IAC and idle problem
 

I have an 85 Vette with a BBK throttle body but otherwise stock. I'm having a problem with the idle. I have tried to set the idle using the idle sequence listed in the forum, but the idle will not go up after reconnecting the IAC. Connecting or disconnecting the IAC seems to make no difference.

I have used the chart in the FSM to check the IAC, and it checks out ok. I have even taken the IAC out and checked it to make sure the pintle extends, and it does. I can ground the ALCL, and hear the IAC buzzing in the TB with the ignition on.

If the IAC checks out, what next? I have changed ECM's, and no diffference. If there was a vacuum leak, wouldn't the idle be high vs. low?

Maybe turning the min idle screw to increase rpm and re-setting TPS, will help.

I just put a BBK/Edelbrock T.B. onmy 86, and the problem I have is the idle is a little high. when I reset the idle with the screw, the throttles are so far closed, the blades stick on the bore. I readjusted the throttle blades, and restaked the screws so they dont come back out, and the thing is closer. It just floats when I get off the freeway, and has trouble slowing down.
BUT:
it has way more punch off idle because the blades open up so quickly, allowing more air to be ingested by the engine really fast.
My gas mileage has suffered since I'm basically performing low power burnouts when leaving the line after the signal lights change.
I can tell it's more torque down low, but less control.
it would be very beneficial if my engine was a 383.

I can turn the idle up to normal, around 650ish, and it runs fine. Frys the tires off the line even in drive. If I basically bypass the IAC is this going to affect anything else?

:iagree:

Also as Coupeguy suggested ensure that the blades are not "fully" closed off in the bores or they can hang up in the TB bores and/or choke the engine at the idle position with only the IAC providing airflow.

:iagree:

Have you checked the IAC count? Sure, you can simply adjust the TB to bypass the IAC and the ECM so that it cannot adjust the idle speed but why? Have you cleaned the TB and the IAC housing?

The TB is pristine, its pretty much brand new. I don't have access to a scanner so I don't know what the IAC counts are.

What could be preventing the IAC from controlling the idle if its operating correctly? Is there a way to check the IAC when the motor is running without a scanner?

I've been going through the same issues with my 85. You will be much better off having a scan capability. Its hard to find an inexpensive scanner that works on an 85. I found that using free WINALDL software with an ALDL to USB cable and my laptop works very well. Total cost was $70 for the cable. Someone here may know of a cheaper option.

I dont know if this applies to you too but I was told to make the hole bigger that it in between the to butterfly valves. A tuner on the west coast said the whole wasn't big enough. So I drilled it to be a hair bigger. If you need a pic I can get one for you. It is quick and easy so it might be worth it to do it.

Talk to Auto X-ray. That is what I have.

I know you say the TB is pristine. Does that include the IAC housing? Without a scanner it is hard to say whether the signal is being sent or not. If it is trying to command the IAC and the IAC does not change, there could be something like a break in the wires, as was my case.

[QUOTE=aklim;1572215720]Talk to Auto X-ray. That is what I have.

Forget about Auto X-Ray for an 85. I already made that mistake. I have a brand new Auto X-Ray scanner that does not work on my 85. The same is true for most data logging software. They typically work with 86 and up. The 85 has a 160 BAUD rate. The WINALDL software is free and works.

Where did you get the cable? Did you make it, like on the website?

Are you talking about moving the blades farther away from the bore, so they're not inside the bore?

I ordered the cable from www.aldlcable.com. It came with a CD with software drivers. The cable comes with a shorting plug to connect different pins for different configurations. Use the "10K Resistor Between A and B" configuration shown on the diagram. The cable will increase your idle slightly when scanning. That's normal.

Do a Google search on WINALDL and download the program. I did a data logging drive today. Recorded 20 minutes of data under various driving conditions. You can see how slow the BAUD is for the 85 ECM. The data refreshes every 1.5 seconds.

To set the IAC, turn the idle screw clockwise to lower counts and counter-clockwise to raise. Measure the counts, turn off the car, adjust the screw, then restart. Allow a few minutes each time for the computer to relearn an settle in. Once you have set the IAC, you may want to readjust the TPS voltage to .54V.

Don't know if this would apply to an '85 or not. Not difficult and may be worth a try.

http://www.hashmarks.com/techtips/throttle_body_mod.htm

He has a BBK ThrottleBody so it should apply, not sure but its a quick fix.

Why someone would buy a performance device which needs further modification to work eludes me - if the Manufacturer can't make it right out of the box, why buy it?

Anyway - the ECM has limitations/design parameters that will only work within a fairly narrow field. First, it has to know that the vehicle is at idle and that signal is from the TPS which for idle, can be anywhere from .48 to .62 volts with .54 volts accepted as the Norm. More or less and the ECM won't control Idle. Start by making sure that TPS voltage is within this range. Next, the throttle body has to have a minimum amount of air flowing through it; prior to IAC control, which is normally given in RPMS. For an '85, the initial spec was 425 rpms +/- 25 rpms. When this Year began to experience stalling issues, a Service Bulletin was issued raising that value to 500 rpms until a redesigned PROM became available as a permanent solution. Whether or not your '85 has the updated PROM will take some research - I no longer have access to '85 Part Numbers. Minimum idle is set by grounding the diagnostic link with the ignition on; engine off. You should be able to hear the IAC pintle extendlng into and seating in it's bore and when fully seated, no air is going to get around/through it. Then to keep it seated, disconnect the IAC harness. You should also disconnect the Timing Connector so that the ECM doesn't advance timing which can increase idle and then start the car. With only the air around the throttle plate getting in, you may have to feather the throttle and it helps to have an assistant start it (turn the key) so that you can manually work the throttle lever. Having the motor warmed up beforehand is also a good idea. Turn the screw (if necessary) while observing RPMS on your scanner or whatever device your using to get this data. When set to specs, turn it off and plug the harness back in. Reconnect the Timing Connector. Readjust the TPS to whatever it was (within specs) before you turned the screw. Slightly depress the accelerator and start it. Let it run for 10 seconds and then off for 5 seconds. Restart normally (with your foot off of the accelerator). Actual Idle should be +/- 50 rpms from Targeted in Drive; +/- 100 rpms from Targeted in Park/Neutral. If within spec, note the rpms and turn it off. Restart. The idle should be higher than what you noted and then return to that prior number. Assuming all of that is correct, it's right - leave it alone and resist any temptation to monkey with that screw again (assuming there's at least the same quality in the aftermarket part that there is in the stock throttlebody). There is nothing that will move that screw or cause the adjustment to get out of spec for as long as you own it. Finally, disconnect and reconnect the battery as running it with the timing connector disconnected should have generated a Code 42 which is now in History. 100 ignition cycles will usually get it out, but leaving it there often leads to another post: I scanned my Vette and found this 42 - how do I fix it?

Manufacturer makes one thing that fits the general buying audience and not everybody. Also, manufacturer has to conform to certain federal regulations like emissions and mpg.

Isn't it easier to set it by IAC counts?

IACs can go bad. A Datalogger will only show you the counts the ECM is asking the IAC to do. You will not know what the IAC is doing. the only check, proper check, is to take it out, and do a resistance check on the IAC.
Use this photo to check it. IAC years arent any different, for an 85-96.
http://shbox.com/1/iac2.jpg

He did say that he has seen it extend and heard it so I am not sure that it is non functioning.