but would first just spray some cleaner in your throttle body. 
Higher than normal idle
Thread Starter
Drifting
Joined: Jan 2011
Posts: 1,853
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From: Hamilton Virginia
Higher than normal idle
My 92 lt1 6speed has a high idle but only for a few seconds. when I am coming to a light and let out the clutch, the idle will be about 1200 rpms. within a couple seconds the idle will start to drop to about 900. Not sure what this is, sounds as if something is sticking. Curious though, what is the warm idle supposed to be ? 650-700 ?
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Try cleaning and resetting your IAC, also check the TPS to be sure its not sticking.
Team Owner
Joined: Jul 2004
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From: Hartford WI
In a L98, I would take the IAC valve off, take the IAC housing off and clean all the passages and reassemble. I suspect you can reset the IAC using a scanner reading the IAC counts like I do with the L98 but can't confirm
Racer
Joined: Aug 2014
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From: Toledo
My 92 lt1 6speed has a high idle but only for a few seconds. when I am coming to a light and let out the clutch, the idle will be about 1200 rpms. within a couple seconds the idle will start to drop to about 900. Not sure what this is, sounds as if something is sticking. Curious though, what is the warm idle supposed to be ? 650-700 ?
Chad
Team Owner
Joined: Jul 2004
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From: Hartford WI
My 95 does exactly what you say here. I tried cleaning IAC, throttle body, etc, with no luck. It does not do it all the time. I chalk it up to a vacuum leak in the intake manifold possibly. I cant find any signs of vacuum hoses leaking. Seems to me it does it mainly when hot, but I'm not 100% sure of that.
Chad
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From: Hartford WI
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From: Anaheim CA
Do you have a modified EPROM?
The ECM has what is known as "throttle follower mode". That means that it opens the IAC as the TPS % rises. The purpose of this is to keep the engine from dying when you take your foot off the gas pedal and the throttle blades slam shut. The air flow basically goes to zero instantaneously when you do this. The ECM has a "throttle follower decay" that (relatively) slowly closes the IAC to get the idle back to normal. If that number has been changed you will get the symptoms you describe. This is especially noticed on manual trans cars -- it gets hidden somewhat by the auto trans when you coast to a stop while in gear.
The ECM uses the MPH from the VSS to help determine when to decay the idle speed. The idea is to drop the idle to normal when speed equals zero. Is your VSS OK? Do you see normal MPH indications on the dash?
The ECM has what is known as "throttle follower mode". That means that it opens the IAC as the TPS % rises. The purpose of this is to keep the engine from dying when you take your foot off the gas pedal and the throttle blades slam shut. The air flow basically goes to zero instantaneously when you do this. The ECM has a "throttle follower decay" that (relatively) slowly closes the IAC to get the idle back to normal. If that number has been changed you will get the symptoms you describe. This is especially noticed on manual trans cars -- it gets hidden somewhat by the auto trans when you coast to a stop while in gear.
The ECM uses the MPH from the VSS to help determine when to decay the idle speed. The idea is to drop the idle to normal when speed equals zero. Is your VSS OK? Do you see normal MPH indications on the dash?
Last edited by Cliff Harris; Nov 3, 2015 at 11:44 PM.
Thread Starter
Drifting
Joined: Jan 2011
Posts: 1,853
Likes: 81
From: Hamilton Virginia
Thanks for the extra info. I do not know if there are any mods on the engine/ecm itself. I just bought it. I am the 4th owner and the last guy I got it from only had it a few months. ( He was a big guy and climbing in and out of it got old pretty fast. ). It does have an aftermarket magnaflow exhaust on it. I do not know what the idle rpms are supposed to be but I would think 900 is still a little high.
Thread Starter
Drifting
Joined: Jan 2011
Posts: 1,853
Likes: 81
From: Hamilton Virginia
Do you have a modified EPROM?
The ECM has what is known as "throttle follower mode". That means that it opens the IAC as the TPS % rises. The purpose of this is to keep the engine from dying when you take your foot off the gas pedal and the throttle blades slam shut. The air flow basically goes to zero instantaneously when you do this. The ECM has a "throttle follower decay" that (relatively) slowly closes the IAC to get the idle back to normal. If that number has been changed you will get the symptoms you describe. This is especially noticed on manual trans cars -- it gets hidden somewhat by the auto trans when you coast to a stop while in gear.
The ECM uses the MPH from the VSS to help determine when to decay the idle speed. The idea is to drop the idle to normal when speed equals zero. Is your VSS OK? Do you see normal MPH indications on the dash?
The ECM has what is known as "throttle follower mode". That means that it opens the IAC as the TPS % rises. The purpose of this is to keep the engine from dying when you take your foot off the gas pedal and the throttle blades slam shut. The air flow basically goes to zero instantaneously when you do this. The ECM has a "throttle follower decay" that (relatively) slowly closes the IAC to get the idle back to normal. If that number has been changed you will get the symptoms you describe. This is especially noticed on manual trans cars -- it gets hidden somewhat by the auto trans when you coast to a stop while in gear.
The ECM uses the MPH from the VSS to help determine when to decay the idle speed. The idea is to drop the idle to normal when speed equals zero. Is your VSS OK? Do you see normal MPH indications on the dash?
Team Owner
Joined: Jul 2004
Posts: 28,472
Likes: 3,290
From: Hartford WI
Thanks for the extra info. I do not know if there are any mods on the engine/ecm itself. I just bought it. I am the 4th owner and the last guy I got it from only had it a few months. ( He was a big guy and climbing in and out of it got old pretty fast. ). It does have an aftermarket magnaflow exhaust on it. I do not know what the idle rpms are supposed to be but I would think 900 is still a little high.
Thread Starter
Drifting
Joined: Jan 2011
Posts: 1,853
Likes: 81
From: Hamilton Virginia
Do you have a modified EPROM?
The ECM has what is known as "throttle follower mode". That means that it opens the IAC as the TPS % rises. The purpose of this is to keep the engine from dying when you take your foot off the gas pedal and the throttle blades slam shut. The air flow basically goes to zero instantaneously when you do this. The ECM has a "throttle follower decay" that (relatively) slowly closes the IAC to get the idle back to normal. If that number has been changed you will get the symptoms you describe. This is especially noticed on manual trans cars -- it gets hidden somewhat by the auto trans when you coast to a stop while in gear.
The ECM uses the MPH from the VSS to help determine when to decay the idle speed. The idea is to drop the idle to normal when speed equals zero. Is your VSS OK? Do you see normal MPH indications on the dash?
The ECM has what is known as "throttle follower mode". That means that it opens the IAC as the TPS % rises. The purpose of this is to keep the engine from dying when you take your foot off the gas pedal and the throttle blades slam shut. The air flow basically goes to zero instantaneously when you do this. The ECM has a "throttle follower decay" that (relatively) slowly closes the IAC to get the idle back to normal. If that number has been changed you will get the symptoms you describe. This is especially noticed on manual trans cars -- it gets hidden somewhat by the auto trans when you coast to a stop while in gear.
The ECM uses the MPH from the VSS to help determine when to decay the idle speed. The idea is to drop the idle to normal when speed equals zero. Is your VSS OK? Do you see normal MPH indications on the dash?
Thread Starter
Drifting
Joined: Jan 2011
Posts: 1,853
Likes: 81
From: Hamilton Virginia
Yesterday morning I decided just to replace the iac unit , instead of taking off the old one and see if cleaning fix the Idle. Well a 15 minute job turned into a 2 hour job. 
While trying to unscrew the valve, it broke with the threads staying in the throttle body. So I had to drain the rad some and remove the throttle body front plate. taking into my Man shed throw it in the vise and chisel out the broken pc without f@cking up the threads. All ended well 
. I got the broken pc out, cleaned the very dirty throttle plate. I installed the new iac valve and put everything back on and back together. Also found a "L" hose going from the throttle plate to the little box next to it,had a split hose. I made a replacement hose and installed it. Refilled the rad, cranked it up and Walaa, she was idling beautiful. Once warmed, she dropped down to about 650 rpms @ idle. Took it for a test drive and every stop sign, the idle was where it should be. Not sure if it was the iac valve or that broken hose, but its fixed and I am happy. 
While trying to unscrew the valve, it broke with the threads staying in the throttle body. So I had to drain the rad some and remove the throttle body front plate. taking into my Man shed throw it in the vise and chisel out the broken pc without f@cking up the threads. All ended well . I got the broken pc out, cleaned the very dirty throttle plate. I installed the new iac valve and put everything back on and back together. Also found a "L" hose going from the throttle plate to the little box next to it,had a split hose. I made a replacement hose and installed it. Refilled the rad, cranked it up and Walaa, she was idling beautiful. Once warmed, she dropped down to about 650 rpms @ idle. Took it for a test drive and every stop sign, the idle was where it should be. Not sure if it was the iac valve or that broken hose, but its fixed and I am happy.
Last edited by scrappy76; Nov 15, 2015 at 07:17 AM. Reason: add
Team Owner
Joined: Jul 2004
Posts: 28,472
Likes: 3,290
From: Hartford WI
Yesterday morning I decided just to replace the iac unit , instead of taking off the old one and see if cleaning fix the Idle. Well a 15 minute job turned into a 2 hour job. While trying to unscrew the valve, it broke with the threads staying in the throttle body. So I had to drain the rad some and remove the throttle body front plate. taking into my Man shed throw it in the vise and chisel out the broken pc without f@cking up the threads. All ended well . I got the broken pc out, cleaned the very dirty throttle plate. I installed the new iac valve and put everything back on and back together. Also found a "L" hose going from the throttle plate to the little box next to it,had a split hose. I made a replacement hose and installed it. Refilled the rad, cranked it up and Walaa, she was idling beautiful. Once warmed, she dropped down to about 650 rpms @ idle. Took it for a test drive and every stop sign, the idle was where it should be. Not sure if it was the iac valve or that broken hose, but its fixed and I am happy.
While trying to unscrew the valve, it broke with the threads staying in the throttle body. So I had to drain the rad some and remove the throttle body front plate. taking into my Man shed throw it in the vise and chisel out the broken pc without f@cking up the threads. All ended well . I got the broken pc out, cleaned the very dirty throttle plate. I installed the new iac valve and put everything back on and back together. Also found a "L" hose going from the throttle plate to the little box next to it,had a split hose. I made a replacement hose and installed it. Refilled the rad, cranked it up and Walaa, she was idling beautiful. Once warmed, she dropped down to about 650 rpms @ idle. Took it for a test drive and every stop sign, the idle was where it should be. Not sure if it was the iac valve or that broken hose, but its fixed and I am happy.
Thread Starter
Drifting
Joined: Jan 2011
Posts: 1,853
Likes: 81
From: Hamilton Virginia
Cleaned the throttle plate very well, sprayed cleaned inside all the passages. I can runs codes, but everything is running great. If anything changes, I will tackle it then. She is idling great and running great.
Team Owner
Joined: Jul 2004
Posts: 28,472
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From: Hartford WI
Idling great doesn't show whether you are at the good end of the adjustment area or not. If you are at max adjustment, it can idle well because it is still inside the window. However, tomorrow if conditions call for adjustment that are different, it may now be outside the window of adjustment. so if you are at 0 counts, it can idle well for Today's condition. If the ECM needs to lower it, no can do since it cannot close it closer than close. Similar with it being in the upper end of the adjustment. Hence I give it 30 counts so it has a way to reduce the counts if it needs to and when I give it AC in certain conditions, it can raise the idle because it needs to.