5.0 Bronco wont idle down

[old55pete]

Been trying to figure this one out on my own and it's got me stumped. Start it first thing in the morning and it runs on high idle for a couple of minutes and idles down, that as it is supposed to be. As soon as I hit the foot feed, there is a mechanical popping noise and it goes to 1100 - 1500 rpm and stays there for 15 to 20 minutes then idles down to 850 rpm. I ran a KOEO and a KOER and got no codes. I ran them two times, once with an OTC Genises and a Snap on Modis. Nothing even came up in the KAM. I have changed the thermostat and the throttle cable, I ran it with the cruse cable and down shift cable unhooked. Same thing. The TPS is reading at 0.98 volts. The only thing that seams to make a difference is if I un plug the throttle air by pass valve, then it idles down where it is supposed to be. As soon as I plug it back in, back to 1500 rpm for 15 to 20 minutes.

I am stumped here, even a smoke test showed no vacume leaks. I need some help here as there is something that I am missing.

[Ray Cecil]

Im reaching back into my past...had an 86 5.0 efi Speed density also...seems like I had a similar issue once. Isnt the throttle air bypass valve you speak of called an IAC solenoid? Maybe that is malfunctioning. Im sure you can test the voltage coming to it. I'll see if I can find something.

[old55pete]

Yes sir, some folks call it the IAC valve or the IAC motor. I am guessing that where your standing and to whom you are talking is the difference in the names.

And thank you in advance for your help.

[Whisler]

From what I have experienced that is the classic fault for a dirty IAC. Take it out and clean it and see if that helps.

[Rusty_S85]

Sounds like a sticky IAC.  what you can do is get it to get stuck on high idle again then take a hammer and lightly tap the IAC and see if it instantly drops to idle.  if it does I wouldn't waste time with trying to clean it I would just replace it.

[Ray Cecil]

Rusty_S85 wrote

Sounds like a sticky IAC.  what you can do is get it to get stuck on high idle again then take a hammer and lightly tap the IAC and see if it instantly drops to idle.  if it does I wouldn't waste time with trying to clean it I would just replace it.

Thats what I am thinking. But, before you replace it, make sure all your grounds are good. Battery cable might look good but could be deteriorated inside the insulation.

[Rusty_S85]

Yep grounds are important on something like an EFI system.

Why I always suggest adding grounds on old EFI vehicles.  I prefer the woven strap grounds as they are more flexible, don't corrode like standard battery cables.

[Ray Cecil]

Rusty_S85 wrote

Yep grounds are important on something like an EFI system.

Why I always suggest adding grounds on old EFI vehicles.  I prefer the woven strap grounds as they are more flexible, don't corrode like standard battery cables.

A ground strap has solved more than just a couple of issues for me over the years. I too like the woven straps. Usually anytime I order parts from jegs I throw in a couple ground straps into the order. I use them on my stuff, and friends vehicles.

[Rusty_S85]

Yep I had a problem with one of my tail lights being dimmer than the other on my '82 I added a flat woven ground cable from bed to frame no dice.  For some reason I took the lens off and took the bulb out for something and found out it wasn't an 1157 bulb it was the resistor bulb the 1257 I believe its called which we stock at work for the 90`s vehicles with ABS systems they have a lower candle power rating.  But I left the ground strap on but going to be replacing it with a better ground strap as it was a small cheapie from the local autopart store.

[NZCOBRA]

Hi pete , if none of that works , i had some really high idle issues and it was a plug down beside the  trans , i suppose it tells the computor that the trans is in gear , i ended up disconnecting it and  putting a jumper in . That was one of my high idle experiences!  In my truck,   Crazy computors , also watch that pcv valve it supplys a lot of air and is a percentage of the idle air , a new one that i bought flowed so much air the valve inside sounded like a small motor . , but that would  be more constant high idle , anyway  I used a plastic body pcv that seemed to flow less this meant i could increase my base idle to give the iac less to do , and  my truck wouldn't surge and die .stuff to think about . Cheers

[old55pete]

Thank you gentlemen, I will give it all a try and let you know what worked. It might be a spell as I am in the middle of selling my business. Thank you all.

[Gearhead]

IAC's were a big problem. Never heard of one making a mechanical popping sound. Has anyone figured out where that noise is coming from? Typically the IAC  would sludge up, and so would the throttle plate, due to the crank vent hose location causing higher idle. The sludge is best cleaned by removing the IAC and cleaning it out with carb clean. I believe that prior to 89 the throttle plate could be cleaned with carb clean, after that there was a coating on the throttle plate the carb clean would eat off causing idle issues. You cleaned those by removing them and scrubbing with soap and water. The noise could be the big clue see if you can find where it's coming from.
Another thing causing elevated idle would be a vacuum leak, but those usually are high and stay that way.

[old55pete]

Sorry I haven't done anything with this in a while. It has been a wild couple of months to say the least.

As a couple of you stated, I first checked the grounds and found a couple that needed attention. One was the brand new battery cable on the neg post. Im not sure whats up with that, but after cleaning the inside with a terminal brush, I tested with a multimeter and found it to have no resestance.

I also found the ground between the fire wall and the back corner of the intake was broken and repaired that too

None of these fixed the problem. I again smoke tested it and dident find a single leak so I moved in to the IAC or idle air control valve. I pulled it off and for the most part it was clean as a whistle, both inside the valve and the ports into the throttle body. I sprayed some PB Blaster into the valve and it worked fine for a couple of days and then went back to the same old story. I found a NOS IAC on ebay thanks to Garys extencive parts lists and installed it today, along with a new throttle cable. Another story for another time as I need to sort that one out

[old55pete]

Sorry I forgot, the popping sound when you push the foot feed, I took it to an AC shop to have it sucked down and recharged so it would work right, it would blow cool but not cold. While they had it they took the cover off of the throttle cable and tried to adjust the idle down by the screw by the cables. The screw wasent even touching the tang and was letting the butterflys jamb closed. With the engine off and the key on I hooked a multimeter to the TPS and adjusted it until I got one volt. That is also now solved.

PS

They gave me an estimet for $3000.00 to repair the AC, they wanted to rebuild the whole system. Sad part was it was working. What a ripoff joint that turned out to be.

[Gary Lewis]

Wow!  $3000!!!!  You can buy a running truck for that!

But glad you got the problem solved with the idle.

[old55pete]

Yes sir, the AC shop dident much like it when I told them that I already owned the car and all I wanted was the system evacuated and re charged, a simple enough request I thought.