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Thread: Occasional High Idle Speed - $8D 383 fully modified TPI

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    Fuel Injected! zaut's Avatar
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    Occasional High Idle Speed - $8D 383 fully modified TPI

    I am writing/asking this because of two reasons. One my son has his laptop at school so I canít data log and two I am looking for confirmation of my theory. Occasionally my TPI engine will start idling very high. I do not have a vacuum leak. One time while it was idling at about 1600 rpm, I unplugged the connector to the TPS sensor. When I re-plugged it, the idle went back to normal. It has not gone into the high idle since then. I was never getting a check engine light. (Except when I unplugged the TPS). I am thinking that I was not getting a good ground to the TPS. This in turn was giving the ECM a 5 volt signal to the TPS signal input (Circuit 417). Because the ECM was seeing a wide open throttle signal, the IAC follower function was causing the IAC to open fully allowing a lot of idle air in. Does this make sense? I would think if the ECM was always seeing a WOT condition I would have other issues during the problem but I wasnít seeing any during. The car definitely has better acceleration now. I am thinking that the acceleration enrichment function was not working with the TPS ground problem. Anyhow, I just wanted pick some other brains on this issue. I havenít been here for awhile because I got the car running great thanks to everyoneís help. Thanks.

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    it's a possibility. i recently had to replace a TPS due to it acting badly after the engine warmed up.... when it did, after closing the throttle, the voltage dropped too far below the normal TPS closed voltage position and the ECM learned that position as 0%, so when the throttle was actually closed, the ECM thought it was open by 3% or so and caused a ~1000 RPM idle.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    EFI GearHead ! EagleMark's Avatar
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    Most ECM will set to 0% when the engine is started. Or does some have another learn?

    You seem to have found an issue with the TPS, but it could be the sensor itself or a ground. Can check with voltage meter at idle around .54 volts and swing slowly and smoothly to around 4.5 volts. Any glitch or sticking and you found the issue.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
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    with the MPFI and SFI 60V6 applications i'm familiar with....

    they check voltage at key-on, and the lowest voltage it sees before cranking is considered the 0% TPS point. with the SFI stuff, this is done with every key-on, but with the MPFI, it only does it if the NVRAM failed(battery disconnected).
    while running, if at a high enough RPM and low enough MAP, the TPS 0% point can be changed in a downward direction, but not upwards.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    Fuel Injected! zaut's Avatar
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    I need to keep my mouth shut. I think I jinx myself. The high idle started again. I checked the TPS and it is working fine. 0.54 volts a closed about 4.5 volts at WOT with a linear increase as I open the throttle. I will have a laptop this weekend to dig deeper. What is funny is it is not consistent. I shut the car off and it gets back to normal. I hate intermittent problems. They are the hardest to diagnose.

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    EFI GearHead ! EagleMark's Avatar
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    That sounds more like an IAC issue. Dave and I both found bad IAC valve this week, checked wiring, Ohm sensor etc... IAC would hang... it resets when turned off and mine anyway would work it's way back up in IAC counts and stay there. Replaced and fixed.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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    Fuel Injected! zaut's Avatar
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    EagleMark, You may be correct with the IAC Valve. I finally got a PC connected to the car. The target idle was 600 rpm and the engine was running at 850 - 900. The IAC count was 59. I shut the car off and restarted and the idle went down to 600 and counts were 37. When the IAC hangs, does the counts hang as well? I would think the ECM would pulse the IAC to change and count those pulses, regardles if the valve moves or not. I did not think the IAC gave any feedback of its actual position.

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    the IAC doesn't give feedback.... the counts displayed are what the ECM is commanding it to be and can be incorrect if the IAC isn't acting as expected.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    EFI GearHead ! EagleMark's Avatar
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    Quote Originally Posted by zaut View Post
    EagleMark, You may be correct with the IAC Valve. I finally got a PC connected to the car. The target idle was 600 rpm and the engine was running at 850 - 900. The IAC count was 59. I shut the car off and restarted and the idle went down to 600 and counts were 37. When the IAC hangs, does the counts hang as well? I would think the ECM would pulse the IAC to change and count those pulses, regardles if the valve moves or not. I did not think the IAC gave any feedback of its actual position.
    Like Robert said... seems we are finding more and more IAC issues lately or guys are just beginning to see idle issues and notice IAC counts? I think the count is going up because the ECM is commanding more but the valve is not moving or getting stuck in the move?

    The one I did hands on last week did the same as yours. I still have the bad IAC here and going to tear it aprt to see if it is stripped? Or if I can see what is happening... I think 30 year old IAC are just wearing out!

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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    Fuel Injected! zaut's Avatar
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    I can agree that if the ECM commanded a move, it would assume it was made (no feedback). The count would be what the ECM thinks the IAC is at. However, (please correct me if I am wrong) the counts are not absolute. The ECM counts up and down. What I have seen is low counts meant the IAC was closed or low idle RPM, and high count meant the IAC was open. I would think that I should see a very low count when the IAC was stuck open and the ECM was trying to close it. The data I am missing is what are the "normal" counts for the IAC at any one RPM. Maybe 59 and 37 are very low counts, below what should be normal. I guess I have to make a $50 gamble on a new IAC or keep looking to see if it is anything else. Since my engine is highly modified, I am guessing no one can say what the normal count should be for my configuration. And YES my IAC is 27 years old so it could just be tried. One thing I have noticed since I completed the rebuild was that at cold startup, until a fixed amount of time, the IAC would stay at a fixed number, I think 150. Is that normal? Is there a way to adjust that?

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    EFI GearHead ! EagleMark's Avatar
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    There's a max count of usually 145. Then a park count and depending on ECM the IAC will close all the way and go to that position. Well not if the IAC is stripped.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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    Fuel Injected! zaut's Avatar
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    I replaced the IAC last weekend. I am still getting the high idle. I am now wondering if the canister purge valve is sticking on. I will unplug it for a few days to see if that is the problem. Unfortunately I haven't had the high idle when I had the computer hooked up to watch the IAC counts.

    I did notice another issue when I had the computer hooked up. I fall out of closed loop occasionally. The O2 ready goes away at the same time. Is this indicating a bad O2 sensor?

    On a up note, I finally got the car to the drags Saturday. It run a 13.70 quarter mile with street tires. I was hoping for low 13sec to high 12sec. Since this is my first chance at getting good log data, I am very happy with the 1/4 ET.

    I was seeing some knock retard occurring at high MAP in the 3200 to 4000 rpm range right after shifts. I need to make adjustments. But I want to remind EagleMark that the file I put together for using the EST module instead of the memcal knock module works. I would hope you would consider adding that file to somewhere on the site as a reference. I uploaded it again with this post.

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    Does anyone know what "IAC Command speed open loop offset" does? This bin I started with has that value at 1000 rpm. I am not sure what it is for so I wonder if that may have something to do with my high idle speed. Also how does "IAC Learn" affect things, what does it do?
    Last edited by zaut; 09-18-2012 at 07:35 AM.

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    Fuel Injected! JeepsAndGuns's Avatar
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    Where is your o2? Is it in the stock location, is it the original one, do you have headers? I dont know anything about the chevy cars, is it a heated sensor? If you have headers and its not a heated sensor, that might be your problem with it dropping out of closed loop. The o2 is cooling off. You could try wiring up a heated o2 sensor.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

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    I have Edelbrok headers designed to replace the stock manifold. I was thinking that the O2 was cooling off too. Is that common? It happened when the car was idling at normal speeds. Can you recommend a heated replacement? (year, model, etc) I was thinking about doing that anyways because I can get the car in closed loop quicker. I understand there different types of O2 sensors, one I have heard of is a wide band O2 sensor. Not knowing the difference, I have been hesitant to change it. Thanks.

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