I just recently bought a 94 Z with 75K on it and everything was fine until I drove it back home (120miles) and the next morning I took it for a drive and everytime I stop at a stop light (gear is on drive) waiting to turn green, the car's RPM jumps up and down from 1000 to 1200, seems like its trying to go even though my foot is on the break as hard as I can? After drive it and as soon as I put it on park RPM goes up to almost 2K once and down to like 550 and stays there in park. Please help. The ex owner told me that he had to rebuilt the trans and installed a shift kit while at it. Please let me know whats going on. thanks

 

Possibly an idle air motor, although its not that common for one to go bad on GMs.

 

mine did that when the opti was bad, espicaly when the air was on

 

how much is it to change the opti?

 

I noticed that it is only doing it when my heater is on...whats the deal? Im guessing it has something to do with the fuel/air mixture..HELP!

 

welcome to the unknown cause of the low rpm miss/idle surge club. Seems to especially effect auto cars. My car has it. I havenot really been able to cure it.

Funny you say you took a 120 mile trip, another two poeple that have this problem say they had it start RIGHT after they took trips, one took a 750 mile trip, another didnt specify, and me

My car does the same exact thing yours does, but with the heater ON it makes it much less noticealbe. Did you mean off?

 

This is a problem that reoccurs on my 94 from time to time. I had it bad for awhile, cleaned out the throttle body along with the IAC & egr valve. That cured it for a few months now it seems to be starting again, but not a bad as it was. I did take it to the dealer and the scan showed no codes?? . I would have to say it is either the egr valve, IAC, or a small vac leak somewhere.

 

can someone please tell me how much does it cost to get it check (just check) to see codes etc... at the dealer? newbie sorry

 

can someone please tell me how much does it cost to get it check (just check) to see codes etc... at the dealer? newbie sorry

 

can someone please tell me how much does it cost to get it check (just check) to see codes etc... at the dealer? newbie sorry

 

You can get a plug in scan tool at Autozone, Kragen, etc. for about $30 and check it yourself. Also check all of the various vacume hoses for leaks.
If you don't find anything obvious, you can get a can of spray carb cleaner and spray around the various vacume hose connections and throttle body and intake manifold gaskets while the engine is running and if there are any vacume leaks the cleaner will get sucked into the engine and cause the idle to slow or stumble.
I don't have an auto trans on my Z, but on all of the older auto tranny cars I have had there was a vacume hose that went to a device called a vacume modulator located on the passenger side rear of the tranny. I've had those go bad, and/or the little piece of angled vacume tube that goes to it and it caused eratic idles.
Typically a dealer will charge you $65-$90 to diagnose the problem (will include a code check at a minimum) and will often credit it toward the repair if you have it repaired there.

Good luck

 

Thank you very much for help,, since I dont really know my way to the Z engine bay,, can you please guide me to where the vacumes are.. maybe picture would be nice... again thank you so much for helping, I want my Z to run good. Thanks.

 

I tried the auto zone scanner on my 94Z & they don't work. I returned it and got my $ back. The local GM dealer cost me $76.00 for a diagnostic scan. And the results were negative for any codes???

 

Sounds like the Idle Air Control (IAC) "Motor" to me. Possible getting stuck open.

Go to autozone and buy a can of carb cleaner, rip off your TB and clean out your TB. Make sure to remove the IAC from it and clean that too. Then see if that works.

The IAC is reponisble for keeping an idle when your throttle is closed by opening and adding air. Now if it's stuck it'll be adding air all the time.

That's where the high RPMs come from.

I just want you to understand how it works, it's better than just telling you how to fix the problem. Now you could help others out, like they helped me out!

Oh you have OBD-I, you could scan it yourself with a paperclip! I'm unsure of how it works exactly, but it does. Ask around, someone would be ahppy to point you in the direction of the place to find the CODES and How-To.

 

Do I have to take the whole TB out, or just where the intake is connected? where and how to remove IAC? Please help more.. thanks. How about vacume leaks,, how to fix them and where are they located in the car?