I have a 1994 F150 4x4 with a stock 5.8L engine (about 108k miles) that starts and idles well when cold (not up to normal temp.) but when warm, it idles too slow, to the point that it dies, especially with A/C running. It will generally always restart and then require a bit of throttle, but once under way it runs great. I've checked and no codes are set. Using a Haynes manual, I followed the procedure to checked the Idle Air Control (IAC). First thing they recommend is checking the voltage from the PCM (key on engine off), should be about 10.5 volts; mine reads 12.33 volts. I pulled the IAC off and cleaned with carb cleaner, it didn't look too bad, but I noticed that the "rubber" looking valve wasn't completely closed, so I gently pushed with a screw driver and "shut" it. Put all back together and it seemed like the idle was even slower (~450 rpm). If I opened the throttle by hand just a little the engine would fluctuate in rpms. finally I adjusted the stop for the throttle so the engine would a least idle at about 500 rpm. First question, is the voltage from the PCM way too high (12.33 instead of "about" 10.5 volt)? If this is the problem what do I do about it (replace PCM)? Second, is it more likely that I need to replace the IAC? The resistance check on the IAC was within spec at 10.5 ohm, but I'm not sure about it not closing. Also the Haynes manual indicates something about an o-ring; is this for other models because mine doesn't have any o-ring??
Thanks,
Bryan

 

Test the circuit wht-lt blu pin 21 pcm harness connector to same at IAC for continuity.

On my 94 I had to run a new wire from the PCM to the IAC due to the same condition, computer had no control of idle/low idle.

Computer did not have connection with the IAC, engine harness plugs into the main harness over on the drivers side inner fender. That circuit did not have continuity through that connector.

New wire run from PCM to IAC, has control over engine idle again.

 

If no continuity, it would seem to me that I would not be able to read a voltage of 12.3V. Can you confirm that you had a voltage reading even though there was no continuity? I can check continuity but it seems that I wouldn't get a voltage reading if there was a continuity problem.

 

Right, no continuity would mean no power there. As for your Haynes manual telling you to look for approx. 10V there... they don't explain anything worth a damn lol.

That valve actually receives full voltage (12.6+) from the engine harness, and the PCM output is connected to the ground side of it. The PCM uses a method called Pulse Width Modulation (PWM) to control the position of the valve.

The valve by itself is only capable of being either open or closed:
No voltage applied = valve 0% open.
Full voltage applied = valve 100% open.

So, the PCM pulses the power on and off to the valve several hundred times per second, which drives the valve to an intermediate position. Adjusting the pulse width (ratio of ON time to OFF time, i.e. duty cycle) adjusts the position of the valve.

I say all that so I can explain to you why the manual says you should see 10V or so. The reason is that when you use a standard digital multimeter to measure the voltage, then due to the pulse width modulation happening here, the meter would average this out and display is as slightly lower than actual voltage. An o'scope would let you see the actual voltage as well as the pulse train output.

Anyhow, I'd say your valve is sticking. Take it off and clean it again with some Berryman's. I use to have to do this ALL the time on my 4.6L.

 

Thanks Dixie460, I think I've got some Sea Foam in the back, I can try that or get some Berryman's if that is the magic juice. I appreciate the info on the operation of the IAC valve and think you are right that it is sticking. I'll let you know.
Bryan

 

Computer provides ground to open valve.

I had key "on" power to IAC yea just like bunruh however that circuit between computer and IAC valve was dead, ground path provided by PCM was dead. The computer could not open the valve via that circuit to allow idle air pass nor high idle engine warm up.

On mine it was dead where it passes through engine harness to main harness connector at DS inner fender well, connector is about 2x2" and 3-4" long or so. Wire might be damaged at some other point on yours or the pin on the computer connector is corroded (be the first thing I'd look for, need pull connector to test circuit anyway)

If needed I could post couple pictures what I did, its just a simple matter of running a new wire though.

 

I took couple pictures a minute so you could see exactly what I was talking about,

First one tap into wht lt-blu wire at pin 21 PCM connector (see black wire coming out from behind cover on connector).



Second one, Tap into same wht lt-blu wire at IAC valve by passing all of the trucks harness.



Third is engine harness to main harness connector, everything on the engine passes through this connector.



Test your circuit first verify the cause of the problem, yours might not be the same issue.
If you end up running a new circuit be sure and solder the connections.

 

danr1, thanks for the pics of your truck and the work you did. I will do some more inspection and continuity checking.

 

Well I wouldn't call Berryman's "magic juice", I reserve that for PB Blaster... that stuff is amazing.

You could try some PB, but Berryman's what I used to have around to clean my IAC valve on the 'ol 4.6L Thunderbird and it always seemed to work pretty good. Either that or if I was lazy I'd just whack it with a screwdriver handle a couple times, that would get it to unstick for the day usually. You could try that and see if the problem goes away for a bit. Just give it a good solid thump or two. I mean, don't bust it off the engine or anything but... you get the idea.

Eventually I got tired of cleaning it and coughed up the $ for a new one. Not that I was trying to be cheap, I was just broke back then due to being newly married (and now divorced ) so non-critical repairs had to wait.

 

Nothing better than PB Blaster on exhaust jobs, it will do amazing things anywhere on the undercarriage.

I spent time trying to free up the IAC with Sea Foam and carburetor cleaner to no avail. I made some pigtail leads so I could apply 12 V across the leads of the IAC valve but other than a small bumping noise, there was no real movement. When I replaced the old IAC, there was no difference between the IAC plugged in compared to unplugged. (See <http: oldfuelinjection.com="">Fuel Injection Technical Library).

I was going to try and check the continuity between the IAC and the PCM as suggested by danr1. However, I tried to remove the plug from the PCM at the firewall and the retaining bolt first loosened and then got tight again. I tried to separate the plug but was unsuccessful, so I just tightened the retaining bolt again.

I decided to go to Autozone and pick up a replacement IAC. After installing, there was a noticeable improvement in idle speed and there was a difference between the new IAC plugged in and unplugged (the engine died). I was happy with that but I didn't have time to drive the truck.

Today I drove the truck about 30 minutes to my destination, then after about 30 minutes I got back to the truck to head home. The truck started but then stumbled and died (bummer ). The engine restarted and with some throttle it cleared and idled again. Apparently there is still something not quite right, or is it a matter of a 20 year old engine not being quite like new?</http:>

 

Loosening the bolt retracts the connector from the PCM, the bolt does not come out of the harness connector it stays with it.

The connector is long and skinny with the bolt dead center, because of that the connector doesn't want to come off perfectly straight all by itself, tends to hang up without little help.

So its...... Loosen bolt till gets harder to turn wiggle connector, Loosen bolt till gets harder to turn wiggle connector, loosen bolt till..... repeat until the connector is free of the computer.

 

This is exactly the kind of information I was needing, thanks danr1!! If I need to disconnect, I will follow your instructions.