This problem just started last week.

The engine starts fine, races normally when started then idles down to a normal rpm as it warms up, and as long as I'm running (in the first "D" position of course), all is fine.

But when I get to where I'm going, or simply stop along the way somewhere and shift the lever from "D" into "P" (Park), the idle drops usually low to a point where the engine runs quite rough and stalls within a few seconds.

The engine starts immediately upon turning the key to the START posiiton and has the idle rpms returned to normal.

What would cause that?

 

Possibly a sticky IAC?

 

x2.

The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAB to function properly. Remove the IAB and clean it.

 

Glc, Ymeski.... Thank you both. My mechanical expertise lies in the marine line (Evinrude/Johnson), but I do my best to keep my vehicles running with help from people like you. The picture reveals what to look for (IAB) and I'll clean that item as soon as possible, then get back to you with the results.

THe mention of a "IAC"... Is that a typo or another component I should look at?

 

It's IAC - idle air control.

Clean it with WD-40 or Deep Creep. Spray it out good and work the plunger against the spring with a screwdriver till it moves freely and smoothly.

 

Okay, thank you. I just read up on it and found that it is a component that is known by at least eight (8) names. I have the WD40 on hand and will clean that item as soon as the weather warms up.

 

Which engine do you have? New ones range from $40 or so for a generic to close to $80 for OEM.

 

My engine is the 302 V8, auto transmision, AC.... Loaded 1988 F150 Lariat.

 

I'm having a very similar problem. How can I find the IAC on an 89 f150 302?

 

It looks just like the pic in post #3. It's attached to the throttle body and has an electrical cable plugged into it. 2 bolts to remove it, be careful and you can reuse the gasket.

 

As mentioned, the part goes by many different names:
•Idle Air Bypass
•Idle Air Control
•Idle Speed Control
•Throttle Bypass Air
•Idle Bypass
•Inlet Air Controller
•Inlet Air Bypass
•Intake Air Bypass
•Intake Air Control

On this diagram, it's referred to as the " Throttle Air Bypass Solenoid".

 

Well, the weather finally warmed up enough for this old guy to get back to that possible faulty IAC problem.

It came of the engine easily with no gasket damage, and the solenoid assembly portion came off of the assembly without any O Ring damage. The plunger/rod/valve assembly was not stuck and moved freely, however I did clean it (the plunger assy) as good as possible in my cleaning tank (mineral spirits), via a brush and compressed air. I did use WD40 also.

There didn't appear to be any major leakage from one chamber to the other... BUT there was a whisper of a leak between the two that I couldn't correct. No problem assembling the unit back to the engine, using gasket sealer on the gasket and the O Ring.

While at it, I changed the spark plugs too. The interior porcelin surrounding the vertical electrode of the original plugs appeared to be a brilliant white in color EXCEPTING one plug... that one plugs porcelin appeared to be an Off White, actually gray in color. I'm sure that means something to you fellows but I have no idea what.

Running the vehicle and changing from "drive" to "park" as per my original post #1 here, the rpm still drops very low but has yet to actually stall out. Shutting the engine down, then restarting results in a higher, normal idle rpm.

 

Warm curb idle for your truck is only supposed to be 650rpm.

 

I have no idea what the idle rpm actually is (no Tach on vehicle) but since using the vehicle a few times, it does seem to be smoothing out idle wise. Now when shifting from drive to park, the idle does drop quite low BUT hasn't stalled yet as it did before.

The idle rpm is (after warmup) obviously higher during the start up period then after the vehicle has been running awhile and being shifted into park. Possibly something else is affecting the idle that is in conjunction with the IAC valve?

So much has been added to the automotive world since the 53 Flathead models that a lot of us have no idea what we're looking at, let alone knowing what those items actually do, hence the advice we need from the people who do...... on this forum.

EDIT: 05/02/14 - Running the vehicle for the past month along with cleaning that valve, the problem of having it shut down when shifting into PARK seems to have vanished. Apparently that valve, after cleaning, just needed a little running time on it. Many thanks to GLC and Ymeski.