Crazy Idle At Stop Signs

[mikeshel]

Hey folks I just purchased a 97 Olds Bravada it has 4.3 l V-6 and 107000 miles the Service Engine Soon light is now on and when first started the idle shoots up to about 1800 then will slowly come down to about 1000if I turn the A/C on it comes down to about 7 or 800 once I shift to D it goes up to about 1000 again if I drive any distance when I let off the gas the RPM's stay at about 15 -1600 I do not need to apply gas if I want to coast at this speed. When I aply the brakes as coming up to a stop sign it stays around 1200untill I get stopped and then the idle drops to about 7 or 800 and the whole vehicl shakes and jumps. After a couple seconds all that will calm down and it stays steady and smooth at 750. I used a buddys code reader and it has 2 - error codes all they say is Idle speed high and Idle speed low. I have replaced MAP and TP sensors any ideas? Please help I dont have much hair left to pull out !!!
thanks in advance

[DJ Nitrous]

my dad's astro used to have a similar problem...just sitting in park it would idle high then low then go back high then back low then high then low then go back high then back low then high then low then go back high then back low then high then low then go back high then back low then ....u get the idea....it turned out to be a vaccume leak...that might be a factor in urs...not positive though...u might have a garage chk it out though

[Flatrater]

Do a good visual inspection under the hood look for hoses off. Or listen for a hissing noise usually if you have a vaccum leak it will cause a high idle. It would help knowing what code is set in your truck. Could be a stuck IAC motor, dirty throttle plate and so on.

[mikeshel]

Thanks for the advice I did check for vaccum lines everything has a place, no hissing noise I did spray about 2 cans of throttle body cleaner on plate and all areas around it it was really bad by the way ecspecially on the bottom side and around edges. I did remove the IAC valve and cleaned the area where it seats it was also really black. I checked the IAC by turning on then off the key and it would extend then retract I even tried a new one but still acted the same however they allowed me to return that one so I'm not stuck with it. The code is P0507 Idle control system High RPM. Today I replaced Cap, Rotor and wires still no change.
When I start it idle goes to 1500 for about 4 sec. then down to 1100 where it will stay unless I turn on AC then it goes down to about 850 so I shift into gear and go down the road I did notice if I go easy and keep the RPMs low I come to a stop sign and it stays around 850 however if I go further and get the RPMs above 2000 when I come to the sign right as I'm about to stop it surges up to 1200 then slowly comes down to about 850 and starts chugging the whole car shakes for about 2 seconds then it is fine. I'm ready to give up I just dont know where else to look Please Help

[HotRodAJ]

I have a 92 blazer and it used to have a problem similar to this. Sitting at stop lights holding the brakes the motor would rev up and then go back to normal and then rev up again and so on. I read in a magazine that sometimes the EGR valves will go bad after 100k miles or so in blazers like mine and cause the idle to fluctuate like that. So i went out and unplugged my EGR valve and drove the car around for a while (pretty sure this violates emissions laws ) but it ended up getting rid of the problem. So i would recommend changing your EGR valve and seeing if that fixes the problem. If that doesn't work then I dunno what it could be.

[pcv]

With the car running, spray cleaner around the EGR valve and if the idle stabilizes, remove the EGR and clean it. Otherwise keep looking for a vacuum leak. Spray around the intake, base gasket. I only hope no one turned the idle up.