Record the part number of the STC control valve from the data tag on the valve.

Record the upper and lower shift-point values for the valve.

Disconnect the STC oil outlet line from the rocker lever housing.

Remove the elbow fitting from the rocker lever housing.

Install a fitting between the rocker housing and oil outlet line.

Connect a 0-300 psi pressure gauge, Part Number 3375932, to the fitting.

Disconnect the STC fuel pressure sensing line from the fuel inlet passage of the cylinder head. Plug the hole in the cylinder head with a pipe plug.

Connect the STC fuel pressure line (A) to the outlet side of the air pressure regulator, Part Number ST-990-40.

Connect a 0-160 psi pressure gauge (B), Part Number 3375275, to the air pressure regulator.

Connect a shop air pressure line (C), capable of providing 100 psi, to the inlet side of the regulator.

Adjust the air pressure regulator until the air pressure gauge shows 0 psi.

Operate the engine at idle. The oil pressure between the STC valve and the tappets must be at least 103 kPa [15 psi].

NOTE: Because the oil pressure can be less than 103 kPa [15 psi] when the coolant temperature is above 49°C [120°F], these tests must be performed at temperatures below this level.

NOTE: If the oil pressure will not reach 103 kPa [15 psi], the STC oil delivery system is not functioning properly. Perform the STC Oil Delivery System Diagnostics found in the following procedure. Refer to Procedure 006-045 in Section 6.

Slowly adjust the pressure regulator to increase the air pressure until the oil pressure drops below 55 kPa [8 psi]. Record the air pressure at this point. This is the upper shift-point (A).

NOTE: The sound of the overhead will become quieter when the 55 to 69 kPa [8 to 10 psi] point is reached. This is the shift from ADVANCED to NORMAL timing.

Raise the air pressure at least 103 kPa [15 psi] above the upper shift-point then slowly begin to lower it. Record the air pressure at which the oil pressure raises above 55 kPa [8 psi]. This is the lower shift-point (B).

NOTE: The lower shift-point is the shift from NORMAL to ADVANCED timing. The overhead will go from quiet to noisy. This pressure must be lower than the upper shift-point. If it is not, repair or replace the STC valve.

Compare the shift-point values to those specified for the valve. If the values are not within these specifications, recalibrate the STC valve.

NOTE: Some STC control valves do not have an adjusting screw. If the calibration is not within specifications, the valve must be removed and repaired or replaced.

Remove the tamperproof wire from the acorn nut on the top of the STC valve.

Remove the acorn nut from the STC valve adjusting screw.

Loosen the locknut on the STC valve adjusting screw.

Use an Allen wrench to turn the STC valve adjusting screw.

Turn the screw in a clockwise direction to raise the STC valve upper shift-point.

Turn the screw in a counterclockwise direction to lower the upper shift-point.

NOTE: Always start with the air pressure at 0 psi before checking the upper shift-point (A). Always raise the air pressure to at least 103 kPa [15 psi] above the upper shift-point (A) before checking the lower shift-point (B).

After adjusting the screw, repeat the STC shift-point check procedure.

Repeat the calibration and check procedure until the shift-points are within the set values listed in the table. If the valve cannot be calibrated to these specifications, replace it.

Tighten the STC adjusting screw locknut.

Install the acorn nut on the STC adjusting screw.

Install the tamperproof wire between the acorn nut and capscrew.

Clean the STC oil control valve and the surrounding area before removing it from the engine.

Remove the following lines from the STC oil control valve:

• Fuel pressure sensing line (1)
• Oil outlet line to tappets (2)
• Oil vent line (3).

Remove the two capscrews which hold the STC valve to the mounting plate.

Remove the STC control valve seal ring.

Install a new seal ring.

Install the STC oil control valve to the mounting plate with the two capscrews.

Tighten the capscrews.

Connect the following lines to the STC oil control valve:

• Fuel pressure sensing line (1)
• Oil outlet line to tappets (2)
• Oil vent line (3)