Coolant Intrusion Around Injector 1 on the Isuzu 4JJ1 Engine in the Hitachi ZX135-5

[MikePhua]

The Hitachi ZX135-5 and Its Powertrain Design
The Hitachi ZX135-5 is a compact radius excavator designed for urban construction, utility trenching, and precision grading. Introduced in the mid-2010s, it features a short-tail swing for tight spaces and is powered by the Isuzu 4JJ1X engine—a 3.0-liter, four-cylinder turbocharged diesel known for its fuel efficiency and emissions compliance. Isuzu, with over a century of engine development, engineered the 4JJ1 series to meet Tier 4 Final standards using cooled EGR and a diesel oxidation catalyst, without relying on diesel particulate filters.
The ZX135-5 has been widely adopted across Asia, Europe, and North America, with thousands of units sold. Its combination of hydraulic finesse and mechanical reliability makes it a favorite among contractors, but like any machine, it’s not immune to wear-related failures.
Symptoms of Coolant Leaking Into the Oil System
When coolant begins leaking into the oil system—especially around injector 1—the consequences can be severe. Operators may observe:

• Milky or frothy engine oil on the dipstick
  
• Rising coolant consumption without external leaks
  
• White smoke from the exhaust during startup
  
• Engine misfire or rough idle
  
• Elevated crankcase pressure
  
• Coolant residue around the injector bore
  

In one case from a pipeline crew in Alberta, a ZX135-5 began showing signs of oil dilution and coolant loss. After removing the valve cover, technicians found coolant pooling around injector 1, with traces in the oil pan. The machine had logged just under 5,000 hours.
Root Causes of Coolant Intrusion Near Injectors
The Isuzu 4JJ1 engine uses direct injection with injector sleeves (also called cups) pressed into the cylinder head. These sleeves isolate the injector from the coolant passages. If a sleeve cracks, corrodes, or loses its seal, coolant can seep into the injector bore and migrate into the combustion chamber or oil gallery.
Common failure points include:

• Cracked injector sleeve due to thermal cycling
  
• Improperly seated injector allowing coolant bypass
  
• Corroded sleeve from coolant contamination or poor maintenance
  
• Defective O-rings or sealing washers
  
• Head casting flaws or erosion around the sleeve seat
  

Coolant entering the oil system compromises lubrication, leading to bearing wear, piston scoring, and eventual engine failure.
Diagnostic Steps and Inspection Techniques
To confirm the source of the leak:

• Perform a cooling system pressure test with the engine off
  
• Remove the valve cover and inspect injector bores for moisture
  
• Use a borescope to examine sleeve integrity
  
• Conduct a cylinder leak-down test to check for cross-contamination
  
• Analyze engine oil for glycol content and viscosity changes
  

Some technicians use UV dye in the coolant and inspect the injector bore with a blacklight. If dye appears around the injector or in the oil, the sleeve is compromised.
Repair Procedure and Component Replacement
Repairing coolant intrusion around injector 1 typically involves:

• Removing the injector and sleeve from the affected cylinder
  
• Inspecting the sleeve for cracks, pitting, or distortion
  
• Replacing the sleeve with a new OEM part
  
• Cleaning the bore and reseating the sleeve with proper sealant
  
• Installing new O-rings, copper washers, and injector hardware
  
• Flushing the cooling and lubrication systems thoroughly
  

In severe cases, the cylinder head may need to be removed and pressure-tested. If multiple sleeves show signs of wear, replacing all sleeves during the repair is recommended.
Field Story and Practical Insight
In southern China, a contractor operating a ZX135-5 for foundation work noticed coolant loss and sluggish throttle response. After ruling out external leaks, he removed the injector and found a hairline crack in the sleeve. He sourced a replacement from a local Isuzu distributor and completed the repair in two days. To prevent recurrence, he switched to a long-life coolant with anti-cavitation additives and began sampling oil every 250 hours.
Preventive Measures and Long-Term Solutions
To reduce the risk of coolant intrusion:

• Use OEM-approved coolant with proper additive packages
  
• Replace coolant every 2,000 hours or as specified
  
• Monitor injector torque and sealing during service
  
• Avoid overheating by keeping radiators and coolers clean
  
• Sample engine oil regularly for contamination
  

Some operators install coolant level sensors with alarms to detect sudden drops. Others retrofit bypass filters to remove fine contaminants from the oil system.
Conclusion
Coolant leaking into the oil system around injector 1 on the Isuzu 4JJ1 engine is a serious issue that demands prompt attention. By understanding the role of injector sleeves, recognizing early symptoms, and applying precise repair techniques, operators can restore engine integrity and avoid catastrophic damage. The Hitachi ZX135-5 remains a reliable machine when maintained properly—and with vigilance around coolant and injector health, it continues to deliver performance in demanding environments.