Unstable idle

I have a 89 formula 350 the problem is the damn thing never has a smooth idle . When you sit in park the rpm will rum up 200 rpm and back down in never ending secession . i have herd of this from fellow owners of 3
geners but no one has a clue what it is ,do you?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

down
Sounds like a vacuum leak.. If not that, then you've got a bad IAC. Those are the best two possibilities without any better description.
-Bruce
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
IAC, probably just dirty. Spray it with Carb Cleaner, usually fixes this very common issue. Vacuum leaks would cause other issues aside from varying idle.
-- 98 Camaro

Those
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.