Idle Air Controller

My 93 vert is doing a new trick! Sometimes during long idle periods, (traffic light, traffic jam, etc...), it will stumble and start idling rough. It will continue idling rough until I shut the car off and restart it. And if the a/c is on, it acts like it wants to stall. I know the IAC is what contols idle speed when accessories like a/c are on, but it also does this when the a/c is off. Has anyone had this problem from their 5.0? I could use some suggestions...thanks. [img]/phpBB/images/smiles/icon_razz.gif[/img]

On mine it would start & idle about 1500 rpm & then die. Then it would act fine. Power was down as well as economy. Bought a new IAC valve & fixed it. You could easily see why mine was bad.
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Now that you mention it, mine has done that as well! I have an AOD and sometimes it will start and run strong, but when you pull it into Drive, it will die. Then it starts and runs fine.

Try cleaning the throttle body and IAC first. ..if that don't do it, the changing it might help. Some times adjusting the TPS (throttle position sensor) helps.

All right, I pulled off the IAC and examined it. It didn't look too bad...a little black, but no buildup. I also checked the resistance on the pins in the connector, (10.3 ohms), seems to me that it is probably something else. A guy down the street said that it could be my 10-pin connectors...does this sound feasible? He also gave me a plate that goes between the IAC and the throttle body and has two allen screews that control the amount of air going around the throttle plate. He said that with that I could control my own idle speed with this. Has anyone used one before?[img]/phpBB/images/smiles/icon_confused.gif[/img]
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That "plate" was installed on the later model 5.0 liters to "trim" the idle. I put one on mine, it seemed to help, but I couldn't be sure...course, I have abarely smog legal cam, so the difference is probably disguised by the cam profile.

I have talked to many other guys, with stock to near stock engines and they have had good results. The plate lets in a controlled amount of air ... the "bypass", part of the name...to give the throttle a chance to modulate the air flow at a slightly high point.

You may have your ohm meter set incorrectly, reading on the wrong scale...it should be reading between 0.900 and 1.1 ohms. For performance, most articles I have read suggest it be set at just under 1.0...something like 0.995-0.999. Other people swear it should be set at 1.001 to 1.05. Well, nothing new there...anything you get 2 ford guys together, you will get 3 opinions on every subject.[img]/phpBB/images/smiles/icon_wink.gif[/img]

The bypass was to stop peole from having to drill a hole in the throttle plate. Use the bypass to adjust idle, then trim it out with the stock idle adjustment. There should be a "how-to-sheet" included with the plate.

<font size=-1>[ This Message was edited by: Beoweolf on 6/21/04 3:07am ]</font>

First, the idle air bypass valve may not look too dirty, but even small amounts of crud affect it and cleaning with gummout should be a regular part of any tune-up. A clean bypass valve will give you a much smoother idle that snaps right back to where it's supposed to be, and handle the cycling of the a/c compressor with ease. Modified engines benefit even more, since the idle tends to get touchy as you add mods. Note that after '93 and on earlier MN12 platform T-birds Ford used a similar black plastic idle air bypass valve that cannot be cleaned with strong solvents - I know because I melted one. But the regular '86-'93 5.0 Mustang metal valve can be cleaned no problem. I also separate the electric solenoid/motor part before I hose the valve with gummout, and wiggle the exposed shaft around to work the crud off the guts. A new gasket is probably called for, although I have gotten away without it (assuming the existing one isn't cracked and does not disintegrate when you remove it. If it does, a new gasket is definitely in order).

As to the air bleed plate, it was a factory service part that was designed to allow dealer mechanics to raise the idle on high mileage engines without having to go through the resetting of the TPS, or for those with plastic bypass valves that couldn't be cleaned. Once installed, idle speed can be changed by set screws without touching the regular idle speed screw, which also changes the TPS setting. I have one installed because it does allow you to tweak the idle speed without touching the TPS setting. I'll try and post the part number if I remember -- it was about $12.

On TPS setting, I do not buy into the "get it as close to 1.0 without going over" theory. I started out dialing in .97, .98 and .99's, as well as a bunch of other settings but found I didn't like the idle characteristics. I found it soft and prone to hunting. It was a matter of lots of trial and error before I settled on .54, which gives me the snappiest idle. But I have an E303 cam, GT40 heads and other mods. It's my thinking that every car will have a different "best" TPS setting (of course staying under 1.0, which I believe is the spot where the computer recognizes an off-idle situation). But it took me hours of trial and error to get there, which is why the bleed plate is good -- I haven't touched the idle screw or the TPS in years, although I regularly clean the bypass valve and tweak the idle.

Also, I clean the throttle throat and butterfly on my '90, although I believe that the factory says don't because it's coated with something or other. I don't hose it out, but I do use gummout on paper towels to get all the EGR crud off the throat as far as my fingers will allow, and on the butterfly. It's another thing I have found to steady a soft idle. If there was a coating on my throttle body, it's long gone now and I haven't seen any harm from cleaning it (I've got 75k on this motor and 170k on the car, including the throttle body and injection).

Finally, I also remove the EGR valve and clean the pintle (i.e., the actual opening and closing part of the valve). Gummout again, and wiggle that cone-shaped thing around to work that crud off. This has also contributed greatly to improving a bad idle on occasion. I think you will need a new gasket here for sure, because the old one bakes on, and flakes off. Be sure to check the holding ability of the diaphram, by forcing the valve open and putting your finger on the vacuum port (the valve should remain open until you remove your finger).

Hey y'all,

Thanks for the great info. I bought one of those actron scanners last nite for the OBD1. I probably wasted the money though seeing as I haven't gotten a check engine light so probably won't have any codes to pull. It's frustrating trying to troubleshoot an engine w/so many components that I don't understand. Until I bought this car, I only ever owned/maintained/rebuilt a carbureated 302, two 351's, and the 460 I'm working on now. Also bought a book, "The Official Mustang 5.0", by Al Kirschenbaum. Maybe studying this will enlighten me! If nothing else, it's full of interesting stuff...

SamSnyder,
Great ideas! As soon as it stops raining, I'm running to town to buy the gaskets and gumout and try every one of them. BTW, the part number on the plate I have is F2PE-9F942-AA. Not sure if that's a Ford P/N or aftermarket. But I'm gonna try to get 'er done without using that. The guy that gave it to me has a highly modified '95 5.0. Mine is stock, (save the explorer heads and Flowmaster cat-backs that the previous owner installed), so I don't see why I should need to modify it just to get it to run right...especially when until two days ago, it ran great.

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Check the salt and pepper shaker connectors at the back of the manifold. Clean them with electrical connection cleaner as the grease inside them gets hard and prevents a good connection. I removed the dealer installed extension harness, cleaned the male and female connectors and now have a rock steady 900 rpm idle.

Wow did I screw up!!!! To make a long story short, my patience ran out about the same time as the money, so I took my car to a mechanic friend of mine north of here. The codes I pulled were actually 41 and 91! Not 14 and 19! So after I replaced the IAC, (2)o2 sensors, the dist., wires, cap/rotor and plugs, cleaned out the EGR valve and spacer plate, throttle body and MAF sensor, replaced the gaskets for all this stuff and advanced the timing...turns out it was a small vacuum hose waaay under the upper intake plenum that had a split in it!!!

Thanks for all your input just the same...guess it wasn't a total loss - runs better than ever!!![img]/phpBB/images/smiles/icon_razz.gif[/img]
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