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Thread: Engine hunts at idle

  1. #1
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    Engine hunts at idle

    Found a vacuum hose at the back of the engine that was an incorrect size (too big) and was loose.It connects at the back of the left hand intake manifold then to a solenoid. Now that it has been replaced with the correct size hose and does not leak my idle is hunting from 750 to 1000 rpm. Can someone tell me what needs adjusting to get it to idle smoothly. Just a note that it idles smoothly if the hose is disconnected completely.
    As I am in Australia there is not much knowledge base here.
    Looking forwards to some answers.

  2. #2
    Senior Member Bitsyncmaster's Avatar
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    Check that your solenoid is powered via the idle switch. It's common that solenoid connector falls off and that give you ignition advance at idle.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
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    Quote Originally Posted by Bitsyncmaster View Post
    Check that your solenoid is powered via the idle switch. It's common that solenoid connector falls off and that give you ignition advance at idle.
    The solenoid appears to have the wires connected. Should it have voltage at the female plug end?. Aslo what else to check,
    Thanks Bitsyncmaster

  4. #4
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by airfireman1 View Post
    The solenoid appears to have the wires connected. Should it have voltage at the female plug end?. Aslo what else to check,
    Thanks Bitsyncmaster
    It should have voltage at idle.

    Note, one pin has +12 with the key on and the other pin is grounded via the micro switch. So read voltage over both pins.

    You also should not get vacuum on a cold engine so even with a failure of idle hunt, it would only do that with a warm engine.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  5. #5
    Delorean Guru
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    Hunting is caused by 1 or more cylinders not working equally to the others. The motor speeds up and slows down as it hits the uneven cylinder(s). The first thing to check is cylinder pressures by doing a compression test. If they are all within 5% of each other then clean and regap the plugs and put them back. Now you have to find the vacuum leak that is causing 1 or more cylinders to be lean or you could have 1 or more bad injectors. Timing would affect all cylinders equally so it is NOT a timing or timing advance problem.
    David Teitelbaum

  6. #6
    Senior Member Josh's Avatar
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    Quote Originally Posted by David T View Post
    Hunting is caused by 1 or more cylinders not working equally to the others. The motor speeds up and slows down as it hits the uneven cylinder(s). The first thing to check is cylinder pressures by doing a compression test. If they are all within 5% of each other then clean and regap the plugs and put them back. Now you have to find the vacuum leak that is causing 1 or more cylinders to be lean or you could have 1 or more bad injectors. Timing would affect all cylinders equally so it is NOT a timing or timing advance problem.
    What are you going on about....?

  7. #7
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    If you have fixed a vacuum leak then the engine is probably running somewhat richer than it was before. So leaning the mixture via the 3mm adjustment screw would probably sort your hunting.

    But you probably shouldn't touch this screw unless you have some direct measurement (exhaust CO) of what is happening to the mixture.

    In my experience engine hunting is caused by a rich mixture.

    Bear in mind even seemingly small leaks, leaving the bung off the mixture adjustment screw for example, will have a big impact on exhaust CO. For example on my car replacing a "forgotten" mixture bung will increase exhaust CO by 2.0%.

  8. #8
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    Quote Originally Posted by Horsebox View Post
    If you have fixed a vacuum leak then the engine is probably running somewhat richer than it was before. So leaning the mixture via the 3mm adjustment screw would probably sort your hunting.

    But you probably shouldn't touch this screw unless you have some direct measurement (exhaust CO) of what is happening to the mixture.

    In my experience engine hunting is caused by a rich mixture.

    Bear in mind even seemingly small leaks, leaving the bung off the mixture adjustment screw for example, will have a big impact on exhaust CO. For example on my car replacing a "forgotten" mixture bung will increase exhaust CO by 2.0%.
    Vacuum leaks are "sneaky" meaning that they will lean out the cylinder or cylinders closest to the leak while the others are running well. Because the cylinders are not all firing evenly you get the hunting or surging at idle. Adjusting the mixture screw is a "global" adjustment meaning it will affect all of the cylinders equally but if some were firing well and others lean, by making it richer the ones that were OK are now too rich and the lean ones may be OK. Not the way to fix it. You don't usually fix anything by turning that mixture screw but everyone tries to anyway. In fact, that screw is the LAST thing you do once you know EVERYTHING else is 100% right. An example of a vacuum leak is an injector seal. If the ones you have are rock hard and you can pull an injector with no drag, you KNOW that seal is leaking! Very common. A temporary fix is to wrap it with a little teflon tape and see what happens. Other common leaks are the W pipe, the air tube to the idle motor in the bottom of the mixture unit, a missing plug over the mixture screw, old, hard, cracked and split hoses to the intake manifold. "Sneaky" ones include a bolt hole, a plug at the end of the intake runner, the idle motor on top, the hose going into the left pontoon, bad decel valve springs, the list can get long. The best way I know to find them is to use a smoke machine.
    David Teitelbaum

  9. #9
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    Quote Originally Posted by David T View Post
    The best way I know to find them is to use a smoke machine.
    I've been to several garages looking for ideas to do this but nobody knows.

    I guess the idea is to pressurise the intake via the airflow sensor and watch for leaks everywhere and anywhere.

    Any ideas for equipment to help with this?

    Mark

  10. #10
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    Quote Originally Posted by Horsebox View Post
    I've been to several garages looking for ideas to do this but nobody knows.

    I guess the idea is to pressurise the intake via the airflow sensor and watch for leaks everywhere and anywhere.

    Any ideas for equipment to help with this?

    Mark
    Any decent garage should have a smoke machine. It is the only way to find problems on newer cars with evap systems on the fuel tanks. I remove the air filter, seal the intake where the air filter fits onto the mixture unit, and pump the smoke in the hose going to the oil filler.
    David Teitelbaum

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