Long post, so be prepared. Okay, so heres the deal...

First the problem...
My LT1 Z has a very annoying little problem. On a cold start the idle is a bit on the low side, but fine. Once it gets to closed loop though, it slowly climbs to 1600 rpms in park...not very nice for the neighbors with the loud 3" TDs! Sounds simple enough? Its got both me and my speedshop stumped. The kicker is that the idle is only jumping up once it hits closed loop. Open loop is fine, except for a bit of cam surge. Since its only in closed loop, I suspected the o2s right away. Bought and installed the 4-wire heated o2 conversion kit, and they respond much faster then the single wire 93 o2s. Smoothed out a bit of the higher rpms too, but problem still there...but now it goes into closed loop much faster(aka idle jumps up faster!)

Some useful information about the car...
The car is a MAFless Speeddensity LT1.
TPS is brand new, adjusted to .55v at idle.
IAC is new, working properly.
O2s are brand new 4-wire heated.
Unhooking brake booster vacuum line raises idle like it should(not vacuum leak).
TB is fairly recent 58mm, with IAC bleed hole drilled same size as stocker(11/64?) for help on cold starts. Was on the car previously when everything was fine.

What its NOT(AFAIK)...A lot should theoretically have nothing to do with it, but just makin sure everything is covered.

- Not o2s. Brand new 4-wire heated.
- TPS is brand new, verified with DMM, ~.55v idle.
- TB is fairly new, same one that was previously working fine(doesnt neccesarily rule that out though...)
- IAC is fairly new. Counts start at 160(fully open) at cold open loop, and once it warms up and goes into closed loop the idle goes up and IAC counts drop steadily to 0(fully closed) in an attempt to bring rpms down but the idle stays high.
- No vacuum leaks. Removing brake booster line makes the idle jump up quite a bit like it should. Also, all emissions/evap/pcv crap has been deleted and removed and/or plugged off. No real spots for vacuum leaks!
- Not the tune - my dyno tuner and speed shop were pretty stumped to. Car runs great, a little bit on the rich side when cruising but nothing bad at all.
- Not the Opti. New AC-Delco OEM unit with MSD cap and rotor. No missing, everything smooth.
- Not the wires. Fairly new Taylor Thundervolt wires, routed carefully with no arcing or visible damage.
- New Blaster coil.
- Plugs look good, taken out and cleaned.
- All cylinders firing properly - not a fuel or ignition issue!



As you can see, I've just about hit all the bases I can think of here. Typing it out and thinking about it makes me lean towards the Throttle body, even though it was working fine before. I just dont want to drop over $300 on a new TB and the problem still be there! I have searched countless forums and the internet and have seen two or three threads explaining my exact problem to the T, but none ever finish with any solutions!

LT1s are pissin me off! I've never been stumped like this...

 

I don't know if this sounds stupid or not, but you've came to a dead end. Her's a shot in the dark..Do you have a tune? Or recently had one? Could be a ECM error.

List of mods?

 

It went from self tuned, madz28 tuned, to an attempt to dyno tune it. Due to opti problems it was cut short, but I am pretty sure it is not tune related. The mod list is long...

I am leaning towards the TB a little more now, maybe I was just blindly hoping that wasnt it...it works fine for open loop though which drives me crazy

 

Internal vacuum leak, from bottom of intake manifold?

 

do you have a check engine light on

does the car otherwise run ok, ya know lack of power, miss, anything like that

 

What are the IAC counts as it raises the idle and once it hits its high idle?

 

I tested with carb cleaner and their doesnt seem to be any vacuum leaks. When the brake booster hose is taken off the idle goes way higher like it should.

The engine light comes and goes, but I'm pretty sure its form my oil pressure gauge acting up - tunerpro doesnt show any DTC codes currently set, just the history. I'll clear the computer and see if any new ones pop up.

Car runs great besides the idle problem!

IAC counts slowly go from full open(160) down to fully closed(0) as the idle raises. Its trying to bring it down, but cannot. One wierd thing is that if I adjust the IAC counts to 30-40 like they should be on a warm idle, then the engine will not stay on at all during open loop cold starts.

Thanks for the help guys! I PM'd someone who was describing the same exact problem I am having from a year old thread I found searching the internet. Hopefully he gets and email or checks his PM's and can tell me how/if he fixed it. If not, I'll just bite the bullet and try a new throttle body.

 

I'm guessing this is a 93, even though you never stated it was. Assumed that from the fact you needed to do the 4-wire O2 sensor conversion and don't have an MAF.

Did you check the IAC for electrical continuity? Shoebox has a guide, but I'm not sure if it applies to the 93's, because they have a slightly different IAC motor. But what the ECM is telling you about the "counts" is what it is asking for, but there is no feedback to insure the IAC is actually responding.

http://shbox.com/1/iac2.jpg

The fact that it idles too low on cold start, even though the ECM is commanding 160 counts, and too high when warmed up, even though the ECM is commanding 0 counts may indicate the IAC motor is not actually moving, or the IAC passage is plugged up.

 

Wow, I totally wasnt aware of that! I figured the counts were the actual IAC position, never thought that it could just be the commanded position by the computer and not neccessarily where it actually sits. Its a fairly new IAC but its definetly possible that it went bad or got stuck somehow. I'll check that out, thanks

 

DAMMIT! The IAC tested out fine(59 ohms A-B, and same C-D), and took it out to clean and it wasnt bad at all. Just a little bit of carbon "dust" on it. Replaced and same problem still happening.

Thanks for the suggestion though! I wasnt aware that those were commanded counts and not actual, it very well could have been the IAC and I would have never known.

 

What software are you using to get this information out of your car? I've been having trouble like this with mine for almost 2 years now... Resulting in me never driving it. I need to get something to read codes and reprogram the car i cant drive to autozone every time the check engine light comes on anymore..

 

hey puck did you ever figure this out, i would just like to know what it ended up being, and i dont know about on our cars but i know on fords i have seen a few iac's that test fine but end up being the culprit anyway

 

Yea, but I still didnt narrow it down to one piece unfortunatly - it was either the IAC, TB, or or the connector leading to it.

I eventually got a new TB and had to "upgrade" to the 94+ style IAC w/ the two machine screw mounting, and was sent the connector to splice it into my harness so that was all done at the same time and solved the problem. I guess either the TB bearings/spring was letting the blades open too much, the connector was not letting the IAC work all the time, or the IAC itself was bad even though it ohm'd out correctly.

Looking back on everything, I still have the IAC lying around but it is the 93 and older screw in style so I cannot test it. The connector leading to it looked every day of its 14 years of age, but I threw it out before doing continuity tests on each pin to see if it had any bad connections. TB was fine and fairly new, so I have no reason to think it was that even though it is technically possible. One of those three things was causing it though! I have a feeling it might have just been the stupid connector was rusted on the inside, or there was a break in the wire but not the insulation or something not easily visible.

Either way, it idles fine now though

 

He has an OBD-I setup and a replaceable chip, so what he's using isn't going to work for you. For scanning, you would have to get OBD-II software like AutoTap. For programming you would use LT1_Edit for OBD-II or the OBD-II version of TunerCat, which is now sold by JET under a different name. If all you are after is codes, you can get a $50 OBD-II code reader from Harbor Freight.

 

What im really after is some data logging so I can get on here and tell someone more educated what is going on with my car.. I was kindof hoping i wouldnt have to spend a ton of money on software. I also have a broken laptop ... What i was really hoping to find is some sort of datalogging code puller like actron or something but more advanced.. It get live data but not what I want.