Maybe the fuel injection system?!

Apologies for this being so long.  Here's the way I'd tackle it:

You don't say how stable the idle RPM is, or if this is a manual or
automatic car.  If automatic, it's worthwhile to know if the rpm is the
same in Park or Neutral.  If different in park and neutral, skip to
using the scan device and check out the 'P/N' self test.  Also, does it
attempt to run at 1700 rpm in Drive or, in first gear if this is a manual.

Sounds like too much air.  Rule #1 on this is, aside from the throttle
position sensor, don't adjust anything or throw any parts at the problem
until all the diagnostics have been run. (You can quickly go through a
couple of hundred bucks in bits and pieces and not find the problem.)

First, make sure the electrical harness and connectors on the Idle Air
Control and Throttle position sensor are OK.  Both are on the right side
of the throttle body.  The sealed adjustment on the IAC isn't used to
set idle rpm -- leave it alone.

Second, dump the stored codes from the ECM.  (paper clip across
terminals A & B on the diagnostic socket, ignition on, engine not
running, etc.etc.)  If there are codes write them down then disconnect
the negative terminal from the battery for 15 minutes to 'zero-out' the
ECM.  Then reconnect the battery and go for a 'training drive.'
Training drive is in the shop manual.  IIRC, 5-10 minutes of idle, some
highway cruise with easy part throttle accels and decels.  Look for
engine check lite etc.  Bring it home, shut down, start it, check idle.
 Shut it down and write down any codes.  see:  <<
http://corvettebuyers.com/ecm4.html >>

Third, inspect vacuum lines everywhere.  Hunt for a vacuum leaks.  Brake
booster has caused idle problems for some  -- 17 years, maybe it's time.

Fourth, open up the inlet and see that the throttle is closing all the
way.  Look for dirt or a piece of gasket that might have fallen down
into the idle air passageway, wedging the idle air control valve open.

If all of that looks OK and there are no codes, find someone with a shop
manual (section 6E3), a scan device and the software to read out a
1227165 model ECM (a notebook computer, ALDL adapter cable and software
like 'Freescan v2.0 or v2.01' by Whittaker will do this job.) <<
http://www.andywhittaker.com/ecu/ecu_software.htm >>

The scan device will show what the IAC is doing, help in calibration of
the throttle position sensor and show a couple dozen other readings.

Note:  There is a sealed adjustment screw on the IAC.  Don't let anyone
turn it unless you are ready to spend $ 85 for a new one.

Best of luck.

--
PJ
'89 Hookercar '02 e-blu coupe