IAC and idle problem
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Using IAC counts to set minimum idle isn't precise and though that information is nice, a functioning system simply moves the IAC to whatever number that gets it within Targeted and that number can vary by engine, make and model. Get it wrong and idle quality, tip in acceleration can/will suffer.
A throttle body that you have to take a Dremel or Drill Press to isn't worth the money - just my opinion - but speaking of which, make sure the IAC Pintle fits - there are different sizes - reputable aftermarket manufacturers should know what works and have a live person you can talk to.
A throttle body that you have to take a Dremel or Drill Press to isn't worth the money - just my opinion - but speaking of which, make sure the IAC Pintle fits - there are different sizes - reputable aftermarket manufacturers should know what works and have a live person you can talk to.
Le Mans Master
Joined: Feb 1999
Posts: 9,926
Likes: 38
From: South Carolina
St. Jude Donor '03 & '05
You can tell the IAC what to do, but you cant see what it is doing, as GM doesnt have that figured into the ECM datalogging for some reason. You either OHM it or replace it with a known good one, and see what happens
Team Owner
Joined: Jul 2004
Posts: 28,464
Likes: 3,288
From: Hartford WI
You can apply voltage to it and it will move. But, if one of the 4 wires isnt working, it may not go in and out during operation.
You can tell the IAC what to do, but you cant see what it is doing, as GM doesnt have that figured into the ECM datalogging for some reason. You either OHM it or replace it with a known good one, and see what happens
You can tell the IAC what to do, but you cant see what it is doing, as GM doesnt have that figured into the ECM datalogging for some reason. You either OHM it or replace it with a known good one, and see what happens
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
I just centered the throttle blades in the bore, and tightened the screws. I did not change anything really, just made the original throttle blades move to a .002"-.003" movement.
Thread Starter
Instructor
Joined: Oct 2009
Posts: 202
Likes: 9
From: Tucson
I did everything that SunCR said above, and...
Same bad idle.
To recap, this is what I've done so far:
1) Checked TB and IAC housing. TB is basically new, IAC housing looks good. Put in new gaskets.
2) Checked IAC out of the car. Pintle extended correctly when ALCL grounded.
3) Checked IAC connector with ALCL grounded. Power to all 4 plugs.
4) Checked IAC on car, with ALCL grounded. Can hear buzzing from IAC motor.
5) Performed idle procedure from SunCR.
6) Car still idles around 500 RPM, which is what I set with IAC unplugged according to procedure.
6) With car warm, can remove IAC with no difference in idle RPM.
Is there any way to check the IAC with the car running without a scanner? I've checked the IAC every way the FSM says and it appears to be good. IT JUST DOESN'T WORK!
Same bad idle.
To recap, this is what I've done so far:
1) Checked TB and IAC housing. TB is basically new, IAC housing looks good. Put in new gaskets.
2) Checked IAC out of the car. Pintle extended correctly when ALCL grounded.
3) Checked IAC connector with ALCL grounded. Power to all 4 plugs.
4) Checked IAC on car, with ALCL grounded. Can hear buzzing from IAC motor.
5) Performed idle procedure from SunCR.
6) Car still idles around 500 RPM, which is what I set with IAC unplugged according to procedure.
6) With car warm, can remove IAC with no difference in idle RPM.
Is there any way to check the IAC with the car running without a scanner? I've checked the IAC every way the FSM says and it appears to be good. IT JUST DOESN'T WORK!
Melting Slicks
Joined: Aug 2003
Posts: 3,399
Likes: 394
From: Lakeville MI
Target idle speed for an 85 auto with warm coolant in gear is 550 rpm. +50 rpm offset for park or neutral.
If it will maintain target idle speed with and without the A/C on, I'd say the IAC is working.
If the tps voltage at idle is too high, closed loop idle PID control will be disabled and the IAC position will be based instead on the warm park position table (coolant temperature dependent) and throttle position without regard to the actual or target rpm.
Check the idle in gear, in park or neutral and with and without the A/C on (if it still works) to verify idle speed in the 550-600 rpm range.
It sounds like you're setting the min air close to the target rpm and therefore not seeing much of a change with the IAC connected.
If it will maintain target idle speed with and without the A/C on, I'd say the IAC is working.
If the tps voltage at idle is too high, closed loop idle PID control will be disabled and the IAC position will be based instead on the warm park position table (coolant temperature dependent) and throttle position without regard to the actual or target rpm.
Check the idle in gear, in park or neutral and with and without the A/C on (if it still works) to verify idle speed in the 550-600 rpm range.
It sounds like you're setting the min air close to the target rpm and therefore not seeing much of a change with the IAC connected.
Melting Slicks
Joined: Mar 2008
Posts: 2,141
Likes: 76
From: Santa Maria, CA
Forgive me if I send you in the wrong direction. The IAC has two coils, one extends the pintle and the other retracts it. You said you verified the pintle extends. The pintle needs to retract to increase RPM. Is this a new IAC? It is possible one coil is bad and you were able to get it to idle by adjusting the idle screw to increase flow through the throttle blades.
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Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
If the IAC has set the idle, disconnecting the connector won't change the idle.
turning on the A/C will cause the engine to die when disconnected if already idling.
putting it in gear may have the same result if idling with the IAC disconnected.
If nothing happens when you turn on engine loads means the IAC is doing it's thing.
If the Idle is too low, you have to redo the idle sequence again, and maybe try to be as accurate as possible.
turning on the A/C will cause the engine to die when disconnected if already idling.
putting it in gear may have the same result if idling with the IAC disconnected.
If nothing happens when you turn on engine loads means the IAC is doing it's thing.
If the Idle is too low, you have to redo the idle sequence again, and maybe try to be as accurate as possible.
Burning Brakes
Joined: Apr 2004
Posts: 762
Likes: 2
From: Huntington ma
Just a coupe of thoughts..
I also have a BBK TB on My '85, for about 4 years now. When I first installed it there was an "interesting" clearance issue between the throttle shaft and the throttle position sensor - the retaining rivet in the TPS was hitting the end of the throttle shaft, and although Both should be rotating together it caused a binding that prevented the TB from closing at idle. A different brand of TPS fixed it, some TPS manufacturers apparently use different dimensions for the protrusion length of the device, either that or there Is no specification for it..
Also - the BBK TB's are known to have a casting issue where the "finger" or hook on the TPS will rub on the side of the TB casting causing the TPS to stick, and not return from wherever you opened it up to last/furthest. The Throttle blades WILL return fine, just that the TPS lever will be stuck and not follow. This will drive you nuts if you don't see it - the ECM never knows that the Throttle came back closed. It's obvious if you are looking at a scan tool, of it you Look at the operation of the TPS itself. I had to grind clearance on My BBK TB to fix this, it happened to me!
I also have a BBK TB on My '85, for about 4 years now. When I first installed it there was an "interesting" clearance issue between the throttle shaft and the throttle position sensor - the retaining rivet in the TPS was hitting the end of the throttle shaft, and although Both should be rotating together it caused a binding that prevented the TB from closing at idle. A different brand of TPS fixed it, some TPS manufacturers apparently use different dimensions for the protrusion length of the device, either that or there Is no specification for it..
Also - the BBK TB's are known to have a casting issue where the "finger" or hook on the TPS will rub on the side of the TB casting causing the TPS to stick, and not return from wherever you opened it up to last/furthest. The Throttle blades WILL return fine, just that the TPS lever will be stuck and not follow. This will drive you nuts if you don't see it - the ECM never knows that the Throttle came back closed. It's obvious if you are looking at a scan tool, of it you Look at the operation of the TPS itself. I had to grind clearance on My BBK TB to fix this, it happened to me!
