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Thread: Vacuum gauge readings

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    Forum Moderator Boudin's Avatar
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    Vacuum gauge readings

    I have a vacuum gauge that I went ahead and added to my gauge cluster. I know what I should see/look for when using it as a diagnostic tool....but I am having trouble finding out what I should be seeing under normal driving conditions. Can anyone help me out here? I know that they have been used before in OEM gauge clusters, but I have never had the pleasure of owning one that did.

    If I understand correctly, they have been used as "economy" gauges to let you know when you are getting the worst gas mileage. Most people recommend avoiding dips below 10 inches if you want to save gas. This makes sense to me since they are basically an "engine load" meter when hooked up all the time....right? I am just looking for some information of what is "normal" as far as what I should be seeing the needle do when driving normally.

    Currently, it idles steady at about 19. When decelerating, it goes up in the 20s and comes back down. When "punching" it....it dips pretty low. What should I look out for here as far as what a "problem" would look like under normal driving conditions?

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    Senior Moderator chevychase's Avatar
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    Gitter done have fun. Running on empty...running blind...running behind.....

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    Forum Moderator Boudin's Avatar
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    Yeah....I have that diagnostic information just about memorized. I actually have that page printed out. Is there anything you can find on what to expect while driving?

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    Forum Moderator Boudin's Avatar
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    The little bit it mentions about the engine braking and "poor man's" fuel mileage indicator. About the lowest I have seen the needle drop was to about 5. I wasn't loading the engine too bad at the time and certainly didn't feel it was close to full output. The max I have seen is about 26 and was with some deceleration off the interstate. It's kinda neat to me how it would change from say 15 to almost 5 with only slight difference in pedal pressure....and really was hardly noticable other than the needle moving.

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    They can really make a difference in mileage too.
    Old gauges often were marked like this one

    Auto Gage Mechanical Vacuum Gauge 2 1 16" Dia Black Face 2337 | eBay

    CC that chart is a good diagnostic tool too.

    DS

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    Since the vacuum is controlled by how much air that is let in the engine(throttle plates) it's always going to be changing when under load. As far as a cruising speed at say 65 you can expect to see about 5 to 8 pounds less than the idle vacuum. In your case Boudin I'd say -11 to -15 would be about normal.

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    Forum Moderator Boudin's Avatar
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    Quote Originally Posted by gpmorgan View Post
    Since the vacuum is controlled by how much air that is let in the engine(throttle plates) it's always going to be changing when under load. As far as a cruising speed at say 65 you can expect to see about 5 to 8 pounds less than the idle vacuum. In your case Boudin I'd say -11 to -15 would be about normal.
    That's pretty much what I understood it to be. You brought up a good point by you mentioning the "negative" pressures. The article mentions that manifold pressure is another term for vacuum....but they don't explain how it is different in this way.

    You guessed it on the nose with what I am seeing when cruising down the road.

    What would you speculate you would see on the vacuum gauge if....say....your advance curves need adjustment.....or you might not have the timing at the most ideal....or any other issue that could be possibly identified by what you are seeing on the vacuum readings while driving?

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    With a retarded condition the vacuum reading, of coarse would be lower, given the engine would be under load (lugging). It's hard to give exact numbers because each engine has it own characteristics. It would take a lot of trial and error of setting timing, changing weight springs, and even adjusting valves to get optimum gauge reading. I'd say if you can go through the entire throttle range, at a slow rate or all at once, without detonation or stumbles then you are close to where it needs to be.

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    Forum Moderator Boudin's Avatar
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    Quote Originally Posted by gpmorgan View Post
    With a retarded condition the vacuum reading, of coarse would be lower, given the engine would be under load (lugging). It's hard to give exact numbers because each engine has it own characteristics. It would take a lot of trial and error of setting timing, changing weight springs, and even adjusting valves to get optimum gauge reading. I'd say if you can go through the entire throttle range, at a slow rate or all at once, without detonation or stumbles then you are close to where it needs to be.
    I agree. From what I have read and understand....steadiness in the needle is what you want to see. Of course, it will fluctuate with the throttle being opened and closed....but as long as the needle doesn't jump around without explanation, idles steady at 17-21", and never reaches 0"....then all is well.

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    Yeah, I think that would be safe to say given there is really no exact set of numbers to say for sure.

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