I'm trying to set the idle on my '90 but when the IAC is unplugged I can't get the car to idle lower then 1200rpm. When the IAC is plugged back in, it idles at about 800. Any idea? Vaccum leak? It doesn't surge at all.

 

Is "A" and "B" jumped with ignition on, when you unplug IAC connector ?

 

Yes, I've followed the tech tip. I've set the idle a few times and never had this issue.

 

If the ECM can close the IAC and keep it at 800, it must not be closing when you jump A and B.
Can you hear and feel the IAC trying to close, when A and B are jumped ?

 

That's a good question. The fans turn on when A and B are jumped so it's hard to hear. I'll try it later.

 

I'd say a vac leak is starting a cascade of events...ECM senses too much air for what the TPS and O2 are seeing...so fuel goes up, rpm rises. OL the o2 is not part of this, so the correct TPS setting is most important in OL and cold starts.
We forget that excess fuel at CS will cause hi-idle too. Like the choke on old antique carbed engines...

Find the vac leak, set base idle (just close the plates all the way) and unscrew the IAC and clean it, without messing with the pintle and screw it back,. it will set itself automatically. There really is NO need to make it reset. It will do that after a short drive and a few throttle up/down cycles. Finding its place is what it does for a living...I've done dozens and never had to do it manually. IACs reset everyday from changing conditions.

 

I spent all morning trying to find a vacuum leak, but couldn't find any. I'm leaning more towards a vaccum leak myself, but why does the car idle fine with the IAC connected? That's throwing me off.

 

Because that's its job. The ECM has what is known as a "target RPM" for various ranges of coolant temperatures. There is a tolerance of 50 to 75 RPM (depending on conditions), so it won't be exactly the target RPM.

When the coolant is cold the target for idle is 1200 RPM (on my car). The ECM adjusts the IAC to achieve this RPM. As the coolant warms up the ECM reduces the RPM by closing the IAC. This works exactly the same way the "fast idle" cam works on carburetors.

The purpose of the minimum idle adjustment is to remove the IAC from the picture (that's why you disconnect it after forcing it to close by jumpering A & B on the ALDL connector) and adjust the throttle blades so that the engine won't die if the IAC is set to completely closed (which shouldn't happen, but this is added insurance). With the IAC completely closed, the only air getting into the engine is through the throttle blades.

I have found that the throttle linkage bends over time and the screw digs into the lever, so the minimum idle adjustment needs to be made periodically. I've never seen anything on this, but every 5 years or so ought to do it.