IAC and how it relates to ECM

[Low Budget]

Can someone please explain this to me? My IAC is not consistant even a little bit! I thought I had it reset and set correctly yesterday, and then I set the TPS and everything seemed good. I shut the car off and restarted it and it's all screwed up again. I HAD the IAC counts around 18 and the TPS was .42 with my idle at 750-800 rpm's in gear. After the restart, the IAC was at 10, but the engine wouldn't even stay running. It just let the rpm's fall off and never even raised the IAC counts. Today, I left the scanner hooked up and went for a 2 leg 100 mile ride. Each time I would come to a stop on the first (20 mile) leg the IAC wouldn't change and I would have to keep my foot in it to stay running. It wouldn't even try to save itself. I shut the car down for an hour. Then for the second leg, (obviously around 80 miles) I raised the minimum air enough to keep it running, but just barely. I also reset the TPS to .42. At times the engine would drop to 650 or so rpm's, and even then the IAC counts remained at -0-. It had changed to -0- when I raised the minimum air.

Here's my real question. Is the computer not telling the IAC to change, or is it telling it to change and the IAC isn't responding? If it's the latter, I know the IAC may just need cleaning and to have the plunger rezeroed. If the ECM's telling it to move, I don't understand why the numbers are not changing on the scanner. Is there no change in the numbers because the ECM needs a response from the IAC it's not getting? In other words, the ECM doesn't show what it's telling the IAC to do, it shows what the IAC IS doing? (What came first? The chicken or the egg?)

I hope someone is able to help me out here pretty quick. I'm going to check my mail at 11am est Sunday for the last time until Friday. (vacation) I'm taking this car and some tools with me. I hope to correct this problem while I'm away, but I won't have any access to you good people where I'm going.

Thanks for any info you can give. Jeff

 

[bruce]

The IAC passage ways need to be clean for the IAC to work, might look there.

Personally, I like setting it so that with the A/C off, the count is 0 in drive.

When adjusting it, the ecm needs to see about 30 MPH for it to do a reset, and recal things. Sometimes diddling with the adjustment too much sitting still without it seeing the 30 MPH can really goof the adjustment up.

 

[TurboDave]

Could be that your adjsutments weren't made under the right conditions??

You should only do the IAC/TPS adjustments after the car is completely up to operating temps, AND more importantly, has been driven around so that BLM's are "smoothed" and where they should be, etc. A short 5 mile srteet drive around should do nicely.

In deference to how bruce does it, I personally find it better for the IAC to be set anywhere from 10-20, in park, A/C off. TPS .38-.44, preferably closer to .40 (Never .46 or higher). I've never cared to let the IAC get to 0 under any conditions, because the ECM has no control at that point, but bruce is something of a wizard with the ECM's and has tried some things that most of us wouldn't and seems to make them work too.

 

[frankg]

Not sure but wondering, could his throttle body shaft be worn causing this?? Maybe sometimes its sucking air in thru the shaft and causing it to adjust wrong then other times when its sealed its not set to that?? It would be just like a vacumn leak. Frank

 

[Intercooler]

Frank,

I thought he had a new TB? His problem still sounds like a MAF problem. The absolute best thing he could try is buy a Translator and 3" or 3.5" MAF and be done with that question mark. Any leak in the system could be wreaking havoc too. Lots of testing to really find the answer.

 

[Low Budget]

Good memory Jesse

The throttle body does definitely make a mess of things when that shaft loosens in the bore. You're right though Intercooler, it is new. I been there, done that.

I'm leaving shortly, so I will be expecting to see lots more help till I get back. Hopefully I won't need it as much till I get online again. I can't count on my luck improving though, so keep it coming guys!

Anyone able to answer the communications part of my question? Does the ECM show the ALDL connector what it's telling the IAC to do or what the IAC has done? (This is the chicken and the egg thingy I was asking about.)

Thanks everyone. I'll clean the IAC and passage at the resort this week and report back on Friday. (I'll make sure I set it under the correct circumstances as well.)

Jeff

 

[bruce]

Re: Good memory Jesse

Originally posted by Low Budgit
The throttle body does definitely make a mess of things when that shaft loosens in the bore. You're right though Intercooler, it is new. I been there, done that.

I'm leaving shortly, so I will be expecting to see lots more help till I get back. Hopefully I won't need it as much till I get online again. I can't count on my luck improving though, so keep it coming guys!

Anyone able to answer the communications part of my question? Does the ECM show the ALDL connector what it's telling the IAC to do or what the IAC has done? (This is the chicken and the egg thingy I was asking about.)

Thanks everyone. I'll clean the IAC and passage at the resort this week and report back on Friday. (I'll make sure I set it under the correct circumstances as well.)

Jeff

The scanner only shows what's being commanded, in ref to the IAC. There is no feedback for the ecm to know how far the pintle is actually off it's seat.

 

[frankg]

Jesse come on you should know by now, new doesn't meen good!!!!!!! His IAC counts wouldnt come into line even with my known good MAF sensor. Frank

 

[Low Budget]

Ok, here's the frustrating update.

Pulled the IAC and was going to clean and depress the plunger. The shaft wouldn't go in, so I gave it a pull to see if it moved out. I yanked the shaft right out of the body! Crap! Bought a new one and installed it per instructions, and it didn't change a thing. Messed with resetting and took drives to get temp right and speed above 30 and on and on. I finally had some success when I pulled it back out. This replacement IAC has the cone shaped piece threaded on the end of the shaft. This is how they have you adjust the initial length from the gasket to the tip of plunger. I adjusted it as short as possible and reinstalled. Everything seemed to work fine then. Went for a ride of at least five miles and shut it down. Went back the five miles and it still worked good. Numbers were variable and linear with the RPM's. Cool. Till the next day. All is crap again. Idle will fall off and the IAC numbers never change. It will let the car shut off if left alone long enough. WTF???

My idle is so bad now it doesn't want to take the gas when pulling out after sitting at idle for a light. It is beyond a tip in stumble. It's like it's loaded up, but the O2 counts and scanner say it's lean. (Can the O2 sensor which is new throw things off like this?) I'm replacing the Mass Air again today, and will let you all know how this effects things. Wish me GOOD luck!
Jeff

 

[mgmshar]

...If your scanmaster (or whatever) is telling you that the ECM is not properly commanding the IAC, then maybe you have a bad chip. When the idle starts to drop, the ECM should command the IAC to open in order to compensate. You should see the IAC numbers going up. If not, then the ECM is not doing its job.

I would try replacing the chip, if possible.

Backgroud, something that I ran into: my car suddenly began to have real problems starting and running after a shut-down. After a lot of pulling my hair out, I figured out that the ECM was not doing its little IAC reset procedure (drops the IAC to 0 and then back up to 150) when I was shutting the car off. Thus, the IAC was WAY off from where the ECM thought it should be when I re-started it. The reason it wasn't doing it was because the socket on my ECM was not making good contact with the chip pins. After a LOT of wasted time, I figured out that was the problem. Sounds like your problem may be something like this.

Good Luck,

 

[chevyII]

o2 volts

The o2 sensor only senses oxygen not fuel. If you missfire a cylinder your o2 should go leaner,if the car runs so rich its not burning there is oxygen going out the tail pipe and by the sensor.

 

[turbo nasty]

Hey low bugd...I did not see you mentioning of having the car in diagnostic mode while doing your adjust.

Try this.

Car at full operating temp turn off engine and put a jumper on the upper row right 2 term on the adl port.

now with the jumper in place. turn key on NO Start and leave it there. Now go and disconnect the IAC elec connector from the IAC. Leave it off, You may have to take off the hard pipe/duct to get to it.
Remove the little cover over the min air blade setting screw if your car still has it on.

Now go back and turn key OFF and REMOVE jumper.

(For this next step I recommend someone to help you) Now start the car, if it will not stay running try turning the screw in a little at a time until you get it to stay alive. Now adjust the RPM to around 650 rpm in gear.

Turn OFF ignition, plug IAC conn back in.

Key on NO start and adjust your tps volts to .45 -.46 @ idle and 4.6 -4.7 WOT.

Now reset the computer (remove power for a few moments).

Now start the car up and see what you have. It may take a few attempts but this works for me and both my GN's and the past 3 others my friends had me tune for them.

 

[Low Budget]

Me again.

I got Advance to bring in a new Micro-Tek Mass Air Meter to replace the one they sold me in July. I picked it up and installed it yesterday. This new one won't even run the engine it's so tits'd. Right out of the box too! I guess I can add myself to the list of people who've had trouble with Micro-Tek. Advance will have another one in for me today. I wish I could afford a translator and LS-1 MAF, but I have too many things going on right now to swing that setup money wise.

mgmshar-
As soon as I get a MAF back on that works, I'll try another chip. My IAC numbers when I shut the engine off and key still on only go to 105 not 150. I thought that was wrong. Huh, I wonder why mine stops at 105? I've had the chip in and out a couple times and it seems to be contacting ok.

chevyII-
I've read your reply about O2 sensors like 10 times and I am embarrassed to say I don't understand. Can you elaborate on or reword your info for me please? I hadn't considered myself having a missfire, but I guess when it runs ruff at idle, that may be what's happening. Is that what's happening when it sits there with an unsteady idle, and if so are you saying that this unburned fuel vapor (due to the lack of combustion) actually serves to raise the O2 content in the exhaust stream? Definitely not disagreeing here, just trying to learn today's lesson.

turbo nasty-
I've done this reset procedure and had no success. As for diagnostic mode, no I wasn't in diagnostic mode. What should I be doing regarding this? As I remember it, the scan tool I'm using does have this mode. I didn't use it for this IAC problem though.

Thanks all!
Jeff

 

[turbo nasty]

I posted the procedure for setting the IAC...read above.

As far as the MAF... A1 Cardone is good. But, you have to get the right one.

No necking down or hourglassing through the sensor.
Elec connector has to be 90 degrees off the body.

 

[Low Budget]

Originally posted by turbo nasty
I posted the procedure for setting the IAC...read above.

As far as the MAF... A1 Cardone is good. But, you have to get the right one.

No necking down or hourglassing through the sensor.
Elec connector has to be 90 degrees off the body.

I know you posted the reset procedure. That is to what I was referring. It didn't solve my problem.

I bought three A1 (my butt) "Car done" MAF's and all three were junk out of the box. I don't doubt some are good, but who will let me go through a warehouse of MAF's to find one I'm happy with? The Microtek is apparently capable of the same hit or miss mentality. What a bunch of crap they ALL are!!

I'm going to pick up the next victim at the parts store shortly.
"I'll be back!"

 

[turbo nasty]

What was GM thinking when they stopped carrying the MAF

Good Luck

 

[BJM]

GM probably decided it was unique and also very obsolete. The newer MAF's are miles ahead in terms of performance and life. They may have had a lot of scrappage in just making them, something the rebuilders just pass on to us to test for them. GM wouldn't allow that to happen so for them it may have been too expensive to bother.

I went the translator route and can now buy used MAF's for $65 CAD any day of the week at a local yard. You don't need a new MAF anymore, they don't fail like our originals.

 

[chevyII]

02 volts

I quess what I was trying to tell you is a general tought of the O2. the sensor reads oxygen not fuel. you can dump fuel from a gas can into an engine and have an exhaust leak or missfire and the O2 will read lean beacause of the oxygen in the exhaust.

 

[Low Budget]

Update


I have a working MAF again.

I am still fighting the IAC problem though. I can watch the IAC number, and when the car is started it is high to match the high idle. As the idle drops, it is plain to see it is following the IAC number down. Just as it should be. Closed loop: The idle goes up as the A/C is activated. The IAC number is also going up as it should. (For the record, I know the number increase is why the idle is going up.) Now here's where the problem starts. With the engine running, A/C off or on, (it doesn't matter) if the rpm's fall off for any reason, the IAC number will not change. The engine can shut off completely, and the IAC is not commanded to open. If it does shut off, the IAC number goes to a quick 188 (don't know the significance of that number) and then settles to 150. (also as it should)

I have had two different chips in and both do the same thing.

Any takers??? What now? The computer DOES know what the rpm's are, as I can see it on the scan tool.

 

[TurboJim]

Post MAF air leak, or large vacuum leak is my guess.

Why else woujld counts be 0, unless it was trying to lower idle caused by a vacuum leak?

Jim