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Thread: Vacuum solenoid

  1. #1
    Senior Member Bitsyncmaster's Avatar
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    Vacuum solenoid

    I was helping Christian with his engine work and found the advance vacuum was only 10 inches. Now the center hose on the solenoid was the normal 20 inches (manifold vacuum). Only thing I could think of is the solenoid has a vent stuck open and the vent "orifice" is the same size as the inlet resulting in half the vacuum.

    Any body ever see this problem before?
    Dave M vin 03572
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  2. #2
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    Quote Originally Posted by Bitsyncmaster View Post
    I was helping Christian with his engine work and found the advance vacuum was only 10 inches. Now the center hose on the solenoid was the normal 20 inches (manifold vacuum). Only thing I could think of is the solenoid has a vent stuck open and the vent "orifice" is the same size as the inlet resulting in half the vacuum.

    Any body ever see this problem before?
    the valve has failed internally. I have gotten away from these completely with our EFI conversions and use a pwm solenoid to control vacuum via the ecu for the distributor. Nice feature with this is that it is based off a CLT map and can be adjusted for different operating climates.
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  3. #3
    DMC Midwest - 815.459.6439 DMCMW Dave's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    I was helping Christian with his engine work and found the advance vacuum was only 10 inches. Now the center hose on the solenoid was the normal 20 inches (manifold vacuum). Only thing I could think of is the solenoid has a vent stuck open and the vent "orifice" is the same size as the inlet resulting in half the vacuum.

    Any body ever see this problem before?
    Yes. They just go bad (leaky) internally. Was it still closing when powered? The vacuum port should be blocked when powered, and the distributor port is open to the vent at that time so that the vacuum advance snaps back to no-advance. When unpowered, the two ports are connected together and the vent is unused.

    If you happen to hook it up backwards the vacuum advance (timing) will "hang" for a short time since it can't bleed off through the vent, and you'll have a slight but un-noticeable vacuum leak at idle. I've had this cause emission test failure.
    Dave S
    DMC Midwest - retired but helping
    Greenville SC

  4. #4
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by DMCMW Dave View Post
    Yes. They just go bad (leaky) internally. Was it still closing when powered? The vacuum port should be blocked when powered, and the distributor port is open to the vent at that time so that the vacuum advance snaps back to no-advance. When unpowered, the two ports are connected together and the vent is unused.

    If you happen to hook it up backwards the vacuum advance (timing) will "hang" for a short time since it can't bleed off through the vent, and you'll have a slight but un-noticeable vacuum leak at idle. I've had this cause emission test failure.
    It still worked, idle no vacuum advance, some throttle had 10 inches vacuum advance.
    Dave M vin 03572
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  5. #5
    Senior Member Trstno1's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    It still worked, idle no vacuum advance, some throttle had 10 inches vacuum advance.
    I know this is an old thread, but I had a couple of questions. I am attempting to solve a idle lumpiness in my engine. I have already replaced all ignition/fuel components on the car and yet it still stumbles a little. One of the things that I didn't replace was the vacuum advance on the distributer or the solenoid valve coming from the distributer. How can I test for vacuum pressure on the engine first, then each component operation second? what am I looking for?

    From what I have been reading, failure of one or both of these component cant create a mild idle stumble.

    Thanks in advance. hehe.....see what I did there...
    You can't buy happiness, but you can buy a DeLorean and that's sort of the same thing....

  6. #6
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    Quote Originally Posted by Trstno1 View Post
    I know this is an old thread, but I had a couple of questions. I am attempting to solve a idle lumpiness in my engine. I have already replaced all ignition/fuel components on the car and yet it still stumbles a little. One of the things that I didn't replace was the vacuum advance on the distributer or the solenoid valve coming from the distributer. How can I test for vacuum pressure on the engine first, then each component operation second? what am I looking for?

    From what I have been reading, failure of one or both of these component cant create a mild idle stumble.

    Thanks in advance. hehe.....see what I did there...
    Idle 'lumpiness" or uneven idle can be caused by a vacuum leak but is most often due to some kind of imbalance between the firing of the cylinders. A vacuum leak can affect the cylinders closest to the leak by leaning out the fuel mixture. A mis-gapped spark plug, a bad ignition wire, a valve adjustment, all can affect a cylinder's efficiency. On one car the gaps of the reluctor to the impulse coil were not all the same and that made the idle "lumpy". To make the idle as smooth and possible your goal is to make all of the cylinders as close to each other as possible. Start with a cylinder pressure test. Check the valve adjustments. If the car hasn't had a tune-up in a long time maybe you are overdue.
    David Teitelbaum

  7. #7
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Trstno1 View Post
    I know this is an old thread, but I had a couple of questions. I am attempting to solve a idle lumpiness in my engine. I have already replaced all ignition/fuel components on the car and yet it still stumbles a little. One of the things that I didn't replace was the vacuum advance on the distributer or the solenoid valve coming from the distributer. How can I test for vacuum pressure on the engine first, then each component operation second? what am I looking for?

    From what I have been reading, failure of one or both of these component cant create a mild idle stumble.

    Thanks in advance. hehe.....see what I did there...
    For your idle problem, just pull the hose off the distributor and see if that corrects the idle problem. If the vacuum advance is on at idle you will get a crappy idle.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #8
    Senior Member Trstno1's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    For your idle problem, just pull the hose off the distributor and see if that corrects the idle problem. If the vacuum advance is on at idle you will get a crappy idle.
    Ok so at idle if i pull off the hose from the distributer and hook it up to a vacuum gauge it should read 0 at idle and 20 once the throttle is activated thus activating the vacuum advance on the distributer right? Right now with that situation I am getting bobbling round the 15 bar mark and 0bar at idle. Is 15 bar pointing towards a vacuum leak? Should it always be 20 bar or am i testing engine vacuum at the wrong spot? Also is there a way to test the vacuum advance valve that the vacuum hose plugs into on the distributer without tearing things apart?

    On a side note, i checked my timing and was getting about 12 degrees at idle. I adjusted the distributer to get 13 degrees and it appears to run a little better, but a little lumpiness remains. Im guessing th next step would be valve adjustment?
    You can't buy happiness, but you can buy a DeLorean and that's sort of the same thing....

  9. #9
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Trstno1 View Post
    Ok so at idle if i pull off the hose from the distributer and hook it up to a vacuum gauge it should read 0 at idle and 20 once the throttle is activated thus activating the vacuum advance on the distributer right? Right now with that situation I am getting bobbling round the 15 bar mark and 0bar at idle. Is 15 bar pointing towards a vacuum leak? Should it always be 20 bar or am i testing engine vacuum at the wrong spot? Also is there a way to test the vacuum advance valve that the vacuum hose plugs into on the distributer without tearing things apart?

    On a side note, i checked my timing and was getting about 12 degrees at idle. I adjusted the distributer to get 13 degrees and it appears to run a little better, but a little lumpiness remains. Im guessing th next step would be valve adjustment?
    If you open the throttle to much your engine vacuum will probably drop to 15 inches. To test the engine vacuum you attach to a hose on the intake manifold. The vacuum does bounce more so with our engines because of the uneven fire sequence.

    Since you have a timing light, you can check the vacuum advance and mechanical advance. Pull the hose off the ignition distributor to check the mechanical advance at the RPMs as shown in the book. Then when the vacuum advance is connected you check a little above idle so no mechanical advance is taking place.
    Dave M vin 03572
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    Quote Originally Posted by Bitsyncmaster View Post
    For your idle problem, just pull the hose off the distributor and see if that corrects the idle problem. If the vacuum advance is on at idle you will get a crappy idle.
    I've messed with vacuum advance at idle on some of my cars that are older than the DeLorean that could be made to use both manifold vacuum advance (vacuum advance all the time) or ported vacuum advance (no vacuum advance at idle, like the DeLorean). Vacuum advance at idle will definitely not give you a crappy idle. In almost every case, it will smooth out the idle because the engine is running more efficiently at idle. A ported vacuum advance setup is only really used for emissions purposes (or with a carburetor on a car that has AC so that the effect on idle RPM on the engine at idle caused by the compressor is minimized. Lower RPM = lower vacuum = less advance = lower RPM, and the cycle continues. Ported advance prevents that cycle).
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