This is exactly what my Z28 is doing. It has a reprogrammed computer, new IAC. I have sprayed throttle body cleaner in areas around vacuum lines. Was it ever decided what the cause(s) of this anomaly is? A GM mechanic said I would have the computer reprogrammed back to original before he could access what was needed. I do not want to do that, but to make the car more drivable in city traffic I would do that.

 

i don't remember exactly bc it was about a year ago since i fixed mine, but i think what i did was close the throttle blades a little further and slotted the holes on the TPS to get the correct reading that i found on shoebox's website for the voltage, and kept playing around with those two things and test driving it until the rpms didn't hang anymore (or allow the car to stall out), the TPS was set at the correct voltage range, and made sure the throttle blades and IAC were fine...since i replaced the TPS, IAC, and had a new throttle body that i cleaned anyway, i didnt really have to worry about the last two...

 

Thanks for the quick reply. What is the reason such procedures are necessary? How would slotting the holes on the TPS help? I would think that would make it run lean. I didn't mention it before, but the car is a 96.

 

my car is a 96 as well....the reason for slotting the TPS is so that you can achieve the correct voltage reading for the TPS...in a sense, you are tricking the TPS into the correct reading while figuring out the correct positioning for your throttle blades to give you a smoother and steadier idle (changing the air/fuel mixture)...the TPS voltage readings correspond to the programmed fuel enrichment tables in the PCM...between a certain range in the TPS, you are at the baseline measurement, however, as you press the gas pedal and open the throttle blades more, the trigger inside the TPS rotates, sending increasing voltage readings to the PCM telling it how much fuel it should be adding for the amount of air being allowed into the throttle body...without slotting the TPS, you will have a higher voltage reading than desired and be running rich...i hope that was easy to follow lol

 

Thanks for the explanation. I do understand (I think). That slotting business sounds kind of drastic. The car didn't come from the factory with this problem. Why should it be necessary. Could the problem be caused by the computer being reprogrammed? I see you replaced several items, IAC and TPS. Is that necessary or did you do this because you thought they could be the problem.

 

At closed throttle, any TPS reading between .3v and .9v is acceptable and technically should not be an issue. Every time the key is turned to ON, the computer looks at the TPS voltage and if it is within the range stated above, it sets whatever it sees as 0% throttle for calculation purposes. People often like to set it at the middle of this range, but slotting the TPS is necessary to make any adjustments. It is usually not needed, except in the case of aftermarket TBs, where it might be out of range.

 

thanks for making that a little easier to understand shoebox!!!
i replaced the IAC as well because i thought it may have been sticking since it was very old and was an original part on the car...my car has little to no original parts on it, including the throttle body, and i needed to slot the holes on the TPS to achieve the voltage i wanted...i dont remember what i set it at, but between the retuned computer, new throttle body, and repositioning of the throttle blades, that is what made it necessary for me to slot the sensor and play around with that and the blades until i had a combination of both the correct positioning for the throttle blades for air/fuel at 0% throttle, and the correct positioning of the TPS for the PCM to recognize 0% throttle at that position

 

Thanks guys for the clear explanations. So, if I make sure my settings are between .3 and .9, my problem is not the TPS?

 

Well my car sat for about a year, and is now having the same problem. The TPS voltage is between the .2 and .9 range to recognize 0% throttle, there is a new IAC valve on the car, and I have not been able to find any vacuum leaks. I took the car for a ride yesterday, and noticed that after it warms up, if I put the car into neutral as I am cruising to a stop, the tach will jump up to and hang just below 3,000 rpms. Once I get to a complete stop, the tach will slowly drop back down to about 1,200 rpms where it seems to idle all the time. It also throws SES code P0507, which is "Idle system high/Idle Air Control (IAC) valve responding." Can anyone point me in the right direction?

 

You need to monitor the IAC readings with a scanner and also check for vacuum leaks. Make sure the throttle blades are not excessively open at closed throttle and that the PCV valve is good and there are no leaks in it's piping.

 

i havent been able to find any vacuum leaks ever, i have adjusted the throttle blades previously to not be excessively open at closed throttle, the PCV valve is new and so is the piping (i replaced both when the new engine went in). i have some screen shots of several parameters that i recorded using AutoTap, but i'm not knowledgeable enough to understand what they mean. is there anyone on here that i can email them to that may be able to decipher them for me?

 

Fred has great page on how to decipher scanner readings.

http://www.injuneer.com/ScanMast.html

 

ok thanks...i will try to use that and help myself out...in the meantime, here are links to see the screenshots in case anyone can notice something that jumps out quicker to them...

http://carphotos.cardomain.com/ride_...4004_large.jpg

http://carphotos.cardomain.com/ride_...4005_large.jpg

http://carphotos.cardomain.com/ride_...4006_large.jpg

http://carphotos.cardomain.com/ride_...4007_large.jpg