The STC control valve uses fuel pressure and spring
force to control the position of an AFC style plunger. The
position of the plunger dictates whether the oil passage
to the hydraulic tappets is open or closed. Fuel pressure
acts on the piston end of the plunger.
During ADVANCED timing (low fuel pressure), the spring
opposes the fuel pressure and holds the plunger in the
open position. Pressurized lube oil flows to the tappets
and initiates ADVANCED engine timing. As fuel pressure
increases, the spring holds the plunger in the open po-
sition until the fuel pressure rises above the certified
switching pressure.
At this certified level, the higher fuel pressure overcomes
the spring. This action shifts the plunger and closes the
oil passage. The oil supply to the tappets is interrupted,
and the engine begins to operate in the NORMAL timing
mode.
The control valve supplies oil to the STC rocker housing
connection through the STC valve oil outlet line (1), the
external oil manifold (3), and three oil manifold connec-
tors (4).
A check valve in the oil line (2) prevents the oil from
draining back into the engine when it is shut off. This
prevents any delay of oil to the tappets during cold starts.
Fuel System General Information Section 5 - Fuel System
N14Page 5-10
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