Hi, I'm having trouble setting the idle speed on a recently transplanted 350 Chevy crate motor. I'm using an old TBI system. Following a procedure I found online, I jump the A&B terminals on the ALDL turn the key to ON and let the IAC bottom the pintle in the valve opening. With the key still on, I disconnect the IAC connector, turn off the key and remove the ALDL jumper. Then I start the engine and set the minimum air speed. I'm using 500 rpm. Then the engine is shut off, the IAC connector reinstalled and then I start the engine, let it run for 5 seconds, shut it off and wait 30 seconds and restart.

The restart yields an idle of about 900 rpm until the transmission is shifted into gear. At that point the idle drops to about 650 in drive and just slightly higher when put back into park. So far, so good.

Once the truck has gone through one accelerate/decelerate cycle, the engine idles roughly, and the IAC counts drop. Sometimes the engine tries to catch itself...the counts come up and the engine picks up speed, only to falter again. Most of the time it stalls. The IAC counts also go to zero while coasting or cruising.

My understanding is that I need to set the minimum air setting low enough so that there is always some component of the idle air introduced through the IAC and some by the throttle plates, which means I should never see zero IAC counts, or at least not while the engine is idling.

I do not know the target idle speed in the PROM for this computer. While I'm sure it would be helpful to know that, I was hoping that by setting the minimum air speed low enough, I could hit the sweet spot and the engine could idle under control of the computer but so far, no luck. Last night at the end of my last data logging run, the CEL light came on with a code for VSS problem. I'm not sure if that's related but I thought I'd mention it.

If there are any old OBD1 gurus out there who care to chime in, please do.

 

It would be helpful to have the GM service manual set for the year and model the engine/computer/PROM came from.

Verify fuel pressure

Connect a scan tool, look at the data stream. Verify that there's no other computer-related problems--such as your VSS issue.

How long has it been since the engine got a proper tune-up? In other words, is there something OTHER than the IAC that's causing rough idle--low compression on one or more cylinders, ignition/spark plug problems, leaking intake gaskets, incorrect base timing, etc.

Have you cleaned the IAC passage in the throttle body?

Throttle body base gaskets are a common problem; and you need the base gasket that matches the throttle body--there's several versions of the base gasket depending on application.

Two examples: