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Thread: Mercruiser 502 hanging idle? Bad IAC?

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    PBN Charter Member 92nsx's Avatar
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    Mercruiser 502 hanging idle? Bad IAC?

    Gen 6 Mercruiser 502 mag MPI. 2000' model. Hours 530
    Mercruiser 502 hanging idle? Bad IAC?-1496967772463.jpg

    Just want to bounce a idea off you guys. I believe my IAC on my port engine is going bad or something. Obviously I have scanned the ECM since it stays in the boat 24-7, and have zero codes/ faults.

    Engine runs and starts fine with zero issues (cold and hot). I just notice every once in a wile once coming off plane and going to idle, the port engine will have an "hanging" idle as I would call it. It will be about 900-1000 rpm when idling in gear. Take it out of gear and in neutral and she is about 11-1200. A quick snap of the stick/ rev it up and down and she comes right down to normal idle speed and I continue on with my day.

    I have verified on with the scanner the TPS is at 0% at idle and 100% at WFO.

    Flame Arrestor is clean

    IAC sounds like it is acting up or need to be re learned? Some thing dirty?
    Kendell do you have an Advanced Auto GM part# the Merc used?
    Any other random ideas. Yes I can flip and flop parts between the engines but that becomes a PITA when working under and on the back side of the throttle body wile floating.

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    PBN Charter Member Kefurr's Avatar
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    Quote Originally Posted by 92nsx View Post
    Gen 6 Mercruiser 502 mag MPI. 2000' model. Hours 530
    Click image for larger version. 

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    Just want to bounce a idea off you guys. I believe my IAC on my port engine is going bad or something. Obviously I have scanned the ECM since it stays in the boat 24-7, and have zero codes/ faults.

    Engine runs and starts fine with zero issues (cold and hot). I just notice every once in a wile once coming off plane and going to idle, the port engine will have an "hanging" idle as I would call it. It will be about 900-1000 rpm when idling in gear. Take it out of gear and in neutral and she is about 11-1200. A quick snap of the stick/ rev it up and down and she comes right down to normal idle speed and I continue on with my day.

    I have verified on with the scanner the TPS is at 0% at idle and 100% at WFO.

    Flame Arrestor is clean

    IAC sounds like it is acting up or need to be re learned? Some thing dirty?
    Kendell do you have an Advanced Auto GM part# the Merc used?
    Any other random ideas. Yes I can flip and flop parts between the engines but that becomes a PITA when working under and on the back side of the throttle body wile floating.
    BWD # 21755
    Sierra # 18-7632
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    PBN Charter Member hook'em's Avatar
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    When I got the high idle, that was diagnosed as a bad IAC, it turned out to be a broken intake gasket. It wouldn't idle back down even with a quick stab of the stick.

    Don't forget to put the ECM in service mode to calibrate the new IAC.
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    PBN Charter Member Short-Bus's Avatar
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    maybe the throttle plate isn't going all the way closed when you ease off the sticks, could just be an adjustment
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    PBN Charter Member 92nsx's Avatar
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    Quote Originally Posted by hook'em View Post
    When I got the high idle, that was diagnosed as a bad IAC, it turned out to be a broken intake gasket. It wouldn't idle back down even with a quick stab of the stick.

    Don't forget to put the ECM in service mode to calibrate the new IAC.
    Umm... In my Merc manual (#16) it says for reset IAC valve pintle position
    a. Turn ignition key ON for ten seconds.
    b. Turn ignition key OFF for ten seconds.
    c. Restart engine and check for proper idle operation

    Might have to dig into the scanner and see what it has.

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    PBN Charter Member hook'em's Avatar
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    That could also be because mines a MEFI 1. I recall something about that at the time. I called the mechanic from the lake and he talked me thru jumping it into service mode with a paper clip because he learned (after it was installed) it should have been put in service mode. They must not all be the same.

    Edit: I've got the early Gen 6 roller with VST, lift pump on the sea water pump and V belts. MEFI 1.
    Last edited by hook'em; 08-16-2017 at 10:15 PM.
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    PBN Charter Member 92nsx's Avatar
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    Well since I don't have a TB gasket, I gave the TB a cleaning (no to dirty photo) and reset IAC via the scanner.

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    PBN Charter Member Kefurr's Avatar
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    Quote Originally Posted by 92nsx View Post
    Umm... In my Merc manual (#16) it says for reset IAC valve pintle position
    a. Turn ignition key ON for ten seconds.
    b. Turn ignition key OFF for ten seconds.
    c. Restart engine and check for proper idle operation

    Might have to dig into the scanner and see what it has.
    this is the same as my manual shows

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    PBN Charter Member 92nsx's Avatar
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    Forgot photos
    Attached Thumbnails Attached Thumbnails Mercruiser 502 hanging idle? Bad IAC?-1502923397209.jpg   Mercruiser 502 hanging idle? Bad IAC?-1502923407188.jpg  

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    PBN Charter Member Kefurr's Avatar
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    Quote Originally Posted by 92nsx View Post
    Forgot photos
    I pushed the fire button in the second pic
    Did that fix it for ya??????
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    PBN Charter Member 642mx's Avatar
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    Hanging idle makes one think of a vacuum leak, easy to check with some brake clean. But these merc big blocks are funny about having a clean throttle body and the IAC. I clean my throttle body and remove the IAC and clean its port and plunger every winter. When mine gets dirty, the idle wonders around when it's cold.
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    PBN User Drew's Avatar
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    You can use quick silver power tune. Also to find out if it's a gasket leak you can spray ether around the gaskets.

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    PBN Charter Member 92nsx's Avatar
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    Quote Originally Posted by Kefurr View Post
    I pushed the fire button in the second pic
    Did that fix it for ya??????
    Fire!!!!

    I'll find out this weekend.
    Kefurr likes this.

  14. #14
    PBN Charter Member 92nsx's Avatar
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    Quote Originally Posted by 642mx View Post
    Hanging idle makes one think of a vacuum leak, easy to check with some brake clean. But these merc big blocks are funny about having a clean throttle body and the IAC. I clean my throttle body and remove the IAC and clean its port and plunger every winter. When mine gets dirty, the idle wonders around when it's cold.
    I know you also buff and wax the inside of your TB besides just clean it out Hahahaha.
    642mx, hook'em and Kefurr like this.

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    PBN Charter Member Kefurr's Avatar
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    Quote Originally Posted by 92nsx View Post
    Fire!!!!

    I'll find out this weekend.
    Sorry wasn't thinkin, bad choice of word in your world!
    hook'em and 92nsx like this.

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    PBN Charter Member 642mx's Avatar
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    Quote Originally Posted by 92nsx View Post
    I know you also buff and wax the inside of your TB besides just clean it out Hahahaha.
    You can eat a ham sammich off of it when I'm thru cleaning!
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    PBN Charter Member Kefurr's Avatar
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    Quote Originally Posted by 642mx View Post
    You can eat a ham sammich off of it when I'm thru cleaning!
    I could but would you let me?
    642mx and hook'em like this.

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    PBN Charter Member hook'em's Avatar
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    Quote Originally Posted by Kefurr View Post
    this is the same as my manual shows
    What ECM does BB have?
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    Quote Originally Posted by Short-Bus View Post
    maybe the throttle plate isn't going all the way closed when you ease off the sticks, could just be an adjustment
    I'm with SB here. Since you can make a difference just opening and reclosing the throttles, I believe something is causing a throttle blade to stick slightly open. This sounds mechanical, not electrical. I suppose it could be a dirty IAC, but that is really independent of the throttle position.

  20. #20
    PBN Charter Member 92nsx's Avatar
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    ^^^ I have both Merc # 16 and #23 downloaded. 16= gen 5 BBC, 23 newer gen 6 BBC. Weird, neither talk about a scanner.

    Quote Originally Posted by Drew View Post
    You can use quick silver power tune. Also to find out if it's a gasket leak you can spray ether around the gaskets.
    Looks like a fancy can of carb cleaner

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