Idles at 2000 to 500 to 2000 up and down

[Joaa]

I'm working on a 1999 S10 blazer but not having any luck getting feedback from the S10 forum.
It's a vortec 4.3 same system as my 99 silverado 5.7 vortec. It starts and instantly goes to 2000 rpm then will drop to 500 and almost stall then back to 2000 and back.
I scanned it and got no codes so I disconnected the egr and tried it again and got the egr code so I know I'm linked to the truck. Fuel pressure is over 60 all the time and I disconnected the exhaust to make sure it wasn't restricted. I have other S10s for parts and have swapped out the throttle body, ECM, and mass air flow sensor.
I can hear the IAC open when it drops rpm's and suck air and close when it raises the idle rpm's. It has one of the blow me drunk driver things in it and I'm not sure how that effects the motor other than not letting it crank over.
I'll be glad to try any suggestions and give thanks in advance. joaa

 

[Joaa]

Found my problem! With the IAC making so much vacuum noise I wasn't able to hear a major leak in the brake booster hose. A hole big enough to put a pencil into. I found it by spraying carb cleaner around until the motor raced.
I thought it should have given me a code for a major leak or at least running lean but didn't. hope this help anyone else if they have a problem like mine.

 

[bigfootchiro]

Found my problem! With the IAC making so much vacuum noise I wasn't able to hear a major leak in the brake booster hose. A hole big enough to put a pencil into. I found it by spraying carb cleaner around until the motor raced.
I thought it should have given me a code for a major leak or at least running lean but didn't. hope this help anyone else if they have a problem like mine.

You read my mind on vacuum leak