WIRING GP-UNIT INJECTOR 3994012 - Caterpillar

Caterpillar Diesel Engines With Capsule-Type Fuel NozzlesThis article is to be used as a guide to find causes of bulged (distorted) capsule-type fuel nozzles. The information in this article may or may not apply to a specific nozzle failure on your engine.
Too Much Torque On The Adapter Nut
Too much torque on the adapter nut will cause compression on the soft metal case of the nozzle. This causes the nozzle to bulge at the top of the case. See Nozzle B in the illustration.
The Service Manual for your engine shows the correct torque for the adapter nut.
Not Enough Torque On The Adapter Nut
If there is insufficient torque (not enough torque) on the adapter nut, the nozzle may seal engine compression but not have enough force to seal the internal components of the fuel nozzle.
If the internal components are not sealed, fuel will leak between the components and cause the fuel nozzle case to bulge in the middle. An example is Nozzle C in the illustration. It is possible for the high pressure fuel injection pulses to cause the nozzle case to crack. To help find if a bulged nozzle was the result of insufficient torque, look at the top of the nozzle case for a mark. There will normally be a mark on the top of the case if the correct torque was used. There may also be carbon deposits on the bottom of the case and on the tip if the adapter nut was not tight enough, as seen in Nozzle C.
It is important to remember that dirt or other failures in the nozzle can also cause the internal components to not seal properly, even if the correct torque is used.
Too Much Heat
Too much heat can change the heat treat of the nozzle's internal components. A fuel nozzle gets too hot when the engine temperature is allowed to get too high. A nozzle that was too hot may have a "blued" or darkened case and may be bulged. See Nozzle D in the illustration.
It is recommended that a nozzle not be used again if it has indications of too much heat.
Fuel nozzles. Nozzle A is a new nozzle; it was tightened with correct torque.