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Vacuum diagnosing your car.

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  • #16
    What is the Pulse Air Reed Valve that connects to the catalytic converter? What if the cat no longer exists? Should this be plugged and where is it located on a fuel injected engine?


    • #17
      Figured out how to cut and paste the text here.....

      (note: what I get most on my Festiva is a jerky motion in
      the vaccum quage which means it's time to wipe clean
      the igntion wires and distrubutor cap because the engine is


      Diagnosing car engines with a vacuum gauge.

      This is a consolidation of diagnostics from three sources:
      1. instructions for Equus vacuum gauge
      2. Chilton general car care manual
      3. Haynes emissions control manual

      Engine must be warm. Select a vacuum hose connected directly to
      the car's intake manifold, not to throttle or carburetor, or
      select an unused port on the manifold. Connect the gauge.
      (Suggestion: connect with a T-fitting to hose going into manifold
      pressure (MAP) sensor.) Start the engine and read the gauge. The
      gauge will give vacuum readings in inches of mercury (chemical
      symbol Hg) or kilopascals (ka).

      1. Equus instructions
      a. testing at idle speed
      - compare vacuum reading with manufacturer's specification.
      - a lower reading indicates possible incorrect timing,
      incorrect valve timing or adjustment, incorrect setting
      of idle mixture, worn piston rings, or leak in intake
      - readings that change slowly indicate incorrect setting of
      idle mixture screw.
      - readings that change quickly indicate sticky valve guides,
      burned valve sets, or leak in head gasket.
      b. testing at 2000 rpm
      - a lower reading indicates possible restriction in exhaust.
      - oscillating reading indicates possible weak valve springs.

      2. Chilton general car care manual.
      - gauge reading steady 17-22 in Hg indicates normal engine in
      good condition.
      - gauge reading low (15-20 in Hg) but steady indicates late
      ignition or valve timing, low compression, stuck throttle
      valve, leaking carburetor or manifold gasket.
      - gauge reading steady but dropping regularly indicates burnt
      valve or improper valve clearance.
      - gauge reading dropping gradually at idle indicates choked
      muffler or obstruction in exhaust.
      - gauge reading slowly dropping to zero as engine speeds up
      indicates choked muffler.
      - gauge reading fluctuating between 15 and 20 in Hg at idle
      indicates stuck valve or ignition miss.
      - gauge reading drifting indicates improper carburetor
      adjustment or minor intake leak at carburetor or manifold.
      - gauge reading fluctuating as engine speed increases
      indicates weak valve springs, worn valve stem guides.
      - gauge reading vibrating excessively at idle but steady as
      engine speeds up indicates worn valve guides.
      - gauge reading vibrating excessively at all speeds indicates
      leaky cylinder head gasket.

      3. Haynes emissions control manual
      a. testing at various speeds
      - engine starting vacuum should be 1 to 4 in Hg. To test
      disable ignition (ground wire from coil), hold throttle
      wide open, crank engine slowly with starting motor.
      - healthy engine at idle should read steady 15 to 20 in Hg.
      - healthy engine at 2000 rpm should read steady 19 to 21 in
      - healthy engine at open throttle should read close to 0 in
      - healthy decelerating engine reading should jump to 21 to
      27 in Hg as open throttle released.
      b. testing at idle speed
      - low steady reading usually indicates leaking gasket
      between intake manifold and carburetor or throttle body,
      leaky vacuum hose, or incorrect camshaft timing.
      - low fluctuating (3 to 8 in Hg below normal) reading may
      indicate intake manifold gasket leak at an intake port
      or faulty injectors on port-injected engines.
      - regular drops (2 to 4 in Hg) in reading at a steady rate
      indicates probable leaking valves.
      - irregular drops in reading indicates possible sticking
      valve or ignition misfire.
      - rapid vibration (4 in Hg) in reading combined with exhaust
      smoke indicates worn valve guides.
      - slight fluctuation (1 in Hg) in reading indicates possible
      ignition problems.
      - large fluctuation (10 in Hg) in reading indicates likely
      weak or dead cylinder or blown head gasket.
      - slow movement through wide range in reading indicates
      possible clogged PCV system, incorrect idle fuel mixture,
      or gasket leak between carburetor, throttlebody, or
      intake manifold.
      c. testing at higher speeds
      - rapid vibration (4 in Hg) in reading at increased engine
      speed indicates leaking intake manifold gasket or head
      gasket, weak valve springs, burned valves, or ignition
      - reading returns slowly to normal and didn't peak above
      normal (5 in Hg) after dropping to zero when throttle
      quickly snapped open (2500 rpm) suspect worn rings.
      - reading returns to normal after long delay when throttle
      quickly snapped open (2500 rpm) suspect blocked exhaust.
      d. testing for blocked exhaust
      - idle speed reading slowly dropping toward zero indicates
      exhaust restriction.
      - excessive backpressure in exhaust then indicated by
      reading not increasing quickly to about 16 in Hg when
      engine speed slowly increased to 2000 rpm.
      - backpressure also indicated by reading not dropping as
      quickly when throttle quickly released and remaining 5 in
      Hg higher or more than normal.
      - disconnect exhaust manifold from engine and retest. If
      problem disappears exhaust system is blocked.
      - to locate restriction reconnect exhaust system one unit at
      a time testing after each until problem reappears.
      Last edited by WmWatt; 06-27-2011, 04:57 PM.
      Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006. 140k km as of June 2021.


      • #18
        Specifically, this is for a 95 FI aspire, but its the same for some other models as well, figured I'd throw it in this thread, maybe it will help someone someday. It came from an electronics and vacuum troubleshooting manual (evtm)

        Vacuum Distribution Diagram
        Last edited by zoom zoom; 08-26-2011, 10:04 PM.
        2008 Kia Rio- new beater
        1987 F-150- revived and CLEAN!!!
        1987 Suzuki Dual Sport- fun beater bike
        1993 Festiva- Fiona, DD
        1997 Aspire- Peaspire, Refurb'd, sold
        1997 Aspire- Babyspire, DD
        1994 Aspire - Project Kiazord
        1994 Aspire- Crustyspire, RIP

        "If it moves, grease it, if it don't, paint it, and if it ain't broke don't fix it!"


        • #19
          Glad this is a sticky. Simple but great info.
          Go ghostiva, go!

          '88 LX "tiny little ghost" White - 5 speed


          • #20
            Hey guys quick vacuum question. So at the track friday we ran an 18.98 but i know it can run a LOT quicker if it wouldn't lag on me. Right off the line and at idle it has issues accelerating, it lopes really bad for a second or two then drives upward just fine, it happened to me off the line as well as moving into 2nd gear it loped again on me and i had issues driving through it, i shifted a little too early as well but thats another story. My friends told me it sounds like a vacuum problem because there's not enough air going through the MAF to allow it to accelerate quickly. Are they right?

            Car #789
            Sponsors: Williams American Construction, Dewaynes Tire service, Roofing Supply Group
            1992 Festiva L - BP Swapped, Aspire Swapped, Rally America Sanctioned.
            1993 Festiva GL - 4sp Automatic and the bluest car i've ever seen...


            • #21
              Could also be TPS.
              My new engine has a throttle dead spot around 1500-2000 RPMs. The engine will lope when I rev it at that range when sitting still and has a flat spot when accelerating. Especially when just coasting in gear then getting back on the gas slowly. It doesn't respond and then suddenly kicks in.
              I'm going to change out my TPS with the known good one on my old engine soon.
              Red 88 L (Ocho)