Troubleshooting with a Vacuum Gauge

Most engines will have a Normal Gauge Reading of 15 to 21 inches of Vacuum.
Before using a Vacuum gauge, engine must be at normal operating temperature. Distributor advance, Must Be working properly and ignition Timing set to specifications. Use Tachometer To Be Certain That Engine Is Running at Specified RPM. Vacuum Gauge must be connected to intake manifold. [see note**]
Following Chart Indicates Possible Malfunction of Various, Vacuum Readings.

* Average and steady at 15-21 inches = Normal.

* Extremely low holding steady Reading with indicator holding steady = Air leak at intake manifold or carburetor; incorrect timing; under powered boat; fault in hull.

* Indicator fluctuates between high and low reading = Blown head gasket between 2 adjacent cylinders (check with compression test)

* Indicator fluctuates very slowly, ranging  4 to 5 points = Carburetor needs adjustment, spark plug gap too narrow, valves are sticking.

* Indicator fluctuates rapidly at idle, steadies as RPM is increased =  Valve guides may be worn.

* Indicator continuously fluctuates between low and normal reading  at regular intervals =  Burned or leaking valve.

* Indicator drops to zero as engine RPM is increased = Exhaust system is restricted.

* Indicator holds steady at 12 to 16 drop back to zero and back to about 21 as throttle is engaged and released =  Possible piston ring leak.
                     ( check compression )
See Chart of maladies.


Note: ** When connecting a vacuum gauge to an engine; connect directly to a vacuum port at the base of or on the carburetor or throttle body itself, when possible. Readings taken from an intake manifold port, that is located very close to an individual cylinder intake valve and/or at the very front or far rear of the engine, can have a dampening effect on your gauge readings. 

V8, V6, and inline engines can have intake manifolds that will distort vacuum gauge readings. Some intake manifolds are "dual-plane" design [sometimes referred to as split-plenum opening] where each side of the opening, below the carburetor, feeds 4 cylinders on a V8, for example. Tapping your vacuum gauge into one of the two planes will result in almost no reading from the other plane, in some cases, meaning that you are only getting a vacuum reading from 4 of the 8 cylinders. 


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