(ISC) Idle speed control actuator

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Figured I bring this often unknown item to light. It was not installed in all TBI engines only the 454 in certain year/s trucks. For the older TBI engines (454) there is a vacuum can attached to the throttle cable on the drivers side of the TBI.

This vacuum can is the 'Idle speed control actuator' aka as 'Throttle kicker'.

It serves a few purposes...

As for the 'Idle speed control actuator' it was not installed in all models of GM. I believe only in the 454 truck models such as the P-30 chassis. It is the vacuum can on drivers side of TBI attached to throttle cable. It has a vacuum line running to the 'EGR valve control solenoid' which is on the passenger side of the TBI just forward of it attached to a bracket.

See pic of mine for aforementioned items: https://www.dropbox.com/s/ow3nt7k4hf...B1%5D.jpg?dl=0

Idle Speed/Throttle Actuator - Mechanical: Description and Operation

Purpose: The idle speed control actuator system assists the idle air control system in controlling the idle speed, primarily at cold temperatures.

Operation: The idle speed control actuator is a vacuum operated device which opens the throttle slightly (2 to 3 degrees of angle) to increase the cold engine idle speed, to improve the mixing of the air and fuel, and to allow lower warmed up engine idle speeds. The idle speed control actuator is controlled by the EGR valve control solenoid, which is controlled by the power control module. To increase the idle speed, the EGR valve solenoid is turned "OFF," and no vacuum is routed to the idle speed control actuator, allowing it to open the throttle slightly. To decrease the idle speed, the solenoid is turned "ON" and vacuum is routed through the solenoid to the idle speed control actuator, allowing the throttle to fully close.

In conjunction with the idle speed control actuator system, the idle air control continuously monitors and controls the engine idle speed to the desired idle speed. A Tech 1 scan tool will read the control module commands to the idle speed control actuator system.

The throttle actuator provides another function. The larger the TB, the more the bores expand as the engine gets hot. The 92+ 454 equipped truck ran much hotter than the older square body version. And GM had switched to a heated intake instead of the small heater plate, which was much more effective at heating the TB and keeping it warm after the engine was shut off. Well, a larger bore means the throttle plate must be closed more to keep idle speed low. No problem, right? Just reduce minimum air by turning the throttle stop and you're done. But if you do that when the engine's hot, the next day when you go to press on the accelerator while the engine's cold, the throttle plate is stuck. Not so good. The throttle actuator allowed a way to reduce idle speed when hot yet keep the throttle far enough open when the engine was off so a cooling TB wouldn't seize the plate.

The following is the correct OBD-1 code chart for the early 90-95 years: https://www.troublecodes.net/gm/#codesobd1


As for the adjustment of the 'Idle speed control actuator' (Vacuum can) aka throttle kicker...

Idle Speed Control Actuator Adjustment

Procedure: (from manual)

Before adjusting ISC actuator you must first check the minimum idle speed to be within specifications. Refer to SPECIFICATIONS/IDLE SPEED. (ISC actuator must have vacuum applied so that it is not contacting the throttle lever.) See image VIEW A.

Remove vacuum hose on ISC actuator on warm engine.
Adjust ISC actuated idle speed to 1300 +/- 50 rpm. See image VIEW B.
Install vacuum hose.

In the even the vacuum can is leaking and not holding vacuum and you're unable to source another. You'll need to get at the 'Idle adjustment screw' that is covered by GM on the front side of the TBI. Turn it open roughly 1 turn. This will open the throttle blade slightly. The IAC will/should make up the difference.

As for the adjustment of the 'Idle speed control actuator' (Vacuum can) aka throttle kicker...

Idle Speed Control Actuator Adjustment

Procedure: (from manual)

Before adjusting ISC actuator you must first check the minimum idle speed to be within specifications. Refer to SPECIFICATIONS/IDLE SPEED. (ISC actuator must have vacuum applied so that it is not contacting the throttle lever.) See image VIEW A.

Remove vacuum hose on ISC actuator on warm engine.
Adjust ISC actuated idle speed to 1300 +/- 50 rpm. See image VIEW B.
Install vacuum hose.

In the event the vacuum can is leaking and not holding vacuum and you're unable to source another. You'll need to get at the 'Idle adjustment screw' that is covered by GM on the front side of the TBI. Turn it open roughly 1 turn. This will open the throttle blade slightly. The IAC will/should make up the difference.

As for the (ISC) description it is item #16:

https://lt-truck-gmc.7zap.com/en/g15+g1500/49295-1994/0-0-1904162379/0-1623891437-0-0/

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The part number for it is GM #17084953

And of course it is no longer available anywhere.

In the event this can no longer holds vacuum then what? I've heard/been told the work around is to remove the cover of the factory idle set screw and open the throttle approximately 1 turn.

Anyone confirm this?

Won't that be an issue for the ECM/Idle air valve and set an SES?