I no longer have my 84 z28, but now have a 84 el camino with a 350 tip. My idle is not steady and can jump from around 700 rpm all the way up to 2500 rpm. I tried to reset the iac with no luck there. The tps is set where it should be at .54 volts. Now I do have a pretty bad leak in the radiator, which I do plan on replacing friday. I have a feeling that the leek might be so bad that it is causing the computer to get all screwed of from maybe not getting a good reading from the temperature sensor. Although, the gauge on the dash seems to be working well. I notice that the fan comes on right around 220- 240 (estimated). It is currently just sitting in the yard, so it is not being driven at the moment.

 

Ok, so I am not getting any fault codes from the computer. So, I removed the iac valve. I then plugged in the iac valve and jumped the computer into test mode, but the motor did not extend. Consequently it is stuck in the open position, thus allowing more air to bypass the throttle body. I was thinking about it last night. After I cleaned the Iac valve and reseted it like the book says, that is when it started to idle very high. Also, I believe the valve was long gone shot; even before I reseted it. Before, when I would start the cold engine, it would idle low and then idle up as the engine warmed up. When I took off the Iac valve it was extended, thus leaving it fully closed or almost closed. I will be picking up a new one friday (just waiting for payday!), along with a new radiator. If anyone know of the values I could measure at the connector that would be a big help. In the meantime I will be doing a google search for them. I am going to see if I can extend the valve back out so that I could get the idle back down.

 

The IAC is a stepper motor with two coils. The connector has 4 terminals, two for each coil. The ECM operates the coils in tandem(back and forth) to move the pintle in or out. The values at the connector will be 12 volts and zero volts(on-off) as the ECM switches the coils. I never operate an IAC with it removed from the TB. I've had the pintle shoot out of the motor, ruining it. You can test the IAC all you need using a 12v test light and the AC switch. When the AC is turned on, the ECM will command the IAC open to raise idle speed. When the switch is turned off, the ECM will then command the IAC back out to lower the idle. You can do the same thing by snapping the throttle. The factory ECM program has an anti stall feature that holds the revs up momentarily when the throttle is released to prevent stalling. You should hear this as you snap and release the throttle. If not, the IAC isn't working. Another method is disconnecting and reconnecting a vacuum hose. The IAC will close down to try to bring the idle down when air is added through the open port. It will then open back up when the hose is reconnected.

A test light will flash on and off held across the connector terminals, when idle change is commanded. Use paper clips as back probes to contact the terminals with the IAC connected. If you see the light flash and no change in idle, the IAC is bad. If the test light flickers and you verify your backprobes are making good contact, you likely have a bad ECM. Use your wiring diagram to see which terminals go to which coil.

A compatible scan tool is the best thing for this test as you can view desired and actual idle speed as well as IAC counts to see if the ECM is commanding the IAC to move while you watch signal at the valve to see if the command is getting to the IAC and if it is responding.

A surging/uneven idle is almost always caused by unmetered air(vacuum leak).

 

Ok, I will test it later and let you know what I find out.

 

So I did the paper clip light test and I am getting a solid light on.

 

I may have done that test wrong, I will try again tomorrow as I ran out of daylight and it is raining. Also, I got code 36 from the ecm. With the A and B terminal jumped, I have the engine running and it actually seems to run much better.

 

Staring the engine with A & B jumped isn't so good. Never saw actually hurt anything but it's not recommended. Jumping A & B with the engine already running puts it in diagnostic road test mode and the light will flash fast in closed loop, slower in open loop. If the engine is running better with A&B jumped, it's because it's probably it's in default mode and overlooking the problem.

Code 36 is a MAF burn off fault. The MAF sensor is the soup can looking sensor in the intake air duct . It measures intake air mass of the engine. The MAF sensor uses a hot wire to measure air mass and in an effort to keep the sensing wire clean in service, the engineers designed a burn off cleaning function into the system. When the engine is shut down, the burn off relay activates and heats the sensing wire red hot for a few seconds to burn off any deposits from the wire. The ECM sees the voltage across the wire during burn off and knows that burn off occurred. Code 36 means that the ECM isn't see the burn off voltage when it expects to. It could be the burn off relay, the MAF sensor, or a few other things. It could also just be wiring. You need to have a good service manual and a DVOM or even just a test light and follow the test sequence for the code.

EDIT: BTW, the MAF sensor is next to god for the ECM. If it is just a little off, you will have runnability issues.