Idle Air Control operation?

[ltmark]

I'm fighting a rough idle on my 85'. I have the manual and have gone through just about everything! I just removed the TPI and the lower cover (the one that holds the IAC valve) The IAC pintle was at the stated beginning distance, 1 1/8 inch. When I install the IAC in the housing (all of it being removed as I stated) I can see that the face of the Pintle is about 1/4 " from being seated against the port. If I hook up the IAC to the connector and turn the key to the "on" position, the pintle does not move. It is a new IAC valve (about 3 months old). Measuring the voltages at the 4 pin connector with the Key "on", there is 12V on on two of the opposite pins and 4.5v on the remaining opposite connectors. My questions are:

1. When adjusted to the 1 1/8" setting, should the pintle "seat" all the way when installed?
2. Should the pintle move when I turn the key to the on position, or does the IAC only move the pintle when the engine is running?

I understand that the ECM moves the pintle in small steps to adjust for proper idle, based on rpm's, etc. but since the face of the pintle has 3 distinct angles, I would think that the starting position when cold should be much closer than 1/4" away from fully seated.

Obviously, the difficulty arises in that when the TPI is installed an the engine is started, there is no way of knowing where the pintle actually is in relation to the fully seated position.

I should say that the air passages were pretty carboned up, although no way close to being restricted, so maybe that could create the roughness, but I don't think so. I won't be able to put it all back together for a week or so because I had to order a new gasket, since I was unable to get one locally!

Does not throw any codes, just "real" rough idle when cold. I have checked all voltages and operation of the cold start injector, TPS, MAF, done the IAC setting procedure, etc. etc.

Thanks for any thoughts!!

 

[TedC]

You may find my old post of help...
http://www.corvetteactioncenter.com...ottle-body-adjustments-1989-l98-corvette.html

Answers:
#1: No. This is to set the pintle far enough away so that it doesn't damage the seat during IAC installation as you thread it in. You could use 1".
#2. Yes. If you follow the steps outlined in this old post, you can actually hear the IAC buzz as it seats it self.

Since that post, I now use an OBD I scanner to properly set min idle speed.

 

[aklim]

I use a very simple method. Clean the IAC passages and make sure the timing is spot on. Assuming the IAC is working, you run the car till it is at operating temp, shut everything else down except the engine. Hook up your scanner. See how many IAC counts. You are looking for a range of 15-20 IAC counts. If it is more then you back out the adjustment screw. Less you open the screw. Each adjustment you make, you rev the motor up and take readings when it calms down.

After that, set TPS if possible and drive.

 

[Hib Halverson]

The IAC pintle should not be on the seat when the engine is idling. If it is, then you've got a pretty big vacuum leak somewhere. If that happens, that means the engine cannot attain the calibrated idle speed even if it closes the IAC fully.

The IAC may or may not "buzz" after you turn the key on. Some engines may reset the IAC for starting. Others might not.

DO NOT observe the instructions which suggest adjusting the minimum air adjustment screw on the throttle body to bring IAC counts into range. That IS NOT what controls idle speed. Unless the TB is new or has been disassembled, you do not change that adjustment and, if the TB is new or has been disassembled, the minimum air adjustment procedure in the factory service manual should be observed. Next adjust the TPS per the FSM. After that, idle speed is controlled by the ECM using the IAC. If the system cannot maintain idle speed control, then there is another problem somewhere.

If you have adjusted the min. air screw in an attempt to control idle speed, first you must readjust the min. air screw using the procedure in the FSM. After that set the TPS. After that, if the system still cannot control the idle speed, you need to find the problem which is preventing that from happening, correct it then verify the ECM and the IAC are properly controling idle.

Again, DO NOT use the minimum air screw to bring your IAC counts into range.

 

[ltmark]

I'm pulling my hair out. I have an 85 and I didn't think I could run software on an 85 via the OBD1? I thought it had to be an 89 or newer?

Also, back to the IAC! I have the housing for the IAC removed, waiting for a gasket. With the A/B jumper in OR out, (with the key in the on position) I have 12 v on caddycorner pins and .45 v on the other caddycorner pins of the connector. If I put either of my two IAC's in the housing and hook up the connector, I get no movement of the pintle or sound, etc. I read that with the A/B jumper on, the pintle should try to seat all the way? Is there a chance that this only happens in the "crank" mode?

Does anyone know how I can manually check the IAC's via voltage, etc? Luckily, I am retired and have the time to mess with this, but I absolutely hate not being able to figure this one out.

 

[aklim]

DO NOT observe the instructions which suggest adjusting the minimum air adjustment screw on the throttle body to bring IAC counts into range. That IS NOT what controls idle speed. Unless the TB is new or has been disassembled, you do not change that adjustment and, if the TB is new or has been disassembled, the minimum air adjustment procedure in the factory service manual should be observed. Next adjust the TPS per the FSM. After that, idle speed is controlled by the ECM using the IAC. If the system cannot maintain idle speed control, then there is another problem somewhere.

If you have adjusted the min. air screw in an attempt to control idle speed, first you must readjust the min. air screw using the procedure in the FSM. After that set the TPS. After that, if the system still cannot control the idle speed, you need to find the problem which is preventing that from happening, correct it then verify the ECM and the IAC are properly controling idle.

Again, DO NOT use the minimum air screw to bring your IAC counts into range.

I have tried it both ways and they come out to about the same. From what I can see, the ECM sends a command idle speed and the IAC tries to achieve it via a vacuum leak. In my car, the idle speed was raised in the PROM so I adjusted for that but found that by using that method, I could get pretty much the same results as the FSM. Can you explain to me why it won't work?

 

[TedC]

I've spent several days playing around with IACs and Min Air, trying to understand the relationships.

The procedure in the factory manual to adjust Min Air is good, but I found key infomation lacking. That is why I documented the steps.

aklim, your steps work as well as long as the desired timing is set to stock and there are no other issues (worn throttle body, leaking gaskets). I know, I tried it, and the results were the same. I prefer to use the factory approach, and then verify that I have 15-20 IAC counts. Some of last years of the L98 like my 1989 have some real hot timing advance built into the prom, and reducing base timing may be required. This is where the factory approach to setting Min Air is more universal. Timing changes impacts Min Air setting, which impacts IAC counts.

Ok, back to ltmark's issue. If you read the steps I detailed out, when you jumper "A" and "B" on the ALDL and turn ignition on (NO NEED TO CRANK), this will seat or fully extend the IAC pintle. If this does not happen, then I would pull out the factory manual and check the wiring, IAC, and ECM. Don't test the IAC out of the TB, or the pintle will fly out. There are three different pintle tip designs, so they need to match when replacing them. I worked on a 1987 vette that idled like crap, even after a good TB cleaning. What I discovered was that the TB was from a newer vette (1989), and someone carried over the 1987 designed IAC. When we installed a 1989 designed IAC and reset Min Air, it was "Miller time".

 

[TedC]

Regarding OBD-1... I found this in my auto x-ray manual... Supports GM OBD-I (for 1982-1995 GM 12 pin ALDL vehicles)