I have a similar problem on my 1990 Escort GT, following the
calibration procedure for my car, I can't get the idle below 1200.
Try plugging all of the vacuum ports on the intake. These are normally
the EGR tap, PCV tap and manifold accessory vacuum tap. I tried this
on my car and there was no difference in idle speed. At least now I
think I can rule out the vacuum/pcv as sources of unmetered air in my
Moving on, the next area of speculation would be the throttle body,
intake manifold or crankcase. I think with the speed density systems
(MAF or vane air meter) the crankcase must be sealed, but I forget why.
Probably because air can get into the combustion chamber through that
route. I was reading this somewhere following a google search on
VACUTEC. This a brand of smoke tester that is used to pressurise an
intake/engine with harmless smoke to determine where leaks can be
found. I haven't found a shop that has this tool yet, but you can get
one for about $1000. <g>
I tried the propane method with no results. I even have a schrader
valve from an old bicycle tube fixed to a 1/4" rubber hose I can
connect to the accessory vacuum manifold, but without a vacuum
regulator to limit my air compressor to 3 lbs. or less I'm not about to
hook it up to the engine.
Writing this, I came up with another idea, restricting the crankcase
ventilation tube. I'll let you know if anything happens.