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  1. #1
    Member
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    Feb 2004
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    Modesto, CA
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    Corvette(s)
    1986 White Coupe

    Default High idle problem on my 86...

    I'm asking for advice on solving the mystery behind my cars high idle problem. I've parked my car for a few months to do some much needed restoration on the interior.

    I thought that I'd take my 86 C-4 out for a drive last weekend after installing new plugs. The car fired up just fine and I pulled it out of the garage to warm up. Once the engine temp reached about 180 degrees, the idle went from 800 rpm's to over 1200 rpm's. As the engine temp climbed to the range of 225 degrees, the electric fan engaged and cycled the temp down to 205 degrees. At this point the engines idle had increased to about 1400 rpm's. I shut it down for about 5 hours to cool off. I started it again, The idle started out at 800 rpm's but once again it climbed after hitting operating temp as it did the 1st time. I checked the OBD-1 for codes but nothing came up. Is there any sensors that could cause the idle to climb like this?

    I also wanted to mention that I replaced the 02 sensor and pulled my batteries cable to clear any settings on the ECM today. I ran a new idle test. Unfortunately, no change. It still idles high when warmed up. Anyone ever had this happen?

    I'm stumped!!

  2. #2
    rrubel's Avatar
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    Glen Burnie, MD, USA
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    Corvette(s)
    1986 Bright Red Coupe

    Default

    If you let it sit, will it eventually come back down? Mine's done that since I got it a couple years ago; was told that when cold it'll rev higher to light the cat off sooner but it does it when warm. It also continues to do it with a *completely* new engine, so it's gotta be something electrical in my case.

    Have you checked for vac leaks? That's one thing that'll cause it.
    [RICHR]

  3. #3
    Jeb
    Member Jeb's Avatar
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    Jan 2005
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    Corvette(s)
    1987 Convertable

    Default

    I suppose it's possible you got a defective O2 sensor. I see where you cleared the codes (if any). Maybe you could read them and get an idea if that's the culprit. If it's the only thing you changed on the engine, I think I'd start there.

    Edit: Maybe I should have asked if you changed the O2 sensor before or after this started occuring.

  4. #4
    Member
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    Corvette(s)
    1986 White Coupe

    Default

    Quote Originally Posted by dzierke
    I'm asking for advice on solving the mystery behind my cars high idle problem. I've parked my car for a few months to do some much needed restoration on the interior.

    I thought that I'd take my 86 C-4 out for a drive last weekend after installing new plugs. The car fired up just fine and I pulled it out of the garage to warm up. Once the engine temp reached about 180 degrees, the idle went from 800 rpm's to over 1200 rpm's. As the engine temp climbed to the range of 225 degrees, the electric fan engaged and cycled the temp down to 205 degrees. At this point the engines idle had increased to about 1400 rpm's. I shut it down for about 5 hours to cool off. I started it again, The idle started out at 800 rpm's but once again it climbed after hitting operating temp as it did the 1st time. I checked the OBD-1 for codes but nothing came up. Is there any sensors that could cause the idle to climb like this?

    I also wanted to mention that I replaced the 02 sensor and pulled my batteries cable to clear any settings on the ECM today. I ran a new idle test. Unfortunately, no change. It still idles high when warmed up. Anyone ever had this happen?

    I'm stumped!!
    I replaced the O2 sensor after the idle problem began. It hadn't been replaced in 4 years and I'm do for an up comming smog test.

    As for the engine idle speed, It starts out at about 800 when the engine is cold. This should be normal since the cold injector is kicking in. Normally the engine would slow down to between 650 and 700 rpms when it's warmed up. Once the engine temp reached about 180 degrees, the idle goes from 800 rpm's to over 1200 rpm's.

    I also removed the idle air control valve and cleaned out the throttle body. The valve it self was replaced a few years back after a code issue. Is it possible for the I.A.C. valve to cause the high idle problem even though I'm not showing an ECM code?

  5. #5
    Member redc4corvette's Avatar
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    1994 Red vert 1986 Blue Silver vette coupe

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    YEP!

    The IAC should measure 1 inch from tip to base.

    I would also check the 3 way vac line behind the head on the drivers side.

    Just a little thing but you might tray it.

    Disconect the IAC wires and go turn your key to the on posession and wait 90 seconds.

    Plug the IAC back in and start car and warm it up .

  6. #6
    Member
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    1986 White Coupe

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    Quote Originally Posted by redc4corvette
    YEP!

    The IAC should measure 1 inch from tip to base.

    I would also check the 3 way vac line behind the head on the drivers side.

    Just a little thing but you might tray it.

    Disconect the IAC wires and go turn your key to the on posession and wait 90 seconds.

    Plug the IAC back in and start car and warm it up .
    I'll check all the vacuum lines and replace as needed. I will also unplug the IAC as you have suggested and see what happens. Do I want to turn the key off before I plug it in an then start the engine?



    Is there any way that the coolant temp sensor could be sending a signal to the ECM that would cause the idle to climb at warmer temperatures? Just reading up on what affect that sensor could have. Keep in mind that I haven't any hard codes from the ECM to support this theory..


  7. #7
    Member redc4corvette's Avatar
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    Turn the Key to start engine and see if it Idles at 450 to 500 Rpm's

    Let the engine run until it is warm you may need to help it along for a few minutes until it will Idle.
    After it warms up shut it off and plug in IAC and start car.


    YES THE COOLENT SENSOR WILL CAUSE A HIGH IDLE.

  8. #8
    JAKEJR
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    Default

    I've tried three times to post some procedures to help track down the cause of your problem. Each time I got a screen telling me I wasn't logged in, but I WAS. The posts turned out to kind of long so I'm posting this just to see if it'll post this time.

    If this one posts, I'll try one more time.

    Jake

  9. #9
    JAKEJR
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    Default

    Quote Originally Posted by JAKEJR
    I've tried three times to post some procedures to help track down the cause of your problem. Each time I got a screen telling me I wasn't logged in, but I WAS. The posts turned out to kind of long so I'm posting this just to see if it'll post this time.

    If this one posts, I'll try one more time.

    Jake
    I've got an 86 too and can probably help solve your problem.

    UPDATE: Well that makes FOUR TIMES that I've typed a response trying to help you with your problem, and each time when I hit the "Submit Reply" button, I get a screen telling me I'm not Logged in, BUT I AM LOGGED!

    So I've just about had it. The test procedures are detailed and take time to type up. I just can't bring myself to type it all up again, only to have the site refuse to accept it. If you want to know how to fix your problem, you'll have to email me at jakejr266@sbcglobal.net and I'll type it up again.

    Sorry
    Jake

  10. #10
    Member
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    1986 White Coupe

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    Jake,

    You've got e-mail.

    Thanks.

  11. #11
    JAKEJR
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    Quote Originally Posted by dzierke
    Jake,

    You've got e-mail.

    Thanks.
    Okay, I sent you a rather long email. You should have it by now.

    Keep me posted.

    Jake

  12. #12
    New Member
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    Mar 2004
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    Norway
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    Corvette(s)
    1985,4+3,Z-51

    Default Hi there. High idle on the 86.

    If the O2 sensor does not function the system is not in closed loop. To monitor the closed loop (if I remember right) you can monitor this in your car without a scanner.
    1 Ground the A and B connector in the ALDL connector. If the engine is not in closed loop the flashing Check engine light will flash with a speed about once every half second and in closed loop it will flash once every 1,5 second. If in closed loop you can also find out if it goes lean or rich. Lean if the check engine light is shorter than the pauses and rich if the check engine light is longer than the pauses.
    the idle is also very dependent of false air or vacuum leak. To check that fast spray start gas or propane around the runner fitting to the manifold an plenum and also around some suspected vacuum lines. Ift the engine increases the rpm there is a vacuum leak there. Spray also on the corrugated plastic ffrom the MAF to the trottlebody as there can be a problem.
    Having checked that clean the throttlebody behind the butterflies because there will be carbon deposites . Clean the IAC valve. If this is not helping check the EGRvalve It might be that this does not close. I think the EGR valve should be closed in the idle mode anyhow. You can check the vacuum membrane in the valve with applying vacuum and see if it holds for more than 20 seconds.

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