Lesson 3/Learning Event 2
If the reading is a little low and the needle floats slowly back and forth from about 12 to 16 inches, the fuel-air
mixture is not correct. This is usually caused by an improperly adjusted carburetor which is covered in later
A plugged or partly plugged exhaust system will result in a normal reading when the engine is first started. But,
the reading will drop off as the engine runs. How quickly the reading drops will depend on how badly the
exhaust system is plugged.
A steady but below normal reading indicates a fault that affects all of the cylinders. Just how low the reading
is will depend on how bad the fault is. Some of the possible faults are incorrect ignition or valve timing, a leak
in the intake manifold or carburetor mounting gaskets, and uniform piston ring leakage.
If all the piston rings are leaking enough to lower the reading, the compression will also be low and blue smoke
will be coming out of the exhaust. The blue smoke is caused by oil leaking from the crankcase past the rings
If the valve timing is incorrect, the low vacuum reading would also be accompanied by low-compression
readings. If an engine uses timing gears, the valves will not get out of time once the engine has been put
together properly. However, if it uses a chain and sprockets to drive the camshaft, it is possible for a worn
chain to slip over one or two teeth on a sprocket. This will upset the relative position of the crankshaft and
camshaft and change the valve timing.
Suppose that the gage needle repeatedly drops back about 3 to 6 inches and then returns to normal. The needle
movement is regular and seems to be timed with the engine speed. This reading indicates a condition that is
affecting one or more, but not all, cylinders. Problems that can cause this are an ignition miss and a leaky
valve or head gasket. If the needle just drops back once in a while, the trouble is probably a sticky valve or an
You can locate the cylinder or cylinders that are affected by disconnecting and connecting the spark plug leads
one at a time while watching the gage. When you disconnect a lead and the action of the needle is not
affected, you know that this is the affected cylinder.