IAC and how it relates to ECM

[Low Budget]

Jim,

I could have the minimum air set too high and end up at 0 but I don't. The counts are not always at 0. As a matter of fact, more often than not, the count is above 0. It could be at say, 12 and just sit there unchanging unless I step on the gas. Then it will go up to much higher numbers depending on how hard I'm on the gas. (Is it supposed to do that?) If I don't touch the gas, it could be sitting at 12, and then as described above, the rpm's will mysteriously start to drop. The count will stay steady and unreactive to what is taking place. It could (although usually doesn't) even shut off it can drop so far. Still no change until it is gone. Then the number changes!

Jeff

(I responded to your mail too Jim.)

 

[mgmshar]

Low Budgit,

You mentioned in an earlier post that your IAC counts are going to 105 with the key on and the engine off. This is not necessarily unusual. The stock chip sets the IAC to 150 when you key the car on (before start). Once the car fires, the IAC quickly closes to bring the engine speed down. Since people don't like the huge engine flare that occurs after a start, some chip makers lower the IAC target position for starting. 105 sounds like a logical number.

I am really wondering about this one. The IAC is sitting at 10, the idle starts to drop, and the ECM does not open the IAC. Strange. It just sounds to me like something is not working right in the chip.

This is a long shot, but worth a look. Pull the chip out of the socket on the ECM. Use a flashlight to look at each metal contact in the socket on the ECM. Make sure that they are all pretty evenly bent outward from the center. In my case, I found that one of these metal contacts had somehow bent inward, and it was no longer making contact with the chip's pin when the chip was inserted. I just used a small screwdriver to very carefully pry the metal contact back out. VERY carefully. Also, before you go anywhere near your ECM with a metal object, disconnect the orange wire by the battery to completely power it down.

Wish I had some better ideas. Good Luck,

 

[Low Budget]

Thanks Mike,

I have checked the chip pretty close, but I'll have another look tomorrow.

Jeff

 

[Intercooler]

Have you checked everything from the turbo outlet back to the throttle body for possible leaks? The IAC numbers changing with the throttle opening/closing is normal. At an idle though it should settle out with your RPM's at a fixed number +/- 15 or so counts. May want to take the time to pull all your IC hoses and go over them with a fine tooth comb. I know long ago I used a spray to test certain areas for leaks. I think it was carb cleaner and it would make the motor race when I hit a leaking area. All your wires and vacuum hoses in good shape? Invest in a Mighty-Vac with pressure/vacuum to check individual lines. This is the most handy tool I spent $80.00 on so far. Great for these checks as well as brakes, hobbs and pressure switches. What kind of vacuum are you pulling? Have you tested your upper TB vacuum block? Sometimes they crack!

 

[Low Budget]

Originally posted by Intercooler
Have you checked everything from the turbo outlet back to the throttle body for possible leaks? The IAC numbers changing with the throttle opening/closing is normal. At an idle though it should settle out with your RPM's at a fixed number +/- 15 or so counts. May want to take the time to pull all your IC hoses and go over them with a fine tooth comb. I know long ago I used a spray to test certain areas for leaks. I think it was carb cleaner and it would make the motor race when I hit a leaking area. All your wires and vacuum hoses in good shape? Invest in a Mighty-Vac with pressure/vacuum to check individual lines. This is the most handy tool I spent $80.00 on so far. Great for these checks as well as brakes, hobbs and pressure switches. What kind of vacuum are you pulling? Have you tested your upper TB vacuum block? Sometimes they crack!

I'll spray around again, but I'm 99.9% sure I have no leaks. My vacuum block is a brand new billet aluminum part. Most hoses are new and others look and feel good and tight. Having just put this engine in, I went over all this then.

Someone please tell me though, how vacuum leaks could affect the IAC, in so much as it doesn't even try to keep up. Vacuum leaks are (in my experiance) consistant not intermittant. If I had one or more, the IAC should be working it's butt off trying to keep up, not sitting on it's butt. Right or wrong??

 

[Intercooler]

So are your numbers moving? That wire I found pulled out of my Translator gave me these kind of problems. Car would just stall or run too rough to idle. Did it just start when you put the motor in? I would almost have to see it to really understand. Are you coming to the gathering in White Marsh? I will be there and so will a bunch of other Buick guys. Maybe if we all had a peek we could find something. I have Direct Scan, SM2 and Mighty-Vac. Bunch of spares too so let me know what to bring if you decide to come. We will just swap it all! LOL

 

[Low Budget]

What and where is the gathering at White marsh?

Jesse,

Maybe. When and where?



This has been (if I am remembering right) a problem for years. When at idle, the only time the numbers change is when you turn the A/C on or step on the gas. Otherwise, the only time they change is on startup or a complete stall.

There are four wires going to the IAC. Does anyone know EXACTLY what each of their functions is? How much power should each have and when? I tried to ohm them all out and couldn't get it done. All I suceeded in doing was checking for voltage instead. (Only one had any with the key on and engine off.) I didn't bother beyond that, because I don't know what I'm supposed to find anyway.

Thanks, Jeff

 

[BJM]

I believe the IAC is a 2 coil stepper motor, the ECM will power one coil and then the other coil, then the first coil inverted, then the second coil inverted, and repeat to make it step continously. At a fixed step position only one wire will have voltage and another will be its ground, the other two would be dead. You should be able to measure a finite resistance off each coil. The manual shows it as two coils. This is why the IAC goes through a reset sequence, it needs to ram into an endstop somewhere to determine where it is and count out from there.

 

[Intercooler]

A nice person from the board sent me a detailed drawing of the wiring per GM. Shoot me an e-mail and I will send it to you.

The gathering is this Sunday at Chili's I think in White Marsh. Check the Lounge for the info. This is MAGNA.

 

[mgmshar]

Just to clarify some things. The IAC number that you see on Directscan or Scanmaster is where the ECM THINKS the IAC is positioned. It is not a read-out of where the IAC actually is positioned. If the IAC was stuck for some reason, then the ECM would still command the IAC to move, but the IAC would not respond. The number shown in DirectScan would still go up, because that number refers to what the ECM is telling the IAC to do, not what the IAC is actually doing.

If the engine is trying to stall for any reason (loose hose, defective IAC, any of the above reasons), then the ECM should command the IAC to open (i.e. increase the IAC steps). You will see the ECM trying to do that on your DirectScan screen - the IAC number will go up. Of course, if the IAC is defective, it won't actually move, and the car will stall.

That's not the problem here. The problem (if I understand it correctly) is that the car tries to stall (for whatever reason), and the ECM is NOT changing the IAC steps to counter-act it. In other words, the ECM is not trying to open the IAC. Either the ECM is not detecting the rpm drop, or something is wrong in the ECM. If the wiring between the ECM and the IAC was bad, then you would still see the ECM trying to raise the IAC steps, but there would be no response from the IAC. That's not what is being seen here.

Make Sense?

In my case, one of the contacts in my ECM EPROM socket was bent. So, when I shut the car off, the ECM would not run the IAC reset procedure (drops the IAC to zero to seat it, and then opens it to 150 steps). So, when I turned the car back on for a re-start, the ECM would show 150 IAC steps (where the ECM thought the IAC was), but in reality, the IAC was only open about 10 steps. Made the car VERY hard to start, and when it did start, it was hard to keep running for the first several seconds. Eventually, the ECM would go to about 190 IAC steps (where it thought the IAC was), but in reality the IAC was only open about 50 steps.

If your IAC actual position and ECM position are getting this far out of sync, this could also cause problems. Watch your IAC counts on DirectScan when you shut your car off. Make sure that they quickly go to zero and then quickly back to 150 when you turn the key off. It also wouldn't hurt to do the IAC reset procedure (putting the car in diagnostic mode, etc.) just in case.

Good Luck,

 

[Low Budget]

Originally posted by mgmshar
Just to clarify some things. The IAC number that you see on Directscan or Scanmaster is where the ECM THINKS the IAC is positioned.

Yep, I get that.

It is not a read-out of where the IAC actually is positioned.

Got that too.

If the IAC was stuck for some reason, then the ECM would still command the IAC to move, but the IAC would not respond. The number shown in DirectScan would still go up, because that number refers to what the ECM is telling the IAC to do, not what the IAC is actually doing.

10-4

If the engine is trying to stall for any reason (loose hose, defective IAC, any of the above reasons), then the ECM should command the IAC to open (i.e. increase the IAC steps). You will see the ECM trying to do that on your DirectScan screen - the IAC number will go up. Of course, if the IAC is defective, it won't actually move, and the car will stall.

Unfortunately mine is not functioning this way.

That's not the problem here. The problem (if I understand it correctly) is that the car tries to stall (for whatever reason), and the ECM is NOT changing the IAC steps to counter-act it. In other words, the ECM is not trying to open the IAC. Either the ECM is not detecting the rpm drop, or something is wrong in the ECM. If the wiring between the ECM and the IAC was bad, then you would still see the ECM trying to raise the IAC steps, but there would be no response from the IAC. That's not what is being seen here.

Make Sense?

You are exactly right in your description of my problem.

In my case, one of the contacts in my ECM EPROM socket was bent. So, when I shut the car off, the ECM would not run the IAC reset procedure (drops the IAC to zero to seat it, and then opens it to 150 steps). So, when I turned the car back on for a re-start, the ECM would show 150 IAC steps (where the ECM thought the IAC was), but in reality, the IAC was only open about 10 steps. Made the car VERY hard to start, and when it did start, it was hard to keep running for the first several seconds. Eventually, the ECM would go to about 190 IAC steps (where it thought the IAC was), but in reality the IAC was only open about 50 steps.

If your IAC actual position and ECM position are getting this far out of sync, this could also cause problems. Watch your IAC counts on DirectScan when you shut your car off. Make sure that they quickly go to zero and then quickly back to 150 when you turn the key off. It also wouldn't hurt to do the IAC reset procedure (putting the car in diagnostic mode, etc.) just in case.

Mine is going to 0 and then to 150 as described. I have done the reset 3 times already. Twice with the old IAC and once with the new one.

Good Luck

It sure sounds like my ECM is bad huh? Thanks for the info and detailed explanations. Jeff

 

[Low Budget]

Update! Still need help!

I have replaced the ECM twice since the last post, the IAC does work when told to, and the chip has been replaced more times than I can remember. No vacuum leaks, new Translator and 3 1/2" MAF installed. A/C commands are made and immediately followed by the IAC. The latest and most annoying problem is the opposite of my original symptoms though. Now, when the rpm's are too high, the IAC is not commanded to lower it's counts! It just pulls you along like you're on the gas. FYI - the TPS is set to .40, so that's not the problem. I have a friend who has a similar problem, but only for a short time after start-up. When he first starts out, his will stay at too high of a commanded IAC count even though the rpm's may be too high for the throttle position. If he sits in gear stopped for a minute, it drops to where it should be. Mine does this too, but where this is the end of his problem, mine continues. Each time I pull out, the counts go up with throttle application as they should. Mine however, do not fall to a low enough level when lifting off the gas pedal. The actual number it stops falling at is irrelevant, and changes from drive to drive. The issue is, that it doesn't fall enough for the actual engine speed. If I sit stopped in gear for a long red light, like my friends car, the counts and rpm's will drop to reasonable levels. As soon as I pull out, it starts all over again! Another FYI - my throttle is not physically sticking.

So...any computer guru's want to take a stab at this one? I am convinced it is certainly electrical, probably ECM related, but... remember, I have had no less than 4 different ECM's and 5 different chips with no less than 10 different programs in this car, all with "some" kind of IAC problem. Whether (like a year ago) it stalls without any higher commanded IAC count, or (now) the engine races without the counts dropping, I have not had perfection once in 6 years!

To appease anyone who would say reset the IAC...I will do that tomorrow...but keep in mind, that has been done to death.

Thanks to anyone brave enough to take on this challenge!
I appreciate your help!

Jeff

 

[TurboJim]

Do ytou have a known good TPS to try? I had this problem with my car when I developed an open spot in the pot. I think what happens is the voltage drops off too abruptly, or too much, the ecm goes into like a limp home mode, but my car never coded.

I could go to move and the car would hiccup like a bad TPS, then the idle would stick high...1200-1500rpm. If you sat at a light it would come down. And stay down, until you hit the bad spot in the tps, and this would happen all over again.

If you could sweep it on a scope you'd see the dropout if it had one. Set to 1v/div and 1 sec/div. Slowly open the throttle while tapping into the signal wire, you'll see the dropout. I dont know if a DVOM will capture it, and you probably would see this on an analog meter, but make sure it has a 10megohm impedance so you dont fry the ECM.

Something to try anyway.

 

[bruce]

The reason I go with a 0 IAC count is that it ensures a base min idle speed even if the ECM/IAC pukes for some reason.

For an engine to stall with a normal IAC of 0 at idle would indicate a fuel problem, ie way to rich or lean. Or possibly an ignition problem, where the module is reporting too high of rpm.

Do you have, or can get access to a Direct Scan?.

 

[Low Budget]

Yes, I have Direct Scan.

I'll take a ride today and get some of this recorded. I just wish it would record longer periods of time. Say, 10-15 minutes would be good. Hint, hint Mr. Chu.

I would prefer to allow the ECM some room for adjustment, rather than the zero count method. I only wish it (the ecm) would take me up on my generous offer!

Hey Jim! This's the problem I've been telling you about for a year. I'll try a new TPS and see what happens. I don't have a suitable meter, so just buying a new TPS is my only real alternative. Thanks for the tip, I'll let you know how it works out.

Thanks for the help!
Jeff

 

[Low Budget]

I am now cautiously optimistic!

I bought and installed a new TPS today. That by itself didn't get it done, but when I reset the IAC, all became good. Since I have reset the IAC many times in the same way with no good results, I must conclude my TPS sensor was the problem. Or at least the last one in a long line of them.

Thanks Jim.

Thanks to everyone who has put in their 2 cents on this subject.

It's nice to be able to stop without my ass lifting off the seat to get it done!



**Special thanks to Mark Clute for all his time and expertise given to me recently. A true friend in the Buick community, with whom I am fortunate to have crossed paths.

Thanks, Jeff