Fast Idle

[traderssd]

Hey guys

I have a '91 XLT with a 351 with an idle problem. It has 55k original miles on it and runs like a champ. It has been well cared for and is clean as a whistle. My issue is that when you first start it up and the engine is cold, it has a very fast idle at about 1500. Then after about 20 or 30 seconds, it slows down to near normal at about 800-900. After it warms up however, the idle goes back up a bit too fast at about 1200. Also every once in a while while driving, the "engine light" comes on but then goes off. I don't know if the two things are related.

I'm thinking the fast idle might be the IAC (idle air control) because if you disconnect the IAC, the idle comes down to about where it should be, but I don't know for sure. Does anyone have any idea what the issue could be? Can a IAC malfunction when it gets hot? Also what about a bad TPS (throttle position sensor) or bad EGR valve or EGR sensor? Can either of these cause my fast idle?

Thank you in advance for any help.

Sincerely,

Stuart D.

Portland, OR

 

[miesk5]

yo STUART,

Curb Idle and Fast Idle are controlled by the PCM and IAC and are not adjustable.

Engine Load, TPS Sensor and the IAC are very closely linked, they must all work together to maintain a good idle.

Try a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19

http://broncozone.com/topic/14269-code-reader/page__pid__74587__mode__threaded

The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.

Run it around to heat the engine up and shift thru all gears incl Reverse. Then turn off all accessories/lights, etc.

Make sure A/C is off and transmission is in Park (automatic) or in Neutral for a Manual & release clutch.

Do Key On Engine Off (KOEO) portion first.

Look Codes up in my broncolinks.com site using the new Search function.

And Post em here according to:

KOEO

&

KOER

==

Idle Air Control (IAC); also called, Idle Air Bypass (IAB), Idle Speed Control (ISC), Throttle Bypass Air, Idle Bypass, Inlet Air Controller, Inlet Air Bypass, Intake Air Bypass, Intake Air Control

try unplugging the IAC motor. If the idle drops, its usually due to an input signal to the computer which results in the computer raising the idle speed. If this is the case, look at all of the computer inputs to make sure everything is up to par. If the engine RPM does not drop when the IAC motor is disconnected, then either the IAC motor is not functioning properly, or, there is a vacuum leak in the engine. Stop the engine, remove the IAC motor and block off the passages. Restart the engine to see if there is any change in engine RPM. An idle speed higher than base idle specs indicates a vacuum leak. If the engine RPM is at base idle specs, then the IAC motor is bad. Resistance specs are 7-13 ohms. A good resistance reading does not necessarily mean that the IAC motor is good. Occasionally, an IAC motor will become weak, and will actually bypass too much air, causing a high idle condition even though everything else is operating properly.

Testing & Cleaning

Source: by Seattle FSB (SeattleFSB)My link

 

[Seabronc]

Yes, the TPS has been known to stick or either could be bad. Pull the codes and see what it is trying to tell you.

Good luck,