Sorry guys, this is a long one. For reference - truck is a 1996 F-150 w/ 4.9L and Mazda 5 spd.

So I'm on the hunt for what is making my truck have a slightly sporadic high idle at operating temperature or while driving. Cold idle after just starting the truck is 1000 rpm. Operating temp. idle is 1100. When I'm on the road and I shift to neutral to come to a stop, idle is always at 1500 until maybe 2-3 seconds after I come to a complete stop, then it drops to 1100.

I'm wondering if this issue is related to the idle air control mechanism on the throttle body. When I unplug it while the truck is idling rpm drops from 1100 to 900-1000. Removing the IAC mechanism all together and taping up the hole on the throttle body gets me to around 700-800 rpm, which I'd consider normal. I have not driven with it like this, so I don't know if it solves my coasting idle issue.

Additionally, I plugged the IAC in with it unbolted and tried restarting the truck, idling it, and revving it. The mechanism made some clicking noises, but nothing moved inside it.

Is this normal, or does it sound like the part is malfunctioning? From what I understand, with a working IAC you can set the throttle plates to let less air in and bring down the idle, but I'm not even sure if mine is still functional. It looks to be the original part that came with the truck in '96. Is there any way to test it or should I just get a new one and go from there? TB has been cleaned out and the gasket has been replaced. IAC has been sprayed inside with TB cleaner. Could the TPS have any fault in this?

Normally I would just google my questions but honestly I'm confused by some of the responses and would rather just ask and be done with it. This is all new to me but with each problem my big white money pit has, more I learn about 1990's Ford engineering and how it all fits together

 

Oh and before anyone brings it up: I've spent probably the last month checking every line on the truck for vacuum leaks. I've used starting fluid, WD-40, you name it. Vacuum lines, reservoirs, throttle body, intake gaskets. I'm fairly certain it isn't a vacuum leak, but then again who even knows.

 

First Run your KOEO codes and see what you get. https://www.ford-trucks.com/forums/1...n-highway.html (second post)

Next check all your vacuum lines. Chart should be on inside of hood. PVC hose is hard to get to but always a likely suspect for vac leaks. Plenty of info on search about how to check for vac leaks, Try these tips and when you are convinced you are good then check back on this same post and you will get plenty of help at that point. Sandy

 

With the IAC valve out and the hole plugged up, the truck should not idle or idle very slowly at best. It's getting air from somewhere else. If you are sure you have no vac leaks, then it's getting past the throttle blades. Have you adjusted the base idle screw? With IAC valve in place and unplugged, adjust the base idle screw to 400 RPM.

On my last 300 (1991 model) I also had a constant high-idle problem and could not get base idle down to 400 RPM. The throttle blades had holes drilled in them like a 70s carburetor. I tapped the holes and put machine screws in them and that fixed my problem.

 

One thing I did that was suggested by VJ was I put duct tape over the throttle body inlets. Of course it will not run like that but we were testing. I put a quarter size hole in one side of the tape and then kept adding tape over the hole until the idle went to where I wanted. After that showed me that blocking some air to the TB helped, I then plugged the holes in the blades with J/B weld and my idle is still a little high but much better than it was. I recommend this be tried before adjusting the screw. If you have to end up adjusting it, mark it so you can go back to original setting if need be. Sandy

 

The screw should not need to be adjusted. It should work as it came from the factory. I have had several (a few from the junkyard) OEM throttle bodies that were worn out. The throttle shaft had play in it and was allowing air past the throttle blades similar to what happens in a carburetor.

 

Sandy - thanks for the tip. My '96 has OBDII and when I hooked up the scanner and pulled codes this morning the only one was for my Secondary Air Injection. Something about incorrect flow. My VECI sticker with all the emissions lines is long gone, so I've been relying on internet diagrams. I'll pull one up again and check all the lines another time. Tomorrow I'll try the duct tape trick and see what that gets me. I appreciate the help!

Jas88 - I appreciate your insight. Its concerning that the truck will still run normally with the IAC blocked off, but for the life of me I can't figure out where the air is leaking in from. Tomorrow I'll try the throttle plate duct tape trick and see if that helps out any. I'm also gonna go back through all the vacuum lines as well. If that fails, I'll adjust the screw.

My4Fordtrucks - Thank you for your input. I would like to avoid messing with the screw if at all possible. I don't remember how the inside of the throttle body was and how worn out the plates were. Any way to check for worn plates and shaft play with it still bolted up to the intake or is it gonna have to come off?

 

My4Fordtrucks - Thank you for your input. I would like to avoid messing with the screw if at all possible. I don't remember how the inside of the throttle body was and how worn out the plates were. Any way to check for worn plates and shaft play with it still bolted up to the intake or is it gonna have to come off?[/QUOTE]

I had to take mine off to see if light would show through. There should be no light coming through but with the age of our trucks, I think you will always see some. Any large gap will not help your problem. You will need to pull the TB if the duct tape works anyway. Always replace you TB and IAC gaskets. TB gaskets turn to trash. Don't stick your fingers in the throttle blades with the motor running or you could be wearing a four finger glove at Christmas. Sandy

 

These are old trucks and you do not know what any of the POs have done so I would not come from the position that the screw does not need to be adjusted. Do not fall into the trap that this is some magical screw that should never be touched. It's there to set what is called the base idle as I described, there was even a spec for it on the early-80s engines (normally 400 RPM), but people would use it to adjust the actual all-the-time idle speed so Ford then put Locktite on it and said "do not adjust this ever".

 

As one poster suggested about the throttle body, you need to pull it and closely look at it for wear in the bores where the blades close.

Also, I have read in the past the past, the TB gasket on the 4.9 blows out. Needless to say, when I had the exact same problem as you, when I pulled mine off, it was blown out.

 

On a couple of my throttle bodies, I have found that I can move the linkage up and down and increase or decrease idle speed. Wiggling the linkage or lightly pushing the stop against the screw will also change the speed. This leads me to believe that the shaft is worn out in the bushings causing 1) unmetered air to pass by and 2) the butterflies to bind up in the bore. Replacing it with another better used throttle body or a new one has solve my idle issues.

 

All that screw does that I could see is it opens and closes the TB blades. You get the screw to tight and the blades will not move. If you adjust it just so the blades barely open, they can bind and go WFO. So, I stay away from bottoming out the screw to lower the idle. I also took my blades out to clean them as the body is not supposed to be cleaned. I used #4 steel wool to LIGHTLY polish the edges of the blades so they move smoothly in the bore.
If you have big air gaps in the blades you can take your TB with you to to Junk yard and pull some TB's and if blades look good take em out and try them in yours to see if they seal better. If so, just buy the blades and if not buy the TB. Sandy

 

The screw I am talking about has nothing to do with the tension on the TB shaft.

 

just curious. You are not talking about the one that will adjust idle that is factory set? If not, which one is it. I can always use help. Thanks Sandy

 

Yes, that IS the one I am talking about, but it has nothing to do with tension on the throttle shaft. It's an idle stop screw, just like on a carburetor and only controls the opening of the throttle blades when the gas pedal is released.