idle rise & fall

hi guys..you might remember me from my overheating post. I replaced the head and all is still ok with temp. Now I have another issue. After car warms up, when I am in park or neutral, the idle will rev. up then fall and repeat this constantly. Its an automatic. Also, it is idling alittle high. Everything is fine when I am driving, only when I get under 35mph coasting, it will rev up & dwn until I apply brake to get under the idle speed, which is about 22mph...LOL Anything I can do to troubleshoot this prob. ? Or anyone know what this is ?
THANKS

After doing the head job, did you time the ignition using a timing light? I would start there.

I would definatly check the Idle Air Control Motor. And make sure you don't have a vacuum leak. Do you know how to check for a vacuum leak?

Check for vacuum leaks around the intake manifold. Pay close attention to the back and hard to see places. Also clean or replace your PCV valve. Also, make sure none of the hoses are crimped or rotten too bad. Good luck...

yea guys, I used timing light. After I put head on , everything was fine for about 2-3 weeks. It just started doing this recently. I will check intake. I searched for vac. leaks, nothing obvious. If my catalyst was blown out, would this effect that ? Cuz it just went too.

the problem is in the computer but the way i fixed mine was i just ran a couple tanks of ethenal throught my geo it made it idle a little rough but when i put it back on regular gas the problem was gone:iceslolan

Does anyone have a definite answer on this rise and fall idle? I have just acquired my 2nd Metro and I'll be damned if that one isn't doing it too.
Now I have 2 with a crappy idle!

I'm not sure if there is a definitive answer. Mine stopped when I replaced the PCV but it would only do it occasionally. It would stop when I plugged the hole for the A/C idle adjuster too. I also had a loose hose on the back of the intake. It could have been one or the other or the combination. I can say I'm sure the problem comes from the intake and thats where you should look.

The definitive answer is it's your IAC valve. That is the valve on the back of the throttle body that gets its air from the bottom of the air cleaner can, thru a small cannister filter, and into the valve, and then directs this clean air into the driver's side of the throttle body just above the idle air control screw (89 to 93 only). I think 94 still has the IAC valve, though, and probably some form of it even later. But the 90-94 models have the most problems. This valve is used to increase idle when headlights, heater fan or A/C is on. The inside of the valve is rubber and copper sheet, and it deteriorates, acting like a reed eventually, or at least, it gets very erratic. I believe the ECM modulates it to get the proper idle. You can buy new ones (watch out, some joints want $137 for them!), but you can also block them off and get about the same results. I pop the bottom rubber hose and cram a pencil tip into it. Tap it in and put a piece of electrical tape over that and then slip the hose back on. You will never get a hunting idle again. The valve throws NO ECM codes, so no worries there. You will find that your idle goes about 50rpm lower for headlights, and 50 more for high heater fan. I never have had A/C, so dunno about that one, but probably more.