A Throttle Return Control system (TRC) is used on some truck engines. When the truck is coasting against the engine, the control valve is open to allow vacuum to operate the throttle lever actuator. The throttle lever actuator then pushes the throttle lever slightly open reducing the Hydrocarbon (HC) emission level during coasting. When manifold vacuum drops below a predetermined level, the control valve closes, the throttle lever retracts, and the throttle lever closes to the idle position.
Control Valve Check and Adjustment
- Disconnected the valve-to-carburetor hose and connect it to an external vacuum source with a vacuum gauge.
- Disconnect the valve-to-actuator hose at the connector and connect it to a vacuum gauge.
- Place a finger firmly over the end of the bleed fitting.
- Apply a minimum of 23 inch Hg vacuum to the control valve and seal off the vacuum source. The gauge on the actuator side should read the same as the gauge on the source side. If not, the valve needs adjustment. If vacuum drops off on either side (with the finger still on the bleed fitting), the valve is defective and should be replaced.
- With a minimum of 23 inch Hg vacuum in the valve, remove the finger from the bleed fitting. The vacuum level in the actuator side will drop to zero and the reading on the source side will drop to a value that will be the valve set point of 21.5 inch Hg. If the valve is not within 1 / 2 inch Hg vacuum of the specified valve set point, adjust the valve.
- Gently pry off the conical plastic cover.
- Turn the adjusting screw in (clockwise) to raise the set point or out (counterclockwise) to lower the set point.
- Recheck the valve set point.
- If necessary, repeat the adjustment.
- Disconnect the valve-to-actuator hose at the valve and connect it to an external vacuum source.
- Apply 20 inch Hg vacuum to the actuator and seal the vacuum source. If the vacuum gauge reading drops, the valve is leaking and should be replaced.
- Check the throttle lever, shaft, and linkage for freedom of operation.
- Start the engine and warm it to operating temperature.
- Note the idle rpm.
- Apply 20 inch Hg vacuum to the actuator and manually operate the throttle. Allow it to close against the extended actuator plunger. Note the engine rpm.
- Release and reapply 20 inch Hg vacuum to the actuator and note the rpm at which the engine speed increases (do not assist the actuator).
- If the engine speed obtained in Step 7 is not within 150 rpm of that obtained in Step 6, then the actuator may be binding. If the binding cannot be corrected, replace the actuator.
- Release the vacuum from the actuator and the engine speed should return to within 50 rpm of the speed noted in Steps 4 and 5.
To adjust the actuator:
- Turn the screw on the actuator plunger until the specified TRC speed range (1475-1525 rpm) is obtained.