Pesky High Idle problem 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 Conntrol 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

 

just try one more test, connect an voltmeter across the IACV, measure the
voltage before ignition on, after ignition on, start and idle (1500) and
monitor the voltage till 1800 RPM, if the voltage is going higher means
that IACV is the cause of this problem, and check the wiring harness for
any shorts, cos you have already replaced the ECU and found the same
problem.
also check if there are any gear/ cluth engage switch/sensor, try
disconnecting it when the RPM is higher, if the wiring harness is short
from this sensor, this problem could happen.

just in case, if you have the diagnostic tool with you, check the value of
Idle CO adjust multiplier if it has that option.

 

Test the TPS, which sends the signal that controls the IAC.

 

If the code 25 is resetting after you clear the code and turn the
key on but before you start the engine, then there has to be an
open or short circuit somewhere in the IAC circuitry.
Start at the SBEC and ohm out the wires with the IAC plugged in.

 

You say no "known" vacuum leaks or other gasket leaks... I suspect
there's an "unknown" one somewhere The code is being set because the
IAC is losing control of the idle speed for some reason, and apart from
the IAC itself (motor) a vacuum leak is a high probability.

Try disconnecting and plugging all the vacuum accessory lines as a test.
A leaking power brake booster (diaphragm or valving) can cause a huge
vacuum leak but the booster may still work if the engine can "keep up"
with the leak. Its harder for the A/C blend doors and other small
vacuum-operated devices to do this kind of thing because of the small
hoses involved, but check/plug them for this test anyway. What about the
PCV valve? If this model has a remote mounted MAP sensor, check its line
(I imagine it doesn't, but not being intimately familiar with the 3.3 in
a minivan I thought I'd cover all bases.

Some engines can develop a vacuum leak into the crankcase- again I'm not
intimate with the 3.3, but if you can disconnect the PCV valve from the
crankcase, do so and see if you get a light blow-by out of the
crankcase. If not, it might be that an internal vacuum leak is
scavenging the crankcase in addition ot the PCV valve. That *usually*
increases oil consumption, though, so I consider it unlikely.


You say you replaced the IAC motor- how is the wiring between the motor
and the PCM?

 

Thanks. I started doing some resistance tests last night and found
that the connector where the IAC motor plugs in is a problem. I should
get around 52 ohms across windings
of the motor and I do when the motor is out. I plugged it in and
started checking downstream at the connector(behind battery) at the
other end of the wiring harness. When I tested across terminals there
I received an open until I wiggled the IAC connector wires at the
motor connector and 'voila' I get the 52 ohms. There is something loose
in that connector plug and I may just replace the whole harness and
see if I still get the Code 25 tripped. My guess is I won't. This wil
bring me back to a baseline to figure what else I need to do as I give
the IAC motor a chance to set idle correctly. Thanks for your help. I
will keep in touch on outcome.

 

It is easy enough to just solder in a replacement plug or just the one
wire. You don't have to stay OEM, it just needs to make a connection.

I just had to bypass one wire of the heater blower plug in my Cherokee
and a really common replacement is the CPS plug.

That trouble is all too common up here in our rust belt, I use spray
contact cleaner, then dielectric grease on the plug seals on my TPS,
CPS, IAC and distributor pick up connections.

Sometimes all they need is a good clean with a spray contact cleaner.
They only pass computer signal current (low) so the slightest hint of
corrosion craps them out.

The Cherokee's idle jumps to 2500 every couple years and a spray clean
of the TPS plug fixes it. It only has 310,000 km on it.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
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also chech the throttle shaft to see if it's worn out causing a vac leak