High idle, pretty stumped

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Yeret

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Ok all, I made mention to this in the GD topic but I think it's about time to make a thread because this issue is starting to irritate me.

Starting about three weeks back, every once in a great while when I shift the tranny into Park, the engine idles up to 1,000 RPM and holds. It drops and rises during this time like the ECU is trying to pull it down but is not succeeding. So, I decided to replace the IAC, which I suspect was faulty anyway due to unresolved cold-start stalling issues last winter. The engine started and idled no problemo, so I decided to rev it. Lo and behold, the engine did not return to it's normal idle of 650 RPM, but instead stuck at friggin' 1,000 RPM again. I revved the engine a little and nothing changed so I revved the engine to 3,000 RPM and what do you know, it returned to proper idle.

So, I spent the next little while revving the engine to see if the issue would happen again. And it did but there was no predicting when it would. It was totally random when the engine would return to a high idle rather than a normal idle.

Earlier this year, I fiddled with the idle screw on my modified throttle body and had no problems with the setting until now. So, I reversed what I did and the issue was improved but the engine still wanted to high idle on occasion. Went for a drive earlier today and when I stopped at the station, the idle was fine. When I got home, however, the engine high-idled again!

This issue has me totally stumped. New IAC, fiddled with the throttle body and the problem as of now remains unresolved. Starting to bug me. What do y'all think? Vacuum leak? Electrical bug? I've got my old throttle body in a box somewhere. If I can find it, I might try putting it back on and see if that does anything if I can't figure this out.

If it could possibly make a difference, this problem started about one week after I got my shorties installed.
 

dudeman2009

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The engine cant rev if its not getting enough air, something is causing it to open the IAC to hold that RPM, which it will do on cold start for a few seconds until things warm up. A vacuum leak unless its huge (like pulling the brake booster) will not affect the idle much at all. If it only happened after shorty headers then I would suspect something to do with what the O2 sensors are reporting and how your tune is deciding to act. Do you have a scantool that will read O2 info and fuel system information? I'd be checking to see if its going open loop when this is happening, and what the O2 sensors are outputting. I'd also be curious to see what the engine temp is reporting via the scantool.

I doubt its anything with the intake, especially since its intermittent its gotta be the computer reacting to a sensor reading somewhere. You might also try the APPS reset procedure and see what the TPS readings are.
 

SYKRAMMAN

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^+1, Yes I would definitely look at the o2's, might need to get a couple non foulers.
 

goodtodoo

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You guys have helped me lots getting my 3 1996 trucks running, I,m thinking. The iac you got isn,t good. Hard to say.
 

Shadow_Death

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I literally cleaned my TB because of my IAC getting stuck open. Turns out there was 122k miles worth of grime built up in there and you could see the lines where the IAC would rest open and closed.
 
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Yeret

Yeret

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Just tried that adjustment procedure. Ended up turning the screw back in at least a few whole turns. Couldn't get as precise as 200 above command (which would have been 850). It was either settle on ~750 or the engine would rev up and hang on to 1,200. Seems to be okay now but I'll drive the truck for a few days before I come to any conclusions.
 

goodtodoo

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I am not understanding this, what did you adjust?
 
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Yeret

Yeret

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goodtodoo: I adjusted the idle screw which holds the throttle blades very slightly open. Normally, this screw is set at the factory and then a plug is pressed over it to prevent it from being fiddled with. However, the TB that I bought had this plug removed.

Pepper: I bought the TB from a guy on eBay who used to sell these regularly, don't remember his name now, though the purchase might still be in my purchase history. Shortly after I bought it, he just disappeared. Maybe he lost access to the machine shop where he says he modified the throttle bodies?
 

dapepper9

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probably Brian Whitely/blkdakotart or somethinh like that for a screen name. He used to do em and he did pretty much disappear. Prison will do that lol
 
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Yeret

Yeret

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That name rings a bell. I'm pretty sure that that's the guy I got it from.

Been a couple days now and...everything seems okay! There was one time where the idle seemed to be a smidge high and rough (could definitely hear it in the exhaust). This occurred while the engine was cold and I had made a very short jaunt from the Wal-Mart parking lot to the Quik Star lot across the street. But overall, unless I come across something else, I think this is as good as I'm going to get. It would be tempting to just nab an F&B 52mm, which I kind of want to do anyway, but there's absolutely no way that I can justify dropping $400 on a throttle body right now, LOL.
 

dapepper9

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Going with larger throttle body you'd be dumb to with f&b for a 52mm. Hipotek machines factory cores to 52mm AND 53mm for less than 200 if i remember correctly. F&b isn't worth unless you go to 52x55 or bigger
 
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Yeret

Yeret

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I was under the impression that OEM units start having warping issues if you punch them to 52mm, let alone 53?

Also, any opinion on The Fastman throttle bodies? They're in the same price range, overall.
 
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dapepper9

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I was under the impression that OEM units start having warping issues if you punch them to 52mm, let alone 53?

Also, any opinion on The Fastman throttle bodies? They're in the same price range, overall.

Warping no but the bore will rupture if you go past 53mm. You're not going to warp cast aluminum. I trust hipoteks machining more but i haven't heard anything bad about fastman
 
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Yeret

Yeret

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Dammit, I might just be back at ground zero again. So, earlier, I carted some crap up to the trash pick-up spot. Engine fired, idled and ran just dandy. Nearing the end of the lane, I decided, being a bit intoxicated, to dig into the throttle and let her eat a bit. I hit the gas, accelerated right up and then, nearing the end, suddenly let off the gas and hit the brakes. As the truck neared stopping, I could hear a noticable hissing from the IAC (I have an open-element intake, so all TB sounds are easy to hear) and the truck was pulling oddly hard. While stopped, in gear, the idle was holding 900 RPM. Shifting into Park, the idle pulled right up to 1,200 RPM and settled at 1,000 with absolutely no attempt to idle down.

So, being irritated, I performed the exact same action in an attempt to "draw out" the issue. After two attempts, nothing odd happened but after driving a little hard up my short, steep hill, I stopped the truck at the crest and sure enough, the idle sat high again.

At some point, I felt a sort of "flat" spot at about 1,900 RPM under moderate throttle. In other words, the engine accelerated but kind of just stopped for a split second at around 1,900 RPM and then accelerated again. My "butt sensors" have noticed this happening on occasion whenever I punch the gas to accelerate quickly, especially when the engine is cold. This seems to have occurred both before and after my most recent adjustment to the throttle body. Not sure if this could be related to this PITA idle problem but I may as well throw it out there.
 
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Yeret

Yeret

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Alright, I gotta admit, this issue is starting to really irritate me. I decided to just unplug the battery the other day to let the ECU reset and I left it unplugged overnight just to make sure that everything was clear. I hooked the battery back up and I swear, the problem is even worse than it was before!

It rained all day today, so I couldn't really do anything. Tomorrow, I'm thinking to dig through my parts boxes and find my old parts (MAP, TPS in particular) and hook 'em up and see if that does anything. If that does nothing, I might swap on the old OEM throttle body. If I still can't get resolution, I might just unplug the IAC and drive without it to see if that helps at all, since the IAC hanging open seems to be causing this problem to begin with.

I've done running tests with the Equus hooked up and haven't noticed anything unusual. The only thing that seems odd to me is the MAP reading of 11-12 inHG at idle but that's only because I've read that the factory setup should yield around 10 inHG at idle. But then, I've got a lot of aftermarket stuff involving the air intake and crankcase ventilation but I've had this stuff for a long time and it's never caused any issues.

The fact that I've modified a lot of the stuff on the engine makes things more complicated but I've never had problems...until I installed my shorties. Honestly, this problem arised after I had them put on but I cannot for the life of me figure out how headers would affect the engine idle. Really sucks because I'm very satisfied with the performance increases from them.

There is one little thing, I guess. When I requested a tune from Ryan, I noted that I didn't have headers because I didn't at the time. Now I do. I had kind of figured that at the time, he just sent me a tune factoring in headers and the ECU would simply adapt if I added them later. Could this be something to consider?
 

dapepper9

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His tune is for full stock to full bolt-on. You said you had luck with opening the idle screw a tad before. Might need to do that incrementally, 1/8 turn at a time then drive for a few days.

Another possible problem/solution:
A while back i had similar issues with my 52x55mm f&b billet throttle body. Apparently they have some issues created by a lack of bearing support in the center of the throttle shaft like factory has. Supposedly the guy who makes em just uses an anti-seize and eventually it dries up. Many guys will hit it with wd40 which soaks in and re-moistens that dried anti-seize. Lasts a few weeks or months depending on driving but comes back. Idle will stick coming to a stop and rpm it picks is random. I was having similar issues and one time it happened to me i left the truck in gear and running, hit the hood latch, put it in park and quickly bailed while idle ramped up to 2200. I popped the hood to inspect the throttle body quickly. I pushed on the throttle manually as if to close it and got a little over an 1/8" of movement and suddenly it idled normal. Goosed it by hand and it diff it again and same thing fixed it. For like 2 weeks i tried various things like shaving the stop on the lever because it almost looked like it was catching, bending the lever, swapping springs lubricating. Nothing worked. Finally i unhooked everything, took it in the house, cleaned everything, removed the throttle blades and noticed what looked like wear on the bore walls. Said **** it, got some 320g and 400g paper out and went to work on the edges carefully. Shaved em a hair and then knife edged em. Had a 52mm f&b as a spare in case i messed it up so i wasn't worried. Worked perfectly. Didn't see the issue for several weeks, sold the 52mm, haven't looked back. At the time I'd forgotten about the idle set screw and that could have maybe helped me but as of yet I'm happy with the route i went. Currently flawless. May pull your tb and just do a basic inspection of everything
 
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Yeret

Yeret

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I might just do that. Might have to wait until tomorrow though since it's supposed to rain all day again.

Another thing maybe worth noting is that whenever the idle hangs up, I can reliably kick it down by revving the engine to at least 2,500 RPM. Anything less does nothing and the idle returns to 1,000 again but if I snap the throttle and rev it to at least 2,500, it settles back to normal.
 
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