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GM Prizm/Nova 1985-1993 Repair Guide

Throttle Positioner System



See Figures 1, 2, 3 and 4

To reduce HC and CO emissions, the throttle positioner (TP) opens the throttle valve to slightly more than the idle position when decelerating. This keeps the air/ fuel ratio from becoming excessively rich when the throttle valve is quickly closed. In addition, the TP is used to increase idle rpm when power steering fluid pressure exceeds a calibrated value and/or when a large electrical load is placed on the electrical system (headlights, rear defogger etc.).

With the engine idling and an electrical load energized, the vacuum switching valve (VSV) is energized. This directs atmospheric pressure through the VSV to the rear TP diaphragm (A). The action of the spring on the diaphragm is transmitted to the push rod. This causes the throttle valve to open slightly and increase engine rpm. If all of the heavy electrical loads are off, the VSV is off, maintaining vacuum on the diaphragm and preventing the push rod from moving.

Click image to see an enlarged view

Fig. Fig. 1: Throttle positioner component location

Vacuum from the TP port of the carburetor acts on a second diaphragm (B), closing the throttle valve. With the vehicle cruising, the vacuum signal in both chambers is low which maintains the TP in its high speed (open) position. However, when the vehicle is decelerated, vacuum on diaphragm A increases quickly which closes the throttle valve somewhat.

The delay action of the vacuum transmitting valve makes the vacuum increase on diaphragm B occur slowly which allows the throttle valve to close at a controlled rate. This slow closing prevents the radical change in emissions caused by the throttle valve slamming shut as the driver suddenly lifts completely off the accelerator, such as when going down a steep hill or preparing to use the brakes.

In the event that power steering pressure exceeds a calibrated value, (such as in a full-lock turn while parking) atmospheric pressure is pulled into chamber A causing diaphragm A to move, increasing rpm. This compensates for the power steering system dragging down the idle speed as it delivers needed fluid pressure.


Throttle Positioner System Operation

Click image to see an enlarged view

Fig. Fig. 2: Throttle positioner operation

  1. Start the engine and warm up to normal operating temperature.
  3. Check the idle speed and adjust if necessary.
  5. Disconnect the hose from the TVSV M port and plug the M port. This will shut off the choke opener and EGR system.
  7. Disconnect the vacuum hose from TP diaphragm A . Check that the TP is set at the first step (electrical load idle up). Throttle Positioner at the first setting speed:

    Manual Trans: 800 rpm
    Auto. Trans: 900 rpm


If not at the specified speed, adjust the speed with the adjusting screw.

The adjustment should be made with the cooling fan (at the radiator) OFF.

  1. Disconnect the vacuum hose from the throttle positioner diaphragm B and plug the end of the hose.
  3. Check that the throttle positioner is set at the second step. The setting speed with the throttle positioner on the second step should be as follows:

    Manual Trans: 1,300 plus or minus 200 rpm
    Auto. Trans: 1,400 plus or minus 200 rpm


  1. Reconnect the vacuum hose to the throttle positioner diaphragm B and check that the engine returns to the first step setting speed within 2-6 seconds.
  3. Reconnect the vacuum hose to diaphragm A .
  5. Reconnect the hose to the TVSV M port.

Vacuum Delay Valve
  1. Check that air flows without resistance from B to A .
  3. Check that air flows with difficulty from A to B .

Click image to see an enlarged view

Fig. Fig. 3: Checking the vacuum delay valve

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Fig. Fig. 4: Throttle positioner operation chart

  1. If a problem is found, replace the vacuum delay valve.

When replacing the vacuum delay valve, side

A should face the throttle positioner.

Vacuum Switching Valve

With the engine at idling at normal operating temperature, turn on the high beam headlights. The throttle positioner should move to the first step positions and the idle should increase slightly.

Power Steering Idle-Up Switch

With the engine at idling at normal operating temperature, turn the steering wheel until the wheels are against their stops. Hold the wheels against the stops and check that the throttle positioner moves to the first position. The idle should increase slightly.