I've found that the " anti-tamper" throttle plate stop screws can move over time. I believe that the correct screw setting is for a very minimal opening of the throttle blade. Be sure to use some Loctite to prevent the screw drifiting out again.
------------------ 87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite. 87GT - 3.4L Turbocharged engine, modified TH125H " I'M ON THE LOOSE WITHOUT THE JUICE "
If everything is working properly, no exhaust leaks, no EGR leaks, and the engine is properly tuned, you can go through the IAC system. Idle is controled by the IAC valve (idle air control) via the computer and is not adjustable.
You can go over the system if your not happy with where it is idleing. Proper idle for the V6 is roughly 900 RPM.
The set screw in the throttle body is for minimum air flow through the throttle plate which is often misunderstood as the idle screw. Any tweaking of the throttle stop screw will give the ECM fits. The ECM wants to be in control of the idle and is not happy when it can't control it. If the normal learn limits of the ECM are exceeded, they will be reset to nominal, causing an erratic idle.
The IAC and associated passages need to be clean to work right. Remove the IAC carefully. You can clean it (the nipple) using carb cleaner and a small brass brush or rag. At this time also clean the throttle palte. Once clean, install the IAC back in the throttle body and reconnect the IAC wires.
For the ECM to properly control idle, the throttle stop screw must be set for "minimum air". This is a process that sets the idle with the IAC fully extended. To fully extend the IAC, jumper ALDL pins A and B together (just like when you check codes) and turn the key on, but do not start the car. With the key on, not running, and in diags mode, the ECM will keep trying to fully extend the IAC. After 30 seconds or so, pull the IAC connector off the IAC *before* doing anything else. This will capture the IAC fully extended.
Now pull out the jumper in the ALDL, and start the car. Typically the "minimum air" idle speed is in the 500 RPM range. I find the car can bearly run at 550. So as long as you can get it to idle on its own between 600 & 700 your good. Set the idle using the throttle stop screw. (The engine should be fully warm to do this.) Now shut the engine off and reconnect the IAC wires. The ECM does not know where the IAC present position is, so pull the ECM fuse (or disconnect the battery) for 20-30 seconds. (This will cause a complete ECM reset of all learned parameters, including the learned IAC ones. Then reinstall the ECM fuse.
Turn the key on, wait 10 seconds or so, and turn the key back off. This will now reset the IAC to a known key-off "park" position. Now start the car. The engine should idle properly under control of the ECM. There are some learned values, such as an IAC offset for A/C, etc that need to be learned, but this will happen under normal driving conditions. I suggest driving the car right away under all conditions. Stop & go, steady cruising over 45 mph, full throttle, and so on. Pull over a few times and turn the car off, then restart it. The IAC can only learn X amount of counts with each run position. If everything else on the engine is in good condition and operating properly it should be around 900 RPM after coming to a complete stop with slight variations and improve over time.
I will say this, you usually don't want to mess with this screw unless someone has already tampered with it or your throttle plate is sticking. If your plate is sticking, back it off just a little bit so it doesn't stick, you should be ok.
I have reset my plate using Skybax article, it was very close but I would recommend using an auto scanner so you can actually "see" the counts on the throttle position sensor, I can't remember the exact range, maybe someone else can chime in here for you.
Welcome to the forum, welcome to the madness.
------------------ jetman Silver 86 SE 2M6 4-speed, with "check wallet light" Now fortified with 8 essential slices of bacon goodness
I used that writeup to reset my TB and it seems to have worked well. You also would want to make sure everything else is good so as to not skew the results. Vacuum leaks, bad timing, etc. will impact the way the engine behaves, so adjusting the TB based solely on idle speed on an engine with other "issues" likely wouldn't work very well. If things are good though, it should get you reasonably close.
I used the writeup as a starting point, and did additional validation via the TPS. My brother's 87 GT hasn't been tampered with, so I put my TPS on his TB, noted the reading on my voltmeter, then reinstalled it on my car and adjusted mine until it read the same as it dd on his car. Short of using the ALDL to monitor things, it should be really close, if not dead on. FWIW, I was only .05v off by using the adjustment writeup earlier in this thread. :-)
I reset the screw on my TB using Skybax's procedure back when I did my 3.4 swap. Worked like a charm. After it was set up properly, I shot a small dab of black RTV into the hole above the screw, almost filling the hole neatly. This was to help prevent any future movement of the screw and to discourage myself and anyone else from messing with it (The RTV can be pried out with a small pick or flat bladed screwdriver if you really need to later)
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12:57 PM
Patrick Member
Posts: 36404 From: Vancouver, British Columbia, Canada Registered: Apr 99
I have reset my plate using Skybax article, it was very close but I would recommend using an auto scanner so you can actually "see" the counts on the throttle position sensor, I can't remember the exact range, maybe someone else can chime in here for you.
It was probably Dodgerunner who told me the proper TPS reading at idle when using WinALDL was 30-50.
