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4JX1 is known for injector sleeves leaking coolant.
The injectors need to come out, to allow the sleeve removal.
Preferably a job for a Holden tech who has done this type of job.
Replace all o-rings , I would also replace the rail pressure sensor and injector harness if there is oil at the harness connector, near the dipstick tube.
How are you going with the problem 4JX1?
Do you need further assistance?