AS84

I remember the tech man at fastchip said my IAC counts should be around 30....well when I first got my chip a few months ago I hooked it up to a snap on scanner and no IAC counts at all. So, I took it apart and cleaned it. I guess that didn't do it becuase it's still not working. The car has a small deadspot in it when I drive it (around 1500 rpms) and whenever I turn on the ac/heater the idle lowers and the car idles way too low. Sound like a bad IAC to you? Should I try cleaning it again, or just get a new one? thanks

PeteL

My forth reply to this post. :mad

Do you get any codes? Does the ECM provide code 12 when you check codes?
What does the scanner tell you about desired RPM vs. actual RPM.

If you get a genune 0 counts (never seen this) then ECM (if it is working) is trying to reduce idle to no effect.

Have you done a minimum idle air adustment exactly as described in the tech tips section on this forum? If not, you need to do so.

You could have a bad IAC, you could have a leak (like TB to Plenum). What else can you tell us.

I think I thought of other things on the other four answers I posted to this, but forget now, sorry.

Good Luck,

Pete

HighHopes85

I just got done tuning my car today and this should really really help you out. Today I set out to fix my idle and I got a very mooth one that I can finally be proud of!!!!!!! :D

I warmed my car up until it was nice and hot. The IAC counts have been at zero previously and they still were. As previously mentioned, the IAC is used by the computer to adjust the idle and just off idle speed of the engine. If the IAC is fully closed, the engine is likely trying to lower the idle speed more than it can. The chip has an idle speed preset in it and it tries to do everything possible to meet this. Worst case scenario, which I had, is that the engine would eventually stall out and give me a code 33 (MAP voltage).

My solution was to screw the throttle plate screw CCW to lower the amount of air that was bypassing the throttle plates. Once I did this enough, the data logger started to show movement of the IAC valve. My ilde became more pleasing to the ear, and also, my actual engine speed was finally equal to the desired idle speed programmed in the chip.

After I adjusted the throttle body blade screw, I readjusted my TPS voltage because it was about .1 volts too low from turning the screw. I got it back to .57 volts (a value that seems to work best for me) and had to tuen the car off because the scanner was now showing 3% throttle even though it was really at 0%. I started her up again and she now showed 0% throttle at idle and purred like a kitten even with a slightly lumpy can and 1.6 RR's.

I think your problem is similar--that your throttle plates are open too far and the IAC is trying to close off all air into your throttle body. Remember that the IAC valve acts as a controlled vacuum leak, so somewhere around 30 counts sounds right. Do ALL of this tuning with the Heater/AC controller in the off position because the A/C compressor command will add about 100 RPM (which will come down to about no addition because of the load). The defrost position will run your A/C compressor, so just make sure it is off.

Then, when all testing is done, turn the A/C on and verify you have a good idle. The IAC counts will generally be higher with the A/C on or the defrost position selected, and this is normal. :cheers: -Matt-

EDIT-The IAC parks at around 60-100 counts. If you have the scanner hooked up as SOON as the key is turned on, the IAC counts should go from the high number to zero (in your case). If there is no movement, the IAC could be wacked.


[Modified by HighHopes85, 7:23 PM 12/15/2001]

PeteL

HIghhopes85, if you adjust the TB minimum idle screw without following the proceedure, you can only LUCK into a proper IAC adjustment.

Here is the link:
http://www.corvetteforum.cc/techtips...D=55&TopicID=2

This is fairly accurate for most L-98 engines, some minor changes by year which is why you really need to have the factory manual to work on a car. http://www.helminc.com

HighHopes85

No offense meant, but I know what I was doing. I burned my chip for 750 RPM once the engine was hot and my idle was still at 825. This led my IAC to be full closed. The 'correct' reset procedures ONLY work if you have a stock setup, as they require you to set Min Air Rate to 400-450 RPM. I do not want to stall at every stoplight so I needed a higher RPM. If I set my Min Air Rate to 450 RPM, my IAC would be at nearly full open and would introduce a low vacuum reading on a gage.

The reset procedure which would be useful for my car would be to set Min Air Rate to about 700 RPM. I could fididdle with it all day long to get the IAC counts down to a reasonable number and up from 0. By using a scanner at the same time, I was able to kill 2 birds with one stone. Like I said, I had to turn the car off, because on a 90-91 speed density setup, the ECM looks at the TPS voltage during open loop. It then takes the lowest value in open loop and uses that for the percentage of 0. Once in closed loop, if the TPS voltage goes lower than what it experienced in open loop, it will not recognize the lower voltage as 0 anymore. That is one of the reasons you aren't supposed to keep your foot on the accelerator pedal while cranking a fuel injected engine.

I did jumper the AB conector after the tuning session and let the IAC fully extend and then disconnected it. The RPM was at 650-675 on the scanner (more accurate than the dash tach) and thus, I can use that as a reference point next time.

To give this information, I noticed AS84 listed an aftermarket chip and a Superram with quite lumpy cam. A stock idle speed will not be ideal for his setup and Ed Wright would have raised the idle speed in the chip accordingly.

I have that reset procedure memorized by heart, but with a non-factory idle, you cannot use it to a T. As far as the Helms Manual goes, I am disappointed with mine. I have found no less than 17 wiring diagram errors, but it is good for a general reference. :cheers: -Matt-

AS84

wow :eek: you guys know your stuff! I read the tech files and between that and highhopes85's reply I imagine she'll be in prefect tune tomorrow. Thanks a bunch guys

PeteL

Great answer, and I don't know the tables in the prom. But I still question a programming setup that would require that you "trick" the ecm into not setting the IAC correctly. I can't argue, as I haven't done what you have done, but I would suggest that either the prom table setup is screwey or there is a paramater that you have to overcome if you have to change the minimum idle adjustment - after having increased the idle parameter that the ecm is seeking.

Think your problem is that you have a lumpy cam that requires a higher minimum idle to even begin to adjust - it just won't idle steady at 450 RPM, so you have a problem. I don't know how you would adjust for this properly, but I'm sure what you have worked out is a decent "patch".

Just so I learn, what happens if you set the minimum idle air to a point that allows the slowest idle you engine will tollerate? Will the ECM/Program/IAC then set the idle you have programmed? Bet it will if you have the tables set up right, but then again I haven't done it so I bow deeply to those that have.


No offence taken at all, and I really learned from your post. :D

Plain old stock - Pete

AS84

If i back the idle screw down further it will die when I kick the defroster on.

HighHopes85

This is a really good topic and I posted a thread on a different site about it before, but no one over there seemed to respond to it. First, here is where I am coming from. I had to do an engine swap in the Vette shortly after purchase due to a TB airfoil screw splitting a piston and the block. I was crunched for time so I swapped out an engine I had just built for my Chevelle-definatly not an engine-friendly cam if no parameters had been changed. I tried and tried for months to fix all my problems and here is what I came up with regarding the IAC and minimum air. Some of it is common knowledge so don't think I am trying to talk down to any of you--I am NOT!!! I just spent more time on my idle, code 42, code 43, and pick-up coils than anyone should ever have to:yesnod:

The throttle plates can not be treated like those of a carb. The idea of the idle circuit of a TPI system is threefold. First you have the throttle plates, then the IAC valve, and lastly the ECM. To tune first, you usually try to decrease the events happening at the same time. This is why you jumper the ALDL connector--to extend and then unplug the IAC valve. By doing this, you effectively remove it from the circuit.

With the valve disconnected, the only air entering the engine will be around the throttle plates. Since there is not an idle-kicker solenoid like the carbs, there has to be some way to raise the idle when the AC is commanded on/compensate for differernt temperatures, ect. This is accomplished with the IAC valve. So with the IAC fully extended (meaning the passageway is closed), the only air going to the engine is from around the blades. If the blades are closed all the way, then it is obvious that all air necessary to keep an engine at 600RPM will have to go through the IAC passageway once it is reconnected. This is a tough thing to ask for. So what happens is the throttle plate is adjusted so the minumum feasible idle RPM that the engine will ever experience, and then down about 50-100RPM below that. Lowering it this extra RPM will keep the valve active and limit the possibility that 0 counts will be reached by the IAC.

If the IAC reaches 0 counts in open loop, the computer will be injecting too much fuel because it does not look at the O2 voltage during open loop. It will then go to closed loop and be able to take out fuel to lower the idle, but it will still be trying to lower the idle. At 0 counts, too much air is still entering the engine and the ECM can not do a thing to compensate for it.

Once the IAC is reset correctly to procedure and the car is once again started, the IAC will retract (when the key is turned to start) and extend to a position that can reregulate the idle speed. The idle speed that the computer tries to obtain is located in a table in the PROM. Normal stock values for a 90 speed density setup with the AUJP PROM is listed below.

<BLOCKQUOTE>code:<HR><PRE>
Deg
C RPM
-40 1050
1050
1050
1050
950
950
32 900
800
700
650
625
92 588
588
600
600
600
152 600
</PRE>[/QUOTE]

I left some temps out so I didn't have to copy them, but it is pretty much a linear interpolation on the Deg Celcius column. The idle is going to try to be regulated based on that table (spark table tweaks it too). Now with a stock setup, the IAC will be able to extend and retract to meet all those cases. But if you have to bump the idle up because of a bigger cam, then most of those values will get scaled up by 100-200 RPM. To keep the IAC in a functioning range now, you have to raise the Min Air slightly to keep approx. the same IAC counts.

It IS tricking the computer if the Min Air is opened up more to let more air in IF the speed isn't raised in the chip. Been there and done that. In this case, using the same flawed theory that I used for a carb, I thought busting the blades open more would result in a higher idle speed, but the IAC would close because of the RPM logged into the chip. I chased my tail on this one and don't want anyone to have to do it. It sucks!!

Unfortunatly I have no clue if Ed Wright raises the idle on his base chips. I guess he sells a base chip and a custom chip. I would really hope that he would let the customer know, but I don't think he does. If the IAC retracts too much (over 70 counts usually), then a vacuum gage will show a noticeable drop in vacuum (on mine it was close to 5-6 inHg drop) and this isn't good especially for MAP cars. MAF cars have a little more leeway because the correct amount of air entering the engine will still be measured even if the vacuum isn't all that great. That part may be worded incorrectly, but it gives a basic idea.

Now, for finding out why the car dies with the defroster on. We gotta find out a ballpark figure of what idle the chip has been programmed for. A scanner will show Desired Idle RPM and Actual Idle RPM. Do you have access to a scanner or do you have to run to the shop for one? Also, do you have a vacuum gage? Most tuning at idle can be done with the vacuum gage.

THe first thing I would do is to jumper the ALDL and go place my finger on the IAC connector. You should be able to feel a little pulsing if it is extending. THis should be for about 5-15 sensods and then the pulsing will likely get softer. If you feel no pulsing, it would likely be a problem in the connector, wiring, or the IAC valve.

I guess I gave a lot more info than anyone wanted, but I was bored and am just hanging around cause I have nothing to do. Have to make a 5 hour trip tomorrow and thought I would give you as much info as I could so you can get your problem fixed before I get home :D

-Matt-




[Modified by HighHopes85, 10:40 PM 12/15/2001]

AS84

Matt-
I do have access to a scanner. I'll take some notes of the desired idle and actual idle. If I remember right my desired idle was much different than my actual idle.

EricVonHa

Guys, very interesting thread. I see that you guys primarily have L-98 Vettes. The LT-1 also uses an IAC... what do you feel the similarities are with proper tuning? Is it identical?

Thanks!

HighHopes85

I have never messed with the LT1, so any information I might be able to give on it could be MISinformation :eek: I better just stick with the L98's :yesnod: -M-