Question about 1989 GMC Sierra
Low / uneven idle problem can cause by a lot of components.
Dirty air filter
Dirty injectors + o-rings
Vacuum leak at intake manifold and vacuum hose.
Bad Idle Air Control Motor (IAC)
Low compression (170 psi + is OK)
Start cleaning and repair these trouble spots and see is there any improvement.
For more advance DIY'er then you may try this.
Use a can of Berryman carburetor clean to locate the vacuum leak.
SEE SAMPLE PICTURE HERE
This is a 20-30 minutes job.
Vacuum leaks can occur in following locations.
Intake manifold AND/OR vacuum lines.
Intake manifold is not accessible unless the engine head if off.
You can still check the potential leak by spraying some Berryman near the intake manifold.
Listen for rpm increase after you introduce the Berryman (FUEL) at idle.
Do the same starting from the brake booster unit (the round unit) on the drive side fire all.
Carefully listen for increase of RPM right after to spray the Berryman to the potential vacuum spot.
Get a friend to HEAR the ROM increase or monitor for RPM on the tech.
DO NOT AIM the Berryman into any heat source.
PLEASE RATE my answer if it is useful to you.
Posted on Dec 14, 2009
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Posted on Jan 02, 2017
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