IAC/High Idle Issues

[digitalsolo]

Hey all,

Hoping someone can give me some direction here; bit of a confusing problem.

Car has a 1998 F-Body PCM, on a 347" LS1 w/ a turbo; 2 BAR OS.

When I hook up my IAC, the car will pull the idle up to ~3000 RPM and hang there; I've replaced the IAC and that made no change. I've toned out the leads from the IAC plug to the PCM, all good.

If I manually adjust the IAC, and just leave it unplugged, the car idles and drives just fine. When I log with the RussK config, it shows my IAC counts dropping to 0; there is no change in the idle RPM, though I can occasionally jump into the VCM controls, force it to 0 there and get a noticable RPM drop (bottoms out around 1250 RPM).

I have reset the TPS using the unplug, power on, power off, plug in, method. Readings and voltages are good.

This happened to me last year, and I just unplugged the damn IAC and tuned idle with the throttle blade (using the set screw, blade is undrilled). I rebuilt the car over the winter, put it in, and it was fine the first several hundred miles. All of the sudden, 3500 RPM idle, no tuning changes.

I'm quite confident there are no vacuum leaks; I've checked with propane, verified with my gauges, and the idle behavior when I bypass the IAC is just as expected (and if I plug the IAC hole with a rubber stopper, the car will, obviously, not run).

From everything I can determine, it seems like the PCM is opening the IAC up, then sending commands to close it, but it's not closing. Again, this happens with 3 IACs.

WTF.

[digitalsolo]

Nobody at all, eh?

I'm going to be changing the car to E85 and am planning on starting with my factory tune to redo it all (I was "learning" during a good portion of my previous tuning, and I'd like to start fresh on some of the things I'm sure I've done wrong).

I'm assuming that I should be making sure my VE table is solid in the idle area? Is there anything else that could be causing this from a software control perspective?

[12secSS]

If you log idle spark advance I can bet it is negative (pulling timing) when you unplug the IAC. This means either the throttle body blade has a hole larger then stock, or you have a vacuum leak some where. When you plug in the IAC, the reason the rpm jumps is because the ECM thinks the hole is OEM size and tries to up the idle. That is until the STIT tell it its too much, so it closes the IAC to the point it can't lower the idle anymore with a closed IAC. That where the Idle Spark Dvance pulls timing to bring idle to match desired idle rpm. Another thing, it could be the blade is being kept slightly open ... either the cable or the throttle stop. Log TPS Voltage, it should be around .47 Volts.

[Montecarlodrag]

Have you tried adjusting the TB screw to close it a little?
When the IAC counts go to 0 and the idle speed is still high, it means the IAC motor has completely closed and there is still too much air entering the engine (causing the high idle speed).

With engine at operating temp, adjust the TB screw to close it until it wants to die, then backup a little. Make sure it doesn't get stuck in the closed position by opening it by hand.
Then do a TPS reset and log TPS volts.
You will want IAC counts somewhere in the middle to let the IAC motor have room to function properly.

You may want to log TPS% when the idle is high. I have seen 98's with faulty TPS sensor go to 2500 RPM at idle with no reason. The TPS went from 0 to 2 or 4% without touching anything.

[nmcqueen469]

Blake, did you ever get this problem resolved?

[digitalsolo]

Blake, did you ever get this problem resolved?

Nick,

Yes, I set the IAC motor where it idles nicely at 900 RPM unplugged the fucker and handled idle speed with spark tuning.

I honestly have no idea why it won't behave. I've swapped the IAC with a known good one, put in a tune that works fine on an identical car, toned out and check resistance on all the wires, sacrificed a chicken, etc. My TPS is at 0 at idle, moves properly with throttle movement, the hole in the throttle blade is unmodified, and I have no vacuum leaks.

The IAC motor COULD control it, since I can go half turns on the plunger and change my idle RPM just as I would expect it to. Apparently I have something buried in the tune that just won't sort it out.

I'm still trying to track that something down, but considering it runs fine with it disconnected, my annoyance level with it hasn't exceeded my care level enough recently to bother with it.

[schitzo]

bump.
I have had this same exact issue for a year now. Cant seem to find a fix for it. Any ideas?

Checked for vacuum leaks
Cleaned idle control valve
Checked for the leaks some more
Reset the TPS......no luck

[schitzo]

At the suggestion of another member, I just replaced my TPS then did the reset and that cured the problem. Idles normal now. You should try it.

[frankmarroquin]

I'm having this same issue, i have a new tps and iac. Car is a bone stock 99 trans am.

[Powerhouse]

I'm having the same issue on my ls1 that I just put a victor jnr and 4 barrell throttle body, also has a 233/233 106lsa cam. Sometimes idles perfect and drops back to idle when slowing down then other times idles right upto 3000rpm and sits there and has cruise control when driving. I've played around with the airflow tables but it still does it. Has brand new tps and iac.

[chevelless502]

Kinda of bringing this back from the dead but this is exactly what im seeing with my 98 Z28 A4 car. Ive been fighting this for a long time. Changed out the intake gaskets, replaced all vacuum lines, Tried 3 different complete throttle bodies on the car that i had, Tried a brand new TPS. Im at a total loss. Car is relatively stock with just a cat back and an SLP lid.

[mowton]

Tuning DBC setups while a bit more complicated is not impossible. You need to follow the procedure closely which means google IAC tuning, add HP Tuners to your keywords and you will many different threads on this process. You need to setup the scanner with the correct PIDs which will include IAC Pos, IAC Desired pos, Idle desired Air, Dynamic air, LTIT (both IG and P/N if auto), STIT, TPS (%), TPS (V), Advance and LT/STFT, RPM, MAP, ECT as a minimum. Setup a Histogram that matches your BRAF table.

Simply put you will want to set the setscrew (good idea is to remove from the bottom and re-install from the top to make on-car adjustments easier) so your IAC Pos is like 40-80 depending on cam size (40-60 mild/big cam and 60-80 stock cam) and TPS volts are < .9/.8 volts. Once set, unplug IAC/TPS connector at TB and put keyon (dont dstart) for 30 sec's. Te attach IAC/TPS connector and validate IAC Pos. May require 2 or 3 iterations to get right. Also if cam is too big, you may have to enlarge the throttle blade hole to get the TPS %/V with in the criteria. Then setup scanner and from a cold start, idle the car from cold to about 200F ECT. Turn off the AC and Fans (via VCM controls). When complete copy and paste the histogram results and fill in the upper and lower data not logged by trending. The goal is to get your LTIT's to -.05 g/sec or so. It is easier also if you disable the LTIT and just use the STIT and you will find how to do that in your search....

Hope this helps

Ed M