caterpillar 3126 engine

This view shows an injector cutaway and an injector sleeve.
The HEUI injector is controlled electronically by the ECM but is actuated
hydraulically. A signal from the ECM controls the opening and closing of
the solenoid valve. The solenoid valve controls the flow of high pressure
hydraulic oil to the injector.
The injector sleeve has four seal grooves. The seals for the two upper
grooves contain the fuel within the fluid manifold. The two lower seals
contain the coolant below the cylinder upper deck. Because the injectors
are mounted in a high temperature area of the engine, the injector sleeves
are surrounded by coolant.
Before installation of a used injector, it is important that both the injector
and the injector sleeve be checked for signs of combustion gas leakage.
Evidence of leakage may appear as carbon on the lower ends of both
components. Leakage of this type will cause low power or misfire on one
or more cylinders. Cleaning of the components, replacement of the seals
and using the correct injector installation procedure will usually solve the
problem.
INSTRUCTOR NOTE: For a complete description of the injector
construction and operation, refer to STMG 672 "3408E/3412E
Engine Controls--Hydraulic Electronic Unit Injection"