86 Cabriolet Weird idle

[Frogbutter]

Understand I have only owned this car for a few weeks. Paid $500 for it and it drove the 130 miles home with no problems (Wife followed in pickup with tow dolly just in case.)
I haven't had any real time to spend with it (Vacation and Midwest Treffen). After it has been running a half hour or so (maybe 10 to 15 miles of driving), if you stop and let it idle the tach jumps to about 1500 rpm and back down over and over again. It sounds as if I'm pumping the accelerator. Any ideas?
Thanks,
Cory

[Briano1234]

Start with replacing all the vacuum lines.
then CIS guru's will assist from here.

[kamzcab86]

CIS hates vacuum leaks, so check and/or just replace all vacuum lines, like Brian said. Check the idle screw O-ring; if it's cracked or missing, it'll cause idle issues. If it is unknown when the oxygen sensor was last replaced or if it hasn't been replaced in the last 30,000 miles (60K at the most), replace it.

Once you've done all that, head over to http://cabby-info.com/cis to check the duty cycle and make necessary adjustments.

[Frogbutter]

Thank you all.
Will start with Oxy sensor. Have checked/replaced all/most of the vacuum lines. Got a little high on carb cleaner but could find no leaks.
Thanks,
Cory

[gull]

What happens if you unplug the idle control solenoid? Does it refuse to idle at all, or idle very low? If so, adjust the idle screw 'till it idles OK without the solenoid plugged in, then hook it back up and see if that helps. If the base idle is too low the idle control system chases its own tail trying to prop it up, and you get surging. Ideally the idle control solenoid should only have to raise the idle slightly.

The solenoid may also be sticking. Disconnect it, blow it out with carb cleaner. Clean the throttle body, too, while you're at it -- carb cleaner and an old toothbrush make pretty short work of all that crud that collects around the throttle plate.