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Thread: Baseline vacuum number

  1. #1
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    Baseline vacuum number

    So I've been concerned about the possibility of a plugged muffler. It's easy to confirm the cat is not plugged, but the muffler is much more difficult. If you google it, they tell you to do a vacuum test. The issue is there is a fairly wide span of vacuum at idle in good running motors. They say you should use your experince to tell you if your perticular motor is reading is low or high. Of course, that doesn't help if you have no experince with your perticular motor.

    In my case, I checked the vacuum at idle and found it to be about 16 which is pretty low. The motor was not warmed up at the time as it should be, so I don't know if it would go up. I was scared to keep running it because I just got done changing a exhaust manifold gasket and I didn't want to change the other one because of high back pressure.

    So I removed the muffler and installed a straight pipe to clear the body. I warmed up the car. (Loud) Now the vacuum is about 18. Im not sure how much, if any, it should go down with the muffler. Do any of you guys know what your vacuum is at idle after warm up?

  2. #2
    EFI'd dn010's Avatar
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    Have you checked your timing before all of this?

    Quote Originally Posted by Helirich View Post
    So I've been concerned about the possibility of a plugged muffler. It's easy to confirm the cat is not plugged, but the muffler is much more difficult. If you google it, they tell you to do a vacuum test. The issue is there is a fairly wide span of vacuum at idle in good running motors. They say you should use your experince to tell you if your perticular motor is reading is low or high. Of course, that doesn't help if you have no experince with your perticular motor.

    In my case, I checked the vacuum at idle and found it to be about 16 which is pretty low. The motor was not warmed up at the time as it should be, so I don't know if it would go up. I was scared to keep running it because I just got done changing a exhaust manifold gasket and I didn't want to change the other one because of high back pressure.

    So I removed the muffler and installed a straight pipe to clear the body. I warmed up the car. (Loud) Now the vacuum is about 18. Im not sure how much, if any, it should go down with the muffler. Do any of you guys know what your vacuum is at idle after warm up?
    -----Dan B.

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    Quote Originally Posted by dn010 View Post
    Have you checked your timing before all of this?
    Yes.

  4. #4
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    So I am confused about your post. You checked vacuum at idle on a cold engine and got a low number. You didn't warm up the engine and leave the gauge attached to see if the needle dropped to indicate a plugged muffler and instead removed the muffler and then took another vacuum reading? Why are you trying to diagnose a muffler issue with vacuum but dismantled the exhaust? It doesn't matter what your initial value is, if the exhaust system is clogged the gauge will show it. Where are you taking readings from? Are you plugging all other vacuum ports during this? When I was running the original engine, I attached the gauge at the large nipple on the back of the passenger side of the intake and if my memory is fine, I had 21in HG when tested living in upstate NY at an elevation around 1,500ft. Have you sent a bore scope into the muffler?

    Also for what it's worth, I blew out every single exhaust gasket on one side of the engine or the other every time I replaced them on my original engine, mostly due to warped manifolds. I now run headers held on by 4 studs per port and I have no problem. If you are changing gaskets and not checking if your manifolds are straight, you are wasting your time.
    -----Dan B.

  5. #5
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    The way you use a vacuum gauge to see if you have a plugged up exhaust is to watch the gauge and if the vacuum starts to get lower as you run the motor it means because the exhaust is causing back pressure the motor can't maintain the level of vacuum it had. Rare on a Delorean for the muffler to clog up but the cat can.
    David Teitelbaum

  6. #6
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    Quote Originally Posted by dn010 View Post
    You didn't warm up the engine and leave the gauge attached to see if the needle dropped to indicate a plugged muffler
    Quote Originally Posted by David T View Post
    The way you use a vacuum gauge to see if you have a plugged up exhaust is to watch the gauge and if the vacuum starts to get lower as you run the motor
    You're almost at 8,000 posts of constant reposting of what has already been said.
    -----Dan B.

  7. #7
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    Quote Originally Posted by dn010 View Post
    So I am confused about your post. You checked vacuum at idle on a cold engine and got a low number. You didn't warm up the engine and leave the gauge attached to see if the needle dropped to indicate a plugged muffler and instead removed the muffler and then took another vacuum reading? Why are you trying to diagnose a muffler issue with vacuum but dismantled the exhaust? It doesn't matter what your initial value is, if the exhaust system is clogged the gauge will show it. Where are you taking readings from? Are you plugging all other vacuum ports during this? When I was running the original engine, I attached the gauge at the large nipple on the back of the passenger side of the intake and if my memory is fine, I had 21in HG when tested living in upstate NY at an elevation around 1,500ft. Have you sent a bore scope into the muffler?

    Also for what it's worth, I blew out every single exhaust gasket on one side of the engine or the other every time I replaced them on my original engine, mostly due to warped manifolds. I now run headers held on by 4 studs per port and I have no problem. If you are changing gaskets and not checking if your manifolds are straight, you are wasting your time.
    In my research, they said a weak vacuum at idle is an indicator of restricted exhaust. If I remove the exhaust and get a reading and then install it and the reading is much weaker, it's plugged. Do you not agree? I attached the same place as you. I did plug the ports. It would appear your motor is in much better condition then mine. (Mine has almost a 100,000 on it) I don't own a bore scope.

    I did resurface the manifold when I had it off.

  8. #8
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    Quote Originally Posted by dn010 View Post
    You're almost at 8,000 posts of constant reposting of what has already been said.
    The way you use a vacuum gauge to see if you have a plugged up exhaust is to watch the gauge and if the vacuum starts to get lower as you run the motor

    Dave is still less annoying than Helirich. Guy made a thread asking for help but it sounds like he just wants people to tell him hes right.

    Supercharged 5.3L LS4 + Porsche 6spd
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    http://lsdelorean.com

  9. #9
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    No, I don't agree. I repeat, the vacuum will appear normal upon start up but the needle will soon drop to 0 if you have a plugged exhaust system. Using a vacuum gauge as a diagnostic tool isn't really common anymore, I work on engines from the 40's & 50's+ so the gauge comes out from time to time but you can't just hook it up, run to the computer asking what 16 means and then ask others to pull out their rulers and compare - you need a chart to know what the values/needle bounce means. You also diagnose before you start taking things apart and while Dave does constantly repeat what is already posted, I do agree that the exhaust getting plugged would be a rare thing. Even when mine was full of the converter pellets it still ran fine.

    I did an engine swap so I don't run that engine anymore but it did have 70K+ on it. Sorry, I've seen multiple times where you mentioned you have a ton of tools so I assumed a bore scope would be one of them, I use that more than the vacuum gauge.

    Quote Originally Posted by Helirich View Post
    In my research, they said a weak vacuum at idle is an indicator of restricted exhaust. If I remove the exhaust and get a reading and then install it and the reading is much weaker, it's plugged. Do you not agree? I attached the same place as you. I did plug the ports. It would appear your motor is in much better condition then mine. (Mine has almost a 100,000 on it) I don't own a bore scope.

    I did resurface the manifold when I had it off.
    -----Dan B.

  10. #10
    EFI'd dn010's Avatar
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    Quote Originally Posted by Josh View Post
    The way you use a vacuum gauge to see if you have a plugged up exhaust is to watch the gauge and if the vacuum starts to get lower as you run the motor

    Dave is still less annoying than Helirich. Guy made a thread asking for help but it sounds like he just wants people to tell him hes right.
    -----Dan B.

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