Regardless of the nozzle type, injectors should
• Discharge cleanly without “after-dribble,” which tends to collect in the cylinder between cycles.
• Atomize fuel for rapid combustion.
• Direct the spray into the far reaches of the cylinder, but without impinging against the piston or chamber walls.
Most malfunctions are caused by carbon buildup on the tip, which distorts the spray pattern and can sometimes stick the needle or pintle in the open position. Automatic injectors can be tested with the apparatus shown in Fig. 5-23. Similar tests can be performed on unit injectors by loading the pump cavity with fuel and applying force to the pump plunger.
There are four standard tests—spray pattern, NOP, sealing effectiveness, and chatter.
Spray pattern
Connect the tester as shown and, operating the pump lever in short, rapid strokes, observe the spray pattern. Pintle nozzles should produce a finely divided spray of uniform consistency and penetration (Fig. 5-24). Multi-hole nozzles generate a wide, fan-like pattern (Fig. 5-25). It can be helpful to direct the discharge against a piece of paper and compare the pattern to one produced by a new injector.
Nozzle opening pressure
Slowly depress the pump lever and note the peak gauge reading. Compare this pressure with the manufacturer’s NOP specification. Some specifications are fairly broad, and engines may not idle properly unless NOPs cluster at one or the other end of the specification range. Most needle-return springs can be stiffened with shims.
Sealing
Dribble standards have become tighter in recent years. The pintle injector illustrated in Fig. 5-26 should remain fuel tight under 80% of NOP for at least 10 seconds. Note and correct any other source of leakage.
Chatter
Injector opening is normally accompanied by a sharp “pop,” which suggests that the needle retracts cleanly. Verify this action as the pump lever is cycled rapidly. However, leaking injectors can be made to chatter and some perfectly good throttling pintle injectors pop open, but refuse to chatter, no matter how adroitly the pump handle is worked.
