Idle around 850-900

[Mike 97]

My truck will idle normally and smooth at 750 with the iac unplugged ( at operating temp). With the iac plugged in it idles around 900 +/- 50rpm. I have replaced the egr ( bad idea) and the iac ( factory part). I have recently replaced the plugs with motorcraft dbl plats and gapped @ .051 -.052" Is there another sensor I am missing? ECT was replaced about 2 months ago and the FPR was relpaced when the fuel pump went bad. PCM is giving no codes - I have the actron scanner so I can't get any other info except for codes. One other thing - if the gas is depressed quickly (similar to kicking down the idle on an older vehicle) it will idle @ 750 rpm for a few sec before going back up to 900rpm. It idles @ 750 in drive - it was lower before but, I don't remember exactly how much lower. Sorry for the length of the post - I wanted to be precise. Thanx.

p.s. the pro-m maf is not on now - it has the stock maf.

 

[Howard]

It sounds like the computer thinks the engine is still cold or under load and so is raising the idle speed to compensate. Could be the temp sender or IAT sensor. Also worth checking for vacuum leaks and sticking throttle linkage. All the best.

 

[Rhett]

i'd check for vac leaks, on the tubes leading to the manifold and in the intake stream.

If you have a 4.0 OHV it should idle at about 500 rpm with the IAC unplugged, and 700 or so with it plugged in.

 

[dogfriend]

What he just said -- it should idle about 800 rpm with the IAC working, about 500 with it disconnected. With the IAC disconnected, you are idling at 750 so you are getting extra air somehow.


Where is the extra air coming from? Probably a vacuum leak.

 

[Mike 97]

Guys -

Here is the link at RPS on resetting the base idle speed Idle speed

Keep the ideas coming !!

 

[dogfriend]

5) Put the truck in PARK (or NEUTRAL if manual trans.) and adjust the idle speed stop screw until the idle is at 700rpm.

This procedure may work , but I found a different procedure in the Ford Service Manual:

CHECK THROTTLE RETURN SCREW SETTING

Key off.

Cables removed from throttle body linkage.

Remove the clean air tube from the throttle body and verify that there is no foreign material or debris preventing the throttle plate from rotating to the fully closed position.

Verify that the Throttle Return Screw (TRC) is in contact with the throttle linkage lever arm when the throttle is in the closed plate (idle) position. Refer to the Intake Air and Throttle Body Systems in «Section 1A» for TRC screw description.

Does the TRC screw contact lever arm?

Yes
GO to «HU6».

No
Only make the following adjustment in the event the TRC screw does not contact the throttle lever arm. Do not adjust TRC screw to try to correct idle quality concerns.

1.) PLACE a 0.002 inch feeler gauge between the TRC screw and lever arm. ADJUST TRC screw until it just contacts the feeler gauge. 2.) REMOVE the feeler gauge and turn the screw clockwise a half turn. 3.) GO to «HU5».

Also, the base idle speed is actually supposed to be higher than 700 rpm with a warm engine

idle speed