Atmospheric pressure at sea level will also support a column of liquid mercury (Hg) 29.92” in height. A more current term refers to the pressure inside the intake manifold as Manifold Absolute Pressure or “MAP.” As currently used, the terms “pressure differential,” “MAP” and “intake manifold vacuum” refer to the difference between atmospheric and intake manifold pressures.Ītmospheric pressure is about 14.7 pounds per square inch of pressure at sea level. Technically speaking, the intake manifold must contain enough liquid fuel and air to support combustion, so what we have is not a complete vacuum, but an atmospheric “pressure differential” between the inside and outside of the intake manifold. Intake manifold vacuum analysis can be a little tricky because the conventional term “intake manifold vacuum” is a technical misnomer. With these thoughts in mind, I began devising a diagnostic strategy that would tell me which bank was misfiring and, with a little luck, which cylinder was misfiring. I suspected a broken valve spring, but removing the valve covers on this particular engine is difficult and time-consuming due to accessory interference. A few moments later, the vacuum reading would stabilize and the engine would idle very smoothly.
When I connected a vacuum gauge to the intake manifold, the needle would jiggle ever so slightly when the engine began misfiring. An ignition scope and compression test yielded no result and neither did spraying the manifold gaskets with aerosol carburetor cleaner.
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