Adjusting idle speed on '89 4.3 v6 (pre-Vortec)

ROKRIG · Aug 25, 2009

I was noticing after I installed a little tach that my motor at idle seems to hover around 500-550 rpm. Does anyone know whether that is correct? It seems like most vehicles tend idle around 650-750.

Is it possible to adjust the idle via the ECU on this vehicle using the OBD harness? I'm assuming that the computer has complete control over the idle speed.

(I've replaced most of the sensors, if not all of them that would be utilized to monitor running conditions and the idle speed is steady but just seems a little low). Granted, it doesn't stumble or dip dramatically when loads (i.e. electric fans, lights are switched on) are placed on the motor.

Just wondering...

Terry

J_SIN · Aug 26, 2009

you sure your tach is set up for a 6 cylinder engine?

nevada · Aug 26, 2009

i say aslong as its working well, and doesnt die out....leave it be.

i prefer mine to idle a bit lower(as long as it doesnt die) then its "supposed" to be. *im not running a 4.3*

ROKRIG · Aug 26, 2009

J_SIN said:
you sure your tach is set up for a 6 cylinder engine?

switch on the back allowing for 4, 6, or 8 cylinder. I moved it to 6. But I can certainly double-check that I didn't bump it during installation

deerlick · Aug 26, 2009

theres a small allen or torx screw on throttle body that adjusts where throttle stops.

Bill USN-1 · Aug 27, 2009

The idle speed is controlled by the chip via the IAC in the throttle body.

The screw on the throttle arm is for setting the minimum idle speed only.
The ECM has a built in keep alive table that will increase the engine rpm should it drop below 475.

You should perform the initial setup procedures(in the FI FAQs on binderplanet) and verify the IAC is actually working when you do it.

At a warm idle you should have very little suction in the port above the IAC. As you open the throttle it should increase.

Verify timing, TPS setting, and min idle screw.

If you give me the BCC letters off the ECM I can tell you what the idle speed should be.

If it's too low for you then you either reprogram the chip or turn up the min idle screw to over ride the IAC idle and then adjust the TPS back to .54v at idle.

ROKRIG · Aug 28, 2009

That's a technical response indeed.

Thank you. I'll try running it and see if the ECM makes the necessary adjustments

Terry

crash2 · Aug 28, 2009

The correct method to setting min air rate(AS per GM specs)---key on and ground A&B on the aldl. You should hear the IAC buing(trying to close it). At this point you need to disconnect the iac. Remove the ground wire from the the aldl and start the engine. You need to set the rmp to 450-500rpms at the set screw on the throttle body. Some systems have a problem trying to reconie where the IAC sits after this procedure. What I do is key off-hook up the IAC--key on and again ground the aldl--this will drive the IAC closed again--but will also give the ecm reference to where it sits since that system is a non feedback for the IAC(computer does not really know where it truely sits. So at this point remove the ground at the aldl. Start it. At this point I like to see the data and make sure the IAC counts are roughly around 15 counts. If its off--you might need to "slighltly" tweek the min air rate screw..

Bill USN-1 · Aug 28, 2009

Once the IAC is connected the ECM resets the IAC position upon turning the key off.
So a double reset is not needed.
If you use winaldl to watch the sensors and log data you will see the IAC position move to closed and then to the park position of 145 after you have already turned the key off.

But as I said before..you must verify the actual position of the IAC because the ECM only sends the signal to the IAC. There is no physical position feed back to the ECM for it to know where the IAC is.

So the numbers can say 5 counts or 200 counts and the IAC still be setting in the closed position.

crash2 · Aug 28, 2009

Bill USN-1 said:
Once the IAC is connected the ECM resets the IAC position upon turning the key off.
So a double reset is not needed.
If you use winaldl to watch the sensors and log data you will see the IAC position move to closed and then to the park position of 145 after you have already turned the key off.

But as I said before..you must verify the actual position of the IAC because the ECM only sends the signal to the IAC. There is no physical position feed back to the ECM for it to know where the IAC is.

So the numbers can say 5 counts or 200 counts and the IAC still be setting in the closed position.

I have had instances where the ECM did in fact not reset--the reason I do that way... There could have been underlying issues with the ECM, that I don't know.