Fast Idle After Highway Driving

[rwaterhouse]

Hi;

I have a 91 Ranger, 4.0 L, 4X4, Auto with the following problem. It starts fine, idles fast for a few seconds and then goes to "normal" around 800 rpm. I then drive 20 miles to work, which is all hiway, and when I get there it is running high (1500 to 2000 rpm in park) and it is driving me crazy. Other than this the truck runs great. The CEL is not on ( I just bought a code reader of ebay and will scan codes when I get it).

Things I have tried:
Replace the IAC - no change. I got two from a pick n pull, neither makes a difference.
Pull the connector on the IAC - If I do that it immediately drops to 5 - 600 rpm, which suggests that the computer is TELLING it to idle that high, right?
Check for vacuum leaks - nothing obvious, and I'm puzzled what would leak only when the motor is hot.
Reset the computer - disconnected battery for 15min. No change.

Other information: This was intermittant, and seemed to happen more when it was cold out, but it has started the last couple of days doing it ALL the time.

I think I have read every post in here that refers to fast idle, but nothing seemed relevant. Suggestions welcome.

 

[shaggymane]

I"ve got a similar problem. looking for solution.

Startup 1800 rpm. other times 1200. Replaced the IAC, no change.
today replaced the ECT (engine coolant temp)sensor, the one that talks to the computer not to the guage. Start up and clutch in at stop signs, 1800 rpm....... but now it at least settles down to 800 after 30 seconds.
MAF's been checked, disconnected battery, followed the idle set procedures.

What to look at next?

 

[shaggymane]

bump

rwaterhouse, have you solved this problem yet?

I've seen this *idles to fast* posted about before and noone ever comes back with a final solution. That is, after the usual attempts don't get it done.

be nice if someone would.

I'd like the idea of "test don't guess", especailly when 60$ parts are involved. Thought i did that. IAC showed incorrect voltage. replacing did no good, though that left ECT, guess not.

TPS is next.

 

[doonze]

well, if the IAC had incorrect voltage, then whatever is sending the voltage to the IAC is screwy.... Does the voltage come directly from the PCM for the IAC control or from some other source???

 

[awood]

I have limited knowledge on this, but it seems to me that there's 3 things that could cause this.
IAC - I know you've changed this twice, but they were not new, and all of them could be full of carbon, causing them to stick. Clean out the valve part of the IAC with Throttle-body cleaner and try it again.
Computer-sending wrong signals to the IAC. The voltage won't vary from 12 volts, but it's a square wave 60 hz frequency that controls this. The valve never seats in one direction or another, but it will "dither" back and forth (60 times a second). Again, the computer only controls the duty cycle of the square wave, not the actual voltage.
Sensors - Sensors feeding incorrect info to the computer, which passes on incorrect commands to the IAC. One possible example of this is the IAT sensor, which is mounted just aft of the IAC on the intake plenum. The Inlet Air Temp sensor (sometimes called ACT - air charge temp sensor) can act in place of a traditional choke. You can test this with an ohmmeter. The higher the temp, the higher the resistance. At 86 degrees you should read roughly 20 to 30 ohms.

 

[Maniak]

A 4th thing that can do it.. bad throttle body..

When our X was doing this I would hop out of the truck, push the throttle linkage.. It would move and viola, the idle was back to where it belonged..

I picked up a used throttle body from a you pull it around here. It was lower mileage (Aerostar showed 150k miles).. I took it apart, cleaned it.. put it back together and all is fine..

What I found on our old one is that the throttle plate was no longer centered in the bore.. The bushings had worn too much over the 350k or so miles it has seen..

~Mark

 

[awood]

Good point. I've also seen the Throttle Body just sticking with carbon build up.

 

[shaggymane]

Here's my update. I reasoned since the new IAC made no difference maybe the old one wasn't bad like i thought. God only knows what I was testing. Memory says it was volts, by back probing the IAC connection. Which, my limited knowldege NOW says is usless here. It's 1.9 to 6.5v

So, i went back to square one. Got the book out for test spec's. Chiltons wants 7 - 13ohms for the 4.0L.

Next i had to learn how to set and use the DVOM correctly. For a while i looked like both were bad. Couldn't get any consistent good readings.

Figured out DVOM setting was the *smallest* setting 200/.)))

My meter has probes so aligator clips helped by grabbing the recessed prongs.
The old IAC reads 9.9 so it is good
The new IAC reads 10.8 ohms


had a look at the TPS next. Book only says measure ohms again.
want consistent smooth flow of numbers but not what the nembers should be.
Book says any jumps or swings back and forth mean it's bad.
Well at first it would start at 3.31 and go down to .50 as I turned the throttle slot.(I took th TPS off).
I did see numbers swing, like from 2.84 back to above 3.0.

Came inside to eat and after that smooth operation. Didn't see those wild swings.

point is, I don't know exactly waht I'm looking at. Good OR bad?

Unscrewed from the intake it'd idle nice and low the more i turned the slot.
bolt it back on with the TPS plugged in, it'd go back to fast idle.


PRices range from $43 @O R's to $29 @AZ.

Frankly I don't wanna spend more $ till I know for sure.

Anyone willing to go test there TPS for numbers to compare?

ok, what'd i forget?

 

[boggs1227]

something you could try .... if it were me and the tps was bad i'd replace it ...but after reading that you said that the idle was fine till you put the tps back then it went to high idle ....but if money is a concern ( have you read the tps mod ? ) you could try it ,but instead of increasing the amount of movement the sensor sees to increase the flow of gas in the injectors .... try turning it back the other way to get to your correct idle rpm

you could give it a try ( but i like i said i would replace it if it's going out ...just me ) ...but if you got a dremal and about 10 minutes of time you could try it ( what could happen ?? you got to go buy another one soon )

this may get you down the road a little while longer ( justa quick fix if you can't go buy a new part )


goodluck


btw : have you checked all of your wire harness connectors ? ( these trucks aren't young anymore and 15-18 year old plastic parts WILL break and go bad!

 

[shaggymane]

hey thanks boggs,
Where is this mod on TPS's? got a link? I went to page 8 in usefull how to threads, didn't see it.
I've got ten minutes and dremel.

my quick fix...because i'm so tired of that high idle, was to try a trick i saw another guy do. Make a new gasket with 2 small holes about the size of 10 penny nail. Worked. Idles at 800 even with all acces runing and turning the steering wheel.

it still tries to spike a high rpm. hits 1200 for half a second.

Can't say what that would do in winter.

I'm still looking for the real cause