First of all, I've already replaced the idle air control valve.
That's not the problem.
The last 3 mornings (first start of the day), the motor starts
normally, then the "check engine" light flickers, and then the idle
goes from about 750rpm to 1500rpm! It stays there for maybe 30
seconds, then it idles normally for the rest of the day.
I've also have had the MFS cleaned recently, new plugs, wires, rotor,
distributor cap also.
it could be just about anywhere. check every hose, and replace any that are
dryrot. it could also be on the intake, and once the engine warms up, it
swells and stops the leak. or it could be in the brake master power booster.
or the heater controls. or the air polution pump.
Actually, that is what it is supposed to do. It is
programmed to do that for the purpose of heating the O2
sensor to get it warm enough to go into closed loop
operation ASAP. On a cold day, it may stay at high idle as
long as 90 seconds before it starts getting the proper sig
from the EGO sensor. The cold start sequence is based on
engine coolant temp. As long as it does not go below the
set point, the cold start sequence does not kick in. On a
cold day, it may go into cold start sequence after only a
few minutes. It is common that they slowly get to a point
that they do not go immediately to high idle, if at all,
after cold because of normal gumming of the IAC. The PCM is
constantly and incrementally adapting to the way your engine
operates. If the PCM was not cleared after replacement of
the IAC, it is simply seeing a startup condition that it has
not yet adapted to and does not perceive as normal for your
engine. Therefore, a CEL flash alerting you to a change on
startup. If, on the other hand, the CEL comes on for 3-5
secs after cold start, that is as it should be to let you
know the system is functioning.
I wouldn't waste any more time looking for a high idle
"problem". Reset the PCM by disconnectiong the neg battery
cable for 10 minutes or, have someone clear it with a
scanner. After it is cleared, start the engine with all
accessories off and not touching a thing including the
throttle. Let it idle for 5-10 minutes and then move the
trans into each gear selectiona couple of time allowing the
idle to settle down in each before moving to the next.
Drive the vehicle a few mile for it to start relearning you
and your engine. It will take a few start/drive cycles
before it completely adapts everything but, you should see
an improvement. Don't bother doing a lot of worrying or
tinkering before doing this. The only thing I would do
first is clean the MAF sensor to make sure there is no
problem there. There are istruction with pics all over the
internet on how to do thi as well as having been posted
numerous times in this group if you want to search it out.
Get some sleep and enjoy your Bronco.
You're right Lugnut, this thing was really bugging me. Well, I
disconnected the battery to reset the PCM (like you said), and it
worked! I just can't understand why it would just start doing that
high idle thing out of the blue. Also, the idle problem started a
couple of days after the ABS light came on. I figured the worst case
senario is the computer(s) had gone kooky.
Thanks to you, I only have one current problem with the Bronco. The
ABS light still stays on. I replaced the rear speed sensor and the
light briefly went out, but it's back on all the time now. I was
hoping the ABS computer would have reset too, but no. Do you have any
ideas about it?
Don't be totally surprised if the high idle returns when
cold weather returns. The high idle is determined by how
cold the engine coolant and the air intake charge is in the
engine in addition to how long it takes the oxygen sensor to
become warm enough to provide a good signal.
The ABS system is one with which I am not terribly familiar.
the main thing to know about the ABS is that it will take
itself completely out of the system such that your brakes
operate as if no ABS were installed if it senses a failure.
I am attaching a link below for you to take a look at the
general troubleshooting procedure and the fault codes for
the ABS system.
You might need a wiring diagram for the system, which you
may also be able to find on the AutoZone site as they now
have some if not limited service materials.
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