I've suspected my 99 Altima has been idling too fast since I got it, but
haven't been able to test or adjust it properly.  I now have the factory
service manual (won't ever buy a Haynes again!) and a proper digital timing
light+tach, and I was able to follow the procedure in the manual for
checking the idle.  What I did was:

- Check idle.  It was about 850 rpm with all accessories off.

- Check timing.  It was around the 20 deg atdc mark, which is where the
manual says it should be at idle.

- Disconnect TPS and restart engine; check idle speed again.  No real change
in idle speed.

The next step, since unhooking the TPS didn't affect the idle, is to turn
the idle adjust screw to set the idle to around 650 RPM (+/- 50).  This is
where I'm having trouble.  I can get a screwdriver down into the screw head,
but it seems to be fairly well locked where it is, and doesn't want to turn.
A big part of my problem is that I don't really have the right screwdriver
to reach all the way down, but I can get one (a slot) to take a bite,
although I only have a 1/4" length of handle left to grasp.

What I'm wondering is a) Is this screw threadlocked from the factory, and b)
is there something else I should look at as a cause of the fast idle?  The
manual doesn't suggest checking anything else, and I have to assume that if
there were any other possibilities, it would cover them.

Any ideas?