Idle advance during engine load
 

Somewhere in my reading I came across what happens to the idle when you put a load on the engine, like turning the steering wheel far left or right. The idea is when you put extra load from the power steering pump the engine is spose to raise idle to compensate. On my 92 when I pin the wheel either left or right the engine does respond with a higher idle but it bobs from about 900 to 400 rpms. This is a real pain in the butt when you are trying to turn with wheel pinned because it causes the truck to lunge. Does anyone know what controls this idle circuit?

Im gonna pull something out of my rear and say its the computer,different things tell it when idles changing,then it sends info to the iac..Maybe im right....someone chime in so i can shutup now lol

Your right chris about the IAC, but what I can't figure out is what tells the IAC to adjust the idle. I am thinking it has something to do with rpms, the computer reads a drop in rpms and sends info to the IAC. So, why can't the engine hold a higher idle, why does it drop back down?

Here's a look at the wiring circuit to the computer

http://shbox.com/1/iac2.jpg

Here's something on the types of control valves

http://www.autoshop101.com/forms/h60.pdf

So there can't only be 2 things causing the idle under load to bounce, either the IAC is faulty or the ECM is faulty. Does that sound right?

Update:

I disconnected the connector from the IAC and pinned the steering wheel. The engine idled down from the increased parasitic drag and held steady. So it seems the bouncing idle under load is a symptom IAC circuit. Idle otherwise is steady, just under increased load does it bounce from 400-900rpm. It will fluctuate in both park and drive, drive being the worse.

Hmm,interesting. Im going to try it on mine tomorrow. I installed a tach so i can see exactly where it stands. Do you have ac? i dont,so try turning it on next time and doing it,and see if it stalls the motor.

Yeah I got AC, but, the compressor isn't engaging when I turn it on. I think the system is empty on freon so the low pressure switch might be working - the AC charge is another project. In theory the same thing should happen when you engage the AC, the extra engine load would cause the ECM to compensate.

It would compensate,i was just wanting to see how it would respond unplugging the iac and turning the air on then pinning the steering wheel.

What I did tonight was retard the timing a few degrees and now when I pin the wheel it holds steady. No more 900-400rpm bounce. So it seems getting the timing perfect is critical on these 4.3 beasts. somewhere, i saw the only way to really time these engines is to put a scanner on it and turn the distributor to get the cam advance to 0 degrees.

You have to manually set the timing with the timing connector (brown wire to the left of the transmission tunnel at the top of the carpet inside under the dash) disconnected. Only the EDI engines (late 95 and up) need to have the CMP retard set with a capable scan tool.