Im not an expert with cars by any means so ask someone who knows befroe you
take my advice but this sounds like a problem with the PCV (postive
crankcase ventilation) Valve.  Should be 3 - 5 bucks US for the part and not
sure if you can replace it yourself or not.  I had a similar problem with a
1995 Nissan Altima.

Top Post!!!!!!!
Is you SES light on ....Any codes.....These things have a bad habbit of
getting clogged injectors....Which may not be noticeable while driving
just at idle....Also the converters clogged up alot  need to know if you
have any codes...Also you can disconnect one plug wire at a time from
the coil pack using rubber insulated pliars or youll shock
yourself...Reconnecting it when your done once you pull off a wire and
there is no noticable change in the idle there is your bad cylinder of
which the misfire ia happing...
S Cook
Rough idle on 97 chevy malibu  

Group: alt.autos.chevrolet.malibu Date: Thu, May 5, 2005, 9:24pm From:
UseLinkToEmail@AutoForumz.com (wyomedic)
I have a 1997 Chevy Malibu with the 6 cylinder motor. Out of nowhere the
motor started to shaking whenit was at a idle. It runs just fine a
highway and in town speeds it just idles really rough. I did have to put
a trany mount in it and I have changed the plugs. All plugs looked the
same and seemed to be burning good. The exhaust pipe is making a puffing
sound and the exhaust smells like raw unburned gas. Any idea of what
could be going on here?