Learned something new about TBI idle adjustment

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Doubld

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Newton, Kansas
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dj
Truck Year
1987
Truck Model
Silverado
Engine Size
5.7
I put a Holley TBI on my '87 with 350. Couldn't get the idle to cooperate. Looked everywhere for how to set it. The Holley TBI has an open idle adjustment screw, but just turning it, wouldn't keep the idle steady and wouldn't raise the idle when cold or a/c was on. The stock TBI adjustment screw was blocked off, but you could drill out the plug, there is a torx head screw there.
Finally found a video that showed something all the shop manuals ignored, Holley ignored in their instructions, every forum I looked on did not have.
It's so simple but not listed. The same 2 OBD1 connections (A and B) that you use to read the engine codes is the answer.
Warm the truck to operating temperature, turn it off. Plug in your jumper wire connecting the A and B terminals. Key on to run without starting, under the hood you will hear the Idle Control Valve clicking, ( The clicking is the ICV closing) unplug the ICV connector. Start the engine, adjust the idle, my truck likes about 600-650. Plug in the ICV.
That's it. If it sets a code unhook the battery for 30 secs to clear it. My truck didn't.
Now the truck raises the idle cold, and drops it as it warms up. Defroster and a/c raises the idle like it should.
 

bucket

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It's in the GM service manuals. At least it was in my Camaro service manuals and probably my Celebrity manuals too. It was in the flow chart for IAC valve diagnostics.

Most people don't seem to mess with it though. Just turn the screw and see if it ever idles too low.
 

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