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  Vacuum gauge


How to read a vacuum gauge?

vac_g1.jpg (5792 bytes) Reading the VACUUM gauge:
Many  engine tuners  optain state-of-the-art diagnostic equipment and  have forgotten one of the simplest, as well as most accurate tuning tools ...

In most cases you can effectively monitor and tune your  car's fuel and timing systems using a quality  vacuum gauge (like the one we provide with our DIY systems).

First a few basics:

  • Connection of the gauge:use a simple "manifold" vacuum source. "This must not be "ported" vacuum that rises as RPM increases, such as vacuum from the distributer that drops when idling". In most cases this will be a direct manifold source or in many cases the vacuum of the servo

  • NOTE: connection to EFI cars is best done "Tee'd" into the source line from the heater system. Other vacuum connections can effect the idle speed as well as information that the ECM needs to see.
  • You must check and if necessary adjust  the ignition timing, valve clearance, sparck plug gap and contact angle of the contact points.
  • After the adjustments are made, you MUST RESET your idle speed setting. You must do this to have a standard or baseline to compare with.
  • If you fine tune the LPG system small adjustments are best, and in fact "optimum"  settings on the vacuum gauge (highest reading) is usually the maximum power. The mixture than is a little richer than it stricktly needs to be. In other words, after the highest reading is reached, the best setting (depending on engine) is to set mixture screws back "lean" approximately 1/16 to 1/4 turn.
  • If your system  uses an  ECOmax, you get the the best milage  when the  vacuum read out will be 5 upto 10 degrees lower. (For more information see the instruction manual.)
  • What does the gauge tell you?

    Let's see .... It can help you find errors in:

    Incorrect mixture, a vacuum leak, leaking power valve, and more ...

    When I was young, I was told to effectively tune an engine with just the vacuum gauge and a timing light. Once the education and practice is there, it is a very effective technique. It takes some patience, but the knowledge of how your engine behaves to tuning changes is worth it.


    Detailed Vacuum gauge needle readings explained below (images at right):

    Steady needle Normal reading (usually 17-22" Hg. in stock engines) Race engines vary "a lot" and in most cases will be considerably less.

    Intermittent fluctuation at idle Ignition miss, sticking valves, lifter bleeding off (hydraulic), or just a BIG camshaft

    Low, though steady reading Late timing, low compression, sticking throttle valve, carb or manifold vacuum leak (remember most fast engines with a big cam and a tight centerline and high overlap will be naturally low ... you must decide your baseline vacuum reading)

    Drifting needle Improper carb setting or minor vacuum leak

    Fluctuating needle as RPM increases Ignition miss, blown head gasket, leaking valve or weak or broken valve spring

    Steady, but needle drops regularly Burnt valve or incorrect valve setting (too tight), "needle will fall when bad valve operates"

    Gradual drop at idle Clogged exhaust, excessive backpressure 

    Excessive vibration that steadies as RPM increases Worn valve guides

Be aware!

some gauges turn clockwise some anti clock wise 

Normal: (stock engine)

vac_g2.jpg (2855 bytes)

Clogged exhaust:

vac_g3.jpg (3067 bytes)

Blown head gasket:

vac_g4.jpg (2960 bytes)

Incorrect valve timing:

vac_g5.jpg (2818 bytes)

Worn out valve guides:

vac_g6.jpg (2830 bytes)

Mixture adjustment needed:

vac_g7.jpg (2841 bytes)


Sticking valves:

vac_g8.jpg (2882 bytes)

Burnt or leaking valves:

vac_g9.jpg (3007 bytes)