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95 LT1 Z28 automatic convertible. Bone stock, no tune.

Did a head gasket job on this car. When I got it all back together, it idled really high. I replaced the TPS and IAC with new units from Rock Auto. Turned out I forgot to tighten the throttle body bolts all the way, so it was sucking in air. This problem started immediately after tightening those up.

Car started out only wanting to idle at 500 rpm. Considering how long it sat, I figured the computer needed to relearn everything. So I drove it around a bit. Every time I'd lift off the throttle completely, it would drop to below 500 rpms and almost die. A few times, it actually did die. I could hit the key and it'd start right back up. Car runs good otherwise.

My vents for the climate controls won't blow out the front no matter the setting, which I've discovered can be a vacuum problem. Is it possible that a vacuum leak is causing both of these issues? Like when I find the leak for the climate controls, could that fix my idle issue? If not, what other problems cause the stalling when throttle lifted?
 

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Yes, I think possible. Rockers all tightened down correctly? First time I did heads, I messed up the the valves by tightening a few rockers a bit too far.


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Discussion Starter #3 (Edited)
Update: I just confirmed these are separate problems. There could still be a vacuum leak so there isn't enough vacuum going to the climate controls, so I'll have to keep looking.

I pulled the line going into the cabin from the 3-way connection under the hood and it had vacuum on it. I put a piece of black tape over the end ( the vacuum helps hold it ) and it still dies or stumbles when lifting the throttle. I also found out I can replicate the issue. In neutral or park, I can rev it all day and it drops to an 800 rpm idle. If I put it in gear, the idle drops to like 600. Then if I rev the engine, it'll drop down and either stutter, or die.

So it only does this under a load. Any ideas why it doesn't compensate for the load and raise the idle when in drive?
 

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Discussion Starter #4
Yes, I think possible. Rockers all tightened down correctly? First time I did heads, I messed up the the valves by tightening a few rockers a bit too far.


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I did them by the book. Turned the motor until the lifter was all the way down, tightened the nut just until the pushrod would no longer spin freely, then did a half turn. Repeated that process 16 times. SHOULD be good.
 

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You need to put car on an obd 1 scanner that does real time sensor data. See what your iac counts are at idle. When you rev engine, iac counts should go down. When you let off the pedal, iac counts should go up to keep engine from stalling. Even if sensor data is correct, the new iac could be defective. Common problem on offshore parts.

Have you pulled the throttle body to check the gasket? Because bolts were loose an air was coming into tb via the gasket surface, gasket could have been compromised.

Also a common place for a vacuum leak is where the intake elbow attaches to tb.
 

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Discussion Starter #6 (Edited)
OBD1 scanner isn't an option. I tried to find one a few years ago with no luck. At least not at a reasonable price.

I found this thread: https://www.ls1lt1.com/forum/lt1-|-lt4-|-l99-engine-tech/96401-iac-trouble-4.html where the guy had messed with the idle screw. I had also before I replaced both the IAC and TPS. I figured my first step is to get that back to factory. That was at least part of his problem.

That's when I found this thread ( on another site ): https://www.corvetteforum.com/forums/c4-tech-performance/922233-iac-reset-procedure-please.html with instructions on how to set it. First thing I gotta do is get the IAC spindle to fully extend closing off the hole so I can then adjust the screw to "minimum air flow" or something like that ( I saw that phrase at another link that I've lost ). 400-450rpm at idle with the car in drive. They say to do it by jumpering post A and B in the ALDL. That will put it in diagnostic mode and the computer will extend the IAC spindle. Problem is, the 95 while still OBD1, has the OBD2 port. Which doesn't have A and B to jumper. So I have no clue how to get that to extend fully.

Another site I went to had someone suggesting jumpering it according to the shoebox site, but I couldn't find any information there how to do it. When I pulled the old IAC, it was fully extended. I figured it was because of the high idle issue. I tried to replicate it by loosening the throttle body bolts, and all it would do is kill the engine, not idle up.. so maybe that was never the issue originally?

SO the TL;DR version is this. How do I reset the throttle screw to factory on a 95? How do I get the IAC to fully extend so I can then unplug it and start the engine and set it according to that second link?
 

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Free obd 1 pc based scan program called "scan9495" is available at the following link. My Files

Download the newest version in the lt1 v8 folder. Also download the dot net framework in folder on same page. Program won't run without it.

Then you need a 16 pin GM obd 1 cable to hook to usb port of pc. Get one at OBD Diagnostics
It's $59.99 plus shipping.

The only way I know of changing the iac pintle length is with a GM tech 1 or tech 2 scan tool. If you've found instructions out in net land, be aware they might be good or they might be bs.

Only suggestion I have in setting the idle adjust screw is the 95 service manual @ My Files
 

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Discussion Starter #8
Thanks cocobobo95. Like I said, 95 scan stuff isn't an option. I'm selling this car as soon as I get it all running right. I don't want to spend $60 on a cable I'm only going to use once.

I'm pretty sure if I get this screw set right and then use the jumper method to see the TPS voltage and adjust that, it should fix the problem. If not, then try another IAC as sometimes new ones are bad.

I've already downloaded your service manual for the 95, but I can't for the life of me locate where it says how to adjust that screw. Any idea what section or page it's on?
 

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From my old days here is how I used to adjust the idle using TB stop screw... not sure how that procedure from corvette forum will work



Idle air is a balance between the TB and IAC.. The IAC controls idle RPM. The TB plate opening up front of intake adds a fixed amount of air that does not change during idle. The IAC under PCM control adds the needed extra air through air runners built into the intake to distribute idle air to all cylinders. This allows a smoother transition from open TB to closed. To adjust the balance between the TB and IAC requires monitoring the TPS voltage at the PCM. Without a scanner this is a PITA.

On my '95 there are 3 wires to the TPS.. GRY is +5 volts from to PCM, BLK is ground to the sensor from the PCM and DK BL is the TPS voltage to the PCM. YOUR wires on the '96 may be different colors, do you have a service manual to verify colors? W/O a scanner you will have to puncture the wires to measure the voltage. With key on engine off set the TB stop screw to show .4 to .65 volts between ground (blk) and TPS voltage (dk bl) ,then verify the voltage goes to at least 4.5 volts at WOT. That verifies correct TB plate opening and the sensor readings of the TPS. The .4 to .65 setting will allow the PCM to self adjust the IAC to maintain the correct idle speed. If the TB stop voltage is set correctly and Idle speed is not correct then you have other problems.. NOTE after setting TPS voltage the IAC may need reset, ref shbox.com 4th Gen LT1 F-body Tech Articles
 
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