Pesky High Idle preventing 300,000 mile mark

Hello all,
On my 92 Grand Caravan 3.3 I have a high idle problem that
consistently trips the Code 25 (IAC circuitry). Idle at startup is
around 1500 RPM's and moves steadily higher(1850+) as engine warms.
Here is what I have checked and replaced.
Idle Air Control Motor
Cleaned throttle body and plate and replaced gasket.
Swapped out Engine Control Module(ECM) from another Caravan(exact
match).
No known vacuum leaks or other gasket leaks
No other codes found
The code 25 immediately sets itself even after I disconnect the battery
for an extended period. I even disconnected the ECM and reconnected.
It will show the code before I ever start the vehicle again. It sets
itself when key is in on position and checks signal voltage. I am
suspecting wiring. Resistance tests appear normal. I would like to get
this car past 300,000 (only 57 miles to go).
It starts great and ran very well before this high idle issue. For
those who know a ton more than me: What would be your next steps if you
were in my shoes? Thanks

 

The most likely culprite here is the EGR valve staying open during idle
which it should not do under any circumstances. Disconnect the vacume
hose from the EGR and start the engine. using a small hammer lightly
tap on the EGR valve if it is stuck it will most likely loosen itself
ck to see that you do not have vacume at the EGR hose at this time as
well. If the idle does not come down use a small mirror and flashlight
to verify that the diaphram on the EGR is in the down position (away
from the vacume port) if it is then the EGR is closed.

NEXT

ck the IAC harness connections at both ends and ck the IAC wires for
continuity while pulling on the wire harness..

There may be carbon build up in the idle channel in the throttle body
as well
generally these vehicles start burning oil at the 100k mark and this
always
fouls op the works.

 

Sorry, a stuck open EGR valve doesn't cause a high idle, in fact
the symptom would be the exact opposite.

He's already done this and found an intermittent connection.

And the symptoms of this (again) would be -low- idle not high
idle.

 

Check the IAC wires for short to ground, short to power and short to each other.

 

I was able to change suspect IAC plug and one wire connection to ECM.
No more Code 25. Idle is very low now around 500 in DRIVE and wants to
stall, but 750 OK in PARK. I may just have to drive it so ECM can
adjust. I will keep you informed as I proceed. Thanks for your
expertise and help.