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  #1  
Old 09-09-2007, 06:10 PM
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3.0L Idle Air Valve Problems

I bought a 98 4x4 Ranger last weekend. It runs pretty well except I have an idle problem. It seems to want to idle a little high when itís just setting. Also when coming up to say a stop light or cruising though a parking lot it doesn't want to settle down. It acts like the throttle is getting stuck but I've checked everything and mechanically it's fine. I also thought vacuum leak or PVC. I checked all of my lines and the valve but I couldn't find a leak and the valve seems to be working. Now after doing a search on here I got to thinking it might be the Idle Air Control Valve. I replace it this morning and it didn't change a thing. Then as a goof, just to see what would happen, I unplugged the valve and drove around like that. The truck ran great! Just like it should. It's a little fussy on startup and the Check Engine Light came on but other than that it runs like it champ. So my question is how is the valve controlled. I know it gets is signal from the ECM but how does it know how to adjust it. Is it possible that there is a messed up sensor that is telling the ECM the wrong thing or is the computer jacked up.

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Old 09-09-2007, 06:28 PM
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Did you reset the computer after replacing the IAC?????
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Old 09-09-2007, 06:57 PM
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No, but I'll go disconnect the battery now and let it set for a little while.
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Old 09-10-2007, 06:54 PM
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Okay, I let it sit for 24 hours. I just reconnetted the battery and took it for a drive. There wasn't a change. It's still doing the samething that it was.
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Old 09-11-2007, 09:04 AM
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OK, have you tested the new IAC????? Just because it's new, doesn't mean it's good, I've seen bad parts right out of the box.

Pull the electrical connector & test the IAC solenoids windings resistance. It should be between 6-10 ohms, with the meters + lead on the VPWR pin & the - lead on the IAC pin. There is a diode in series with the IAC's winding, so it's sensitive to test lead polarity. Post the resistance redings you get.

You should have at least 10.5 volts between the electrical connectors VPWR pin & ground at KOEO.

The computer sends a pulsed signal to the IAC solenoid to cause it to open & close.
The "duty cycle" or number of pulses it recieves will determne the average time it's open & bleeding air around the throttle body, to control the idle speed.

If the new IAC's internal diode is bad it could cause it to act out.

Seeing as how you say it doesn't act out when the IAC is disconnected, it sounds like either the IAC is faulty, or the signal it's recieving from the computer is corrupt. If it's the latter, then something else is fibbing to the computer & causing it to miscontrol the IAC, so maybe the computer trouble code scan will tattle on the culprit.

Let us know what you find.
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Last edited by pawpaw; 09-11-2007 at 09:07 AM.
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Old 09-11-2007, 09:38 PM
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Make sure you clean the connectors too. It may be possible to set the hard idle on the 4.0 (it's easy on the 5.0). Try this:

Run the throttle stop screw in several turns

Disconnect the idle bypass valve

start truck and adjust idle to desired setting with just the screw (make sure truck is warm and in close loop, thermostat will be open)

shut truck off and disconnect battery

reconnect idle bypass valve

after at least 10 minutes reconnect battery

start and drive the truck

What this will do is let the computer use the idle bypass for variances in idle and not rely on it to maintain the idle. Variances are compressor kicking on etc....

I have done this many times on a 5.0. It will work on any ford eec iv ro eec v that has a screw for a mechanical throttle adjustment.
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Old 09-16-2007, 09:13 AM
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Sorry it took me so long to get back, I got pulled away because of work.

Super, I tried to reset it just like you said. I tried three times and there wasn't a change.

PawPaw, I checked the resistance of the IAC. The original would get up to 10.5 ohms. When I checked the new one it come in at 11.5. I took it back and got another one. It read 12 ohms and I havenít installed it yet. The stock one seems to work the best and at least makes the truck drivable.

Voltage coming out of the plug seemed to be on the money. 10.5-11 when the truck wasnít running, 13 running--the same I was getting off the battery (yes I know my battery is getting weak).

There wasn't a trouble code stored in the computer. I had reset it a few days before checking it but I also been driving it up to that time.

So I'm still thinking I have a sensor out of whack. With as many miles that are on this truck I was thinking about doing a "sensor overhaul" anyway. It wouldn't cost that much money just to replace all of them and considering that I need this truck to be reliable it couldn't hurt. The only thing is I hate to be a parts swapper. I pride myself on being a competent mechanic and I should be able to troubleshoot this problem.

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Old 09-16-2007, 11:57 AM
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OK, good feedback.

The IAC's resistance should be between 6-10 ohms & is test lead polarity sensitive. as I noted above.
Are the ones your measuring Motorcraft IAC's, or some afermarket brand?????

Your right about the battery, if it's KOEO reading is 10.5-11.0 volts it's discharged & either needs to be recharged, or replaced.

Low battery voltage can cause all sorts of mischief on computer controlled vehicles, so I'd address this BEFORE beginning to throw parts at the problem.

Either have the battery recharged & load tested, or replace it.

Once you get that squared away, do a idle relearn for the computer. I use the abbreviated one called out for my 94 Taurus & it works fine for my 99 4.0L Ranger.

Start the engine cold & let it idle down some, then shift into ALL gears, including "R", then let it idle, undriven, until it fully warms up on the temp gauge, then shift into ALL the gears again.

This will have the computer quickly relearn it's it's cold & warm start, idle strategy.

The remainder of what it needs to relearn it can do while driving over time.

If your experiencing other than idle driveability problems, you can go through Fords specific relearn sequence, which takes a little more time on the road & requires specific accelleration runs & braking sequences, ect, to have the computer learn all the remainder of what it needs to remember.

So I'd address the known battery problem first, then do the idle relearn & see how it may be affecting the idle speed anomaly.

If there is no change, then have the computer scanned for "pending" trouble codes that might be stored, but not matured to the point of turining the CEL trouble light on, as they may offer up some good troubleshooting clues.

If you don't have a scantool, or code reader, most autoparts stores nowdays will scan the computer at no cost, if your having poblems, even if the CEL is off. Some won't pull codes, unless the CEL is on, so you could just pull the IAC connector to turn the CEL on (wink, wink), then go get the codes scanned. "Any port in a storm" RIGHT!!!! lol

If the trouble code scan provides naught, then inspect more carefully for a vacuum leak somewhere. This could range from the Heater/AC controls, to the vapor recovery system, to a gummed up/stuck open PCV valve, to leaky intake manafold gaskets.
In fact if the PCV valve is due, or past due for change, replace it.

Any of these things can upset the IAC's idle air bleed balance & confuse the computer.

A dirty MAF sensor can muddy the waters too!!!!

Just some more troubleshooting thoughts for pondering.
Let us know how it goes.
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  #9  
Old 09-17-2007, 02:23 PM
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How far open is that butterfly you don't want it open too much I am thinking someone turned up the hard stop to compensate for the flagging IAC, Back it down with the IAC disconnected until it's about 550 600 rpm and then reconnect the IAC, If that's the case! Definately clean your MAF especially if you run a K&N filter!! If that doesn't do it take your multimeter and check out your TPS if it's got a scratchy spot that is throwing off the reading it won't idle right!! If you really want to throw sensors at it just get the TPS first and see how it does!! And run some gas line dryer through it!!!
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Old 09-17-2007, 02:23 PM
 
 
 
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