My newly acquired 1996 Z28 is bone stock (hopefully not for long) but has this annoyingly high idle, especially for a manual trans vehicle. It idles around 1000 RPM no matter what the engine temp or the environmental temps are that the car is in. From the limited amount of reading that I've done about this LT1 engined car, it should idle around 650 RPM or so. Can somebody with more knowledge & experience chime in here and let me know what is possibly causing this and what the remedy is? Thanks in advance!

 

Depends on trans. Auto is 650 in neutral, 550 in gear. Manual is 800 RPM.

There's a table of stock programming - target idle speed vs. coolant temp - in my LT1 scanning guide.

https://www.camaroz28.com/forums/com...-guide-886891/

Stock tach can read 100-200 RPM high at idle. If you can put a scanner on it you can see actual idle speed from PCM. PCM controls idle RPM using idle air control (IAC) valve. The valve is a pintle driven by a stepper motor. The PCM tells the IAC how many steps (counts) it should be opened. There is no feedback as to actual IAC position. The PCM will reduce the counts to lower RPM. Once the valve is completely closed, it can’t reduce idle any lower. Or, if the IAC is nor responding, it can’t pull the RPM down

This could happen if there is a vacuum leak, if the throttle blades are not fully closed, if someone has played with the idle stop screw. Additionally, if the throttle position sensor (TPS) is reading too high, the PCM can not tell the throttle is closed and that it should be controlling RPM.

Check that throttle blades are fully closed.
Check for vacuum leaks.
Check TPS voltage at closed throttle, should be less than 0.90 volts
Check RPM from PCM, to see if tach is accurate
Check accuracy of coolant temp sensor using Shoebox procedure

4th Gen LT1 F-body Tech Articles

Check continuity of IAC, using Shoebox procedure.

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

Reset IAC using Shoebox procedure

4th Gen LT1 F-body Tech Articles

 

wow, thanks for the rapid reply with so much info & things to check!
I had no idea that the stock tachometer isn't accurate at idle. That seems pretty lame that GM engineers couldn't do better than that, but it is what it is, I guess.
I also didn't know that the manual trans cars idle that much higher than the automatics.

Thanks for the tips on looking for vacuum leaks & also to see if somebody has fiddled with the idle stop screw, I hadn't considered either of those scenarios. I will poke around in there and see what all I can find.

 

Check the idle speed in the PCM. You may not have a problem with the tach reading 1,000 RPM. Maybe a previous owner had the needles off the instruments (e.g. to install new gauge faces), and didn't put them back on exactly right. "Whiteface" gauge overlay would indicate that possibility, for example.

The other key measurement is the idle air control (IAC) "counts". If they are bottomed out at "0" the IAC can't lower the idle any more. But I'm not sure if the average OBD-2 scanner will report IAC counts. May be a GM specific value, and require a scanner with "enhanced parameters".

 

I'll have to go back and look, but I think my idle is set at 800rpm. Needle sits around 900-1000. You can see it here at time 2:26....