old55pete Posted May 3, 2020 Share Posted May 3, 2020 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. Link to comment Share on other sites More sharing options...
Ray Cecil Posted May 3, 2020 Share Posted May 3, 2020 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. Link to comment Share on other sites More sharing options...
old55pete Posted May 3, 2020 Author Share Posted May 3, 2020 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. 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. Link to comment Share on other sites More sharing options...
Whisler Posted May 3, 2020 Share Posted May 3, 2020 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. 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. Link to comment Share on other sites More sharing options...
Rusty_S85 Posted May 4, 2020 Share Posted May 4, 2020 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. Link to comment Share on other sites More sharing options...
Ray Cecil Posted May 4, 2020 Share Posted May 4, 2020 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. Link to comment Share on other sites More sharing options...
Rusty_S85 Posted May 4, 2020 Share Posted May 4, 2020 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. 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. Link to comment Share on other sites More sharing options...
Ray Cecil Posted May 4, 2020 Share Posted May 4, 2020 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. Link to comment Share on other sites More sharing options...
Rusty_S85 Posted May 4, 2020 Share Posted May 4, 2020 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. 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. Link to comment Share on other sites More sharing options...
NZCOBRA Posted May 4, 2020 Share Posted May 4, 2020 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 Link to comment Share on other sites More sharing options...
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