Fixing a Bouncing Idle

Date: Tue, 11 Feb 1997 19:33:04 -0400
From: "J.B.Zimmerman" 
Organization: MIT-DACS
Subject: Mk. I 'bouncing' idle speed - diagnosis & fix

Hi Geoff!
  Thanks for your work on the archive...one of the best info sources on 
the web.  I didn't see this info during a fairly cursory search, so I
thought I'd send it in and save others the frustration it caused me...

CAR: '87 Mk. I (USA)
PROBLEM: When cold, the idle speed does indeed start high and then move
     down towards normal, but between 2K and 800 rpm 'base' idle speed,
     the idle 'bounces.'  That is, it drops to 2000, then the car revs
     for a moment until the rpm is around 2200, then it drops back, and
     repeats.  The 'low' rpm number will drop towards 800rpm like normal
     but it's reaaaal irritating to have the puppy bouncing.  Besides,
     if you try to accelerate from rest when it's bottoming out, the
     car stalls sometimes.

EVENTUAL DIAGNOSIS:  Bubbles in the cooling system travelling past the
     temp. sensor causing it to freak out.  First-order fix: bleed the
     cooling system. 

...the problem came back.  At this point, I gave up and turned it in to
my trusted mechanics, who found that I had been correct up to this
point, but that there was a leak in the water pump which was causing
the bubbles.  Fix: replace water pump, bleed cooling system. 

All ok!

Hope this helps someone.

-JBZimmerman!
zimerman@mit.edu
http://uss-repulse.starfleet.com/home.html