Cessna 172 suction gauge tolerances
The Cessna 172 1980 Model 172N Information Manual states the desired range of suction indicated is between 4.5 to 5.4 inches of mercury (see Airplane & Systems descriptions). A suction reading below this range may indicate a system malfunction or improper adjustment and in this case the indicators should not be considered reliable.
Under Normal Procedures before take off there is a requirement to test the magnetos at 1700 rpm and to check the suction gauge. Presumably that means to ensure the suction reading at 1700 rpm is within the allowable range.
Occasionally one sees a C172 where at 1000 rpm the suction gauge is not only below the minimum of 4.4 inches of mercury but so far below as to be off the clock. Normally after engine start it is good airmanship to check the suction system is operating correctly. At idle power even with well below the lower limit readings, in some C172 the gyros are erect and readings of the artificial horizon and directional gyro seem normal. Furthermore, when turning during taxiing gyro indications seem normal even though the suction gauge is almost zero reading.
Question: At idle rpm assuming 1000 rpm set, should the suction gauge read between 4.4 and 5.4 inches of mercury? Or are those limits only valid at 1700 rpm and above? The question arises when the aircraft is certified for IFR and the gyro instruments must be reliable before take-off. Idle power readings below those limits while taxiing for take-off may not ensure the gyros are reliable before entering IMC after lift off.