Step Timing Control (STC) Injectors advance engine timing at engine idle periods for reduced white smoke in cold weather, reduces injector carboning and improves light load fuel economy.The STC system consists of STC injectors, STC oil control valve and STC plumbing. Inside the injector, injection timing is controlled bt the STC hydraulic tappet. In Normal timing, no oil is in the tappet.

In Advanced timing, the tappet is filled with oil and the injector is metering fuel. Engine oil flows from the STC control valve, through the oil manifold and to the tappets. When oil pressure exceeds 10 psi., it moves the tappet inlet checkball and fills the tappet with oil. The STC oil control valve uses fuel pressure to control whether the oil passage to the STC tappets are open or closed. The oil control valve is calibrated to a specific flow and pressure using a fuel pump test stand. The components that control injection timing are based on fuel pump rail pressure (engine load) and “C” brake operation. At the certified level, the higher fuel pressure shuts off the oil pressure to the STC tappet and the engine operates in the Normal timing mode. When the “C” brake is activated, oil pressure in the brake housing is directed to the STC regulator valve and Normal engine timing results and continues Normal while the brake is in use.