Announcement

Collapse
No announcement yet.

Vacuum diagnosing your car.

Collapse
This is a sticky topic.
X
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • Vacuum diagnosing your car.

    Vacuum Gauge Engine Performance Testing

    A vacuum gauge shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. The pistons in the engine serve as suction pumps and the amount of vacuum they create is affected by the related actions of:
    # Piston rings
    # Valves
    # Ignition system
    # Fuel control system
    # Other parts affecting the combustion process (emission devices, etc.).

    Each has a characteristic effect on vacuum and you judge their performance by watching variations from normal. It is important to judge engine performance by the general location and action of the needle on a vacuum gauge, rather than just by a vacuum reading. Gauge readings that may be found are as follows:

    Normal Engine Operation

    At idling speed, an engine at sea level should show a steady vacuum reading between 14" and 22" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 23" or more.

    General Ignition Troubles Or Sticking Valves

    With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, spark plug gap, primary ignition circuit, high tension cables, distributor cap or ignition coil. Fluctuations of 3 to 4 inches may be sticking valves.

    Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, manifold-to-carburetor gaskets, vacuum brakes or the vacuum modulator. Low readings could also be very late valve timing or worn piston rings.

    Exhaust Back Pressure

    Starting with the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present.

    Cylinder Head Gasket Leakage

    With the engine Idling, the vacuum gauge pointer will drop sharply, every time the leak occurs. The drop will be from the steady reading shown by the pointer to a reading of 10" to 12" Hg or less.

    If the leak Is between two cylinders, the drop will be much greater. You can determine the location of the leak by compression tests. Fuel Control System Troubles

    All other systems in an engine must be functioning properly before you check the fuel control system as a cause for poor engine performance. If the pointer has a slow floating motion of 4 to 5 inches - you should check the fuel control.

  • #2
    Any one want to bump this to a sticky? I have found it very useful in the past!

    Comment


    • #3
      Look at www.ncf.ca/~ag384/Cars.htm for a guide I wrote to diagnosing ending performace wiht a hand held vaccum guage. The iginition system on the Festiva is weak and quirky so I stick the vacuum guage on every once in a while to see how it's firing and clean off the components when it's missing.
      Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006.

      Comment


      • #4
        You need to make sure that the vacuum is stronger than 18inches. Mine ran terrible and was moving 16inches of vacuum, but a properly running engine will show about 20 or more inches of vacuum.
        "If you're not first you're last!" -Ricky Bobby

        -91L "Oliver" Now B6DE swapped with an ATX Capri ECU on a MTX!!!!
        The little Festy that could...and did!
        (Gone on to a new life)
        -90 Ford Bronco II Eddie Bauer

        Comment


        • #5
          Also make sure that the engine is fully warmed up to run any vacuum tests. I didn't see that mentioned. Thanks
          Is that someone there ??? 1981 VW diesel pickup---1988.5 Suzuki Samurai tin top---1993 Festiva---1974 Bricklin SV-1 ( very thirsty basket case)---2003 Tracker(bells and whistles)




          )

          Comment


          • #6
            Originally posted by WmWatt View Post
            Look at www.ncf.ca/~ag384/Cars.htm for a guide I wrote to diagnosing ending performace wiht a hand held vaccum guage. The iginition system on the Festiva is weak and quirky so I stick the vacuum guage on every once in a while to see how it's firing and clean off the components when it's missing.
            This one is missing.
            ENFORCER - Midwest Festiva Inc., Iowa

            #1 '90 Sport to modified Lx - RollazX
            #2 .....Cheesehead
            #3 '91 White - Donor Car
            #4 .....Montana Project
            SOLD----Levistiva for $1500
            Bought her back for $450
            Now that's darn near priceless!!

            Comment


            • #7
              Originally posted by WmWatt View Post
              Look at www.ncf.ca/~ag384/Cars.htm for a guide I wrote to diagnosing ending performace wiht a hand held vaccum guage. The iginition system on the Festiva is weak and quirky so I stick the vacuum guage on every once in a while to see how it's firing and clean off the components when it's missing.
              Either this is a dead link or it's mistyped; this goes nowhere.

              Comment


              • #8
                The diagnostics webpage is now here www.ag384bn.bravehost.com/Cars.htm
                Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006.

                Comment


                • #9
                  If I wanted to improvise a vacuum gauge from a saxophone(or clarinet) mouthpiece, with a bleeder screw adjustment for calibration; wouldn't an appropriate pitch signify what were being drawn-assuming reasonable familiarity?(probably an enclosing chamber to protect the reed and mouthpiece tip or perhaps a double-reed would be advised, that likely would also assume the regulation for calibration function)

                  p. s. Plastic reeds are now available which are quite stable and long-lasting.
                  Last edited by bobstad; 07-17-2009, 09:47 AM.
                  '91 Festiva L/'73 Windsor Carrera Sport custom

                  (aka "Jazz Bobstad," "The BobWhan," etc.)

                  Art is the means whereby(a) society advances: Religion is the definition of the parameters of art. Poetry is the actualization of these...

                  Comment


                  • #10
                    Thought I'd add this to the sticky, since the question of vac line diagrams comes up fairly often.

                    http://www.fordfestiva.com/forums/sh...vacuum+diagram

                    Dumb thieves go to prison, smart ones go to work for the Government.

                    1988 L - 232K miles Batstiva
                    1989 L - 247K miles Slick
                    1990 L - 281K miles Orphan Annie
                    Let the hoarding begin!!

                    Comment


                    • #11
                      hi,seems to be a dead lead.

                      Comment


                      • #12
                        heres the diagrams as shown but not all of them













                        FORD BUILT TOUGH SINCE 1987-1993 FORD FESTIVA LX

                        Comment


                        • #13
                          I just vacuum tested both my vehicles.
                          My truck pulled 17 inches Hg and held it steady and it dropped, peaked, and returned as it should when you open the throttle.
                          My Festiva however... At idle it was rapidly moving between 17-20 inches Hg. My Haynes manual says I should perform a leak down test for rapid vibration of about four inches. It also stated for fluctuations around one inch to check ignition system components. I will check the plugs, cap, and rotor when the engine cools off just to get those out of the way.
                          Also, my idle is pretty low- the engine nearly stalls sometimes and its really rough when sitting at a stoplight. I noticed the vacuum gauge drop a tiny bit when the engine idle stuttered. Do I have compression issues (rings, valves), ignition issues, or just a low idle (can it be adjusted on FI cars?). I did the test at operating temp, mileage 18XXXX, elevation approx 2,000Ft.
                          ~Austin
                          Red 88 L (Ocho)

                          Comment


                          • #14
                            Originally posted by WmWatt View Post
                            The diagnostics webpage is now here www.ag384bn.bravehost.com/Cars.htm
                            OK, the main page is there, but the links go nowhere...
                            (At least my slow dial up fails to load...)

                            (Which dropped me when I tried to post this...)
                            Did cenus work, been on "roads" I shouldn't have!

                            Comment


                            • #15
                              That free website provider cut space available so they're not visible now. I tried another free site but they don't allow direct links or something. I have the file on my computer but don't know how to post it anywhere.
                              Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006.

                              Comment

                              Working...
                              X