, showed no cracks and was suitable for grinding. The fuel system required complete regeneration, and the injectors showed incorrect fuel dosing.

Repair and regeneration process

Crankshaft-piston system

As part of the rebuild of the crankshaft-piston system, a full range of work was performed: replacement of pistons, piston rings, regeneration of the crankshaft (grinding), verification and regeneration of connecting rods, and installation of new main and connecting rod bearings. The cylinder liners were replaced with new ones and precisely fitted.

Engine head

The head underwent a full leak verification, followed by machining, replacement of valve guides, and valves. After the regeneration was completed, the head was installed with a new gasket and new studs.

Fuel system

The injection pump was sent for specialized regeneration, and the injectors were tested on a test bench. After regeneration, all components of the fuel system underwent final calibration according to Cummins specifications.

Engine accessories

The turbocharger was regenerated, and the alternator, starter, and all external engine components were checked. Hoses, seals, and cooling system elements were also replaced.

Professional engine assembly

The rebuilt engine was assembled according to Cummins service procedures. New bolts, gaskets, and consumables were used. After assembly, the engine was prepared for testing on the dynamometer.

Testing on the engine dynamometer

Our engine dynamometer allows for a complete check of the unit's operation under load. The engine after repair was started and operated under light conditions for 16 hours, according to the break-in procedure. After this stage, full load tests were initiated, repeatedly reaching maximum values.

The dynamometer enabled a detailed analysis of parameters such as oil pressure, operating temperature, RPM uniformity, fuel system efficiency, and response to load. All parameters were within Cummins standards, confirming the correctness of the rebuild.

Engine installation in the machine

After testing, the engine was transported back to the client. Installation in the JLG G12-55A telehandler required precision due to the limited space in the engine compartment. Our mechanics installed the unit using the client's crane, replacing all air and cooling hoses, mounts, and accessory components.

After installation, start-up tests were conducted in the presence of the machine operator. The telehandler regained full power, stable operation, and correct operational parameters. After a successful trial, an assembly protocol was signed.

Summary

The regeneration of the Cummins QSB4.5 (Tier 4 Interim) engine in the JLG G12-55A telehandler restored the machine to full operational efficiency. Thanks to precise tests on the dynamometer and professional assembly, the client received assurance of the unit's reliability. The next service inspection should be performed after 50 operating hours to confirm operational stability and replace filters.

Linking information

Also check: Cummins engine service, engine regeneration in JLG machines, off-highway engine dynamometer, fuel system repairs.