what exactly is a vacuum leak. Are engines supposed to be air tight? ?


But engines suck air in to mix with fuel so it's not air tight? the term "vacuum" is just confusing to me

Update 2:

what's being leaking? air? fuel mixture? Or some kind of liquid? maybe oil?

17 Answers

  • Anonymous
    1 month ago

    The pistons taking in air during the induction phase creates a partial vacuum in the intake manifold that sucks in the fuel/air mixture.  Part of this vacuum is tapped off to operate various other ancillary systems like the power brakes, air conditioning controls, some emission controls, and in some very old cars to operate the windshield wipers.  A tiny split or crack in an old vacuum line can cause performance issues in any systems actuated by manifold vacuum and causes the engine to perform poorly due to the fuel/air mix being diluted.  A vacuum gauge can tell you if you have the correct vacuum which should be around 19-21 millimeters of mercury (mm/hg) in a 4 cylinder engine.  Run an unlit propane torch around the vacuum lines to find the leak.  The engine will speed up when the propane is sucked into the leak. 

  • 1 month ago

    Air sucked into intake has to be precise, under any and all changes, constant tuning every time you add or remove something.. a vaccum leak can be detected by a spray bottle full of water..  Run vehicle and then spray water, usually intake areas will detect water and stumble the motor..   Don't dump cold water on motor...   

  • 2 months ago

    Vacuum leaks are leaks in the intake manifold and can be bad intake manifold gaskets or a bad runner hose that is attached to the intake manifold. The amount of air going into the motor is monitored by the MAF sensor and it's part of the system that tells the fuel injectors how much gasoline to give the motor. Any leaks going into the intake manifold messes the fuel injection up.

  • Anonymous
    2 months ago

    Engine is suppose to be air tight and the suction is suppose to be going through one opening.

    . Lets use a car with a carburetor. Air gets sucked in past the air filter and into the motor .  As it goes past the venturi (the narrow part of the carb, it sucks in some fuel and now that air fuel mix goes to the cylinders.  As you have many hoses connected to the carburetor, each of these smaller hoses also have a vacuum. None of them deliver fuel, they are for other purposes, like power assisted steering or windshield wipers or other.   After many years of driving with a hot car engine the stretch in the rubber hoses gets cooked out so they do not fit as tight as they should and you get a vacuum leak.  You need to replace those hoses(about every 7 years) to keep the vacuum only going through the upper part of the carb so it can draw in gasoline to feed the engine.

    Most times it is sucking in extra air from the side which is not going in from the top so the engine idle gets erratic

    See  a mechanic and watch what he does.

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  • Anonymous
    2 months ago

    Vacuum controls many Functions on a car if there is a Vacuum leak some functions won't work

    I can adjust my headlights up or down depending on Fuel and Passengers it is operated by Vacuum

    the Old Charcoal canister was Purged into the Fuel system by a Vacuum Valve Distributors advance and retard by Vacuum

  • fuzzy
    Lv 6
    2 months ago

    The throttle controls how much air enters the inlet tract; under light throttle the tract is at a partial vacuum. The amount of fuel needed by the engine is controlled by how much air enters the inlet tract via the throttle body. If there is a leak between this body & the inlet valve there will be more air than needed & the car will run rough & eventually burn out valves.

    This is what is meant by a vacuum leak.

  • 2 months ago

    Yes. Any air leak is bad.

  • Anonymous
    2 months ago

    YES, the engine is supposed to be airtight between the air intake and the exhaust pipe. As each piston sucks air and fuel into its chamber, it creates a vacuum that draws air from the outside through the filter. In between is a port in the side of the throttle body (or carburetor) where that vacuum can be tapped. The vacuum hose not only sucks out exhaust gases that leaked past the pistons to be re-burned but is also used to boost the power brakes. (It has also been used in the past to run things like windshield wipers and concealed headlights.) A vacuum leak throws off the air/fuel ratio and can cause rough running among other things.

  • 2 months ago

    Air is supposed to go in through the air filter and mix with the fuel.  But it is not supposed to go in anywhere else.  There are tubes running all over the engine for various functions, including power brakes and pollution control.  These run off the low pressure of the vacuum (engine sucking air).  Sometimes these hoses crack or come loose and allow air to enter where it is not supposed to. (vacuum leak letting air leak into the hose).

         Sometimes it is difficult to find the exact leak, but usually not too difficult to fix.


          Vacuum leak = air leaking in where it is not supposed to

  • Barry
    Lv 5
    2 months ago

    It can mean a leak around the inlet manifold or the brake servo. It's not strictly a vacuum more a low pressure inward air leak.

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