GxT Ferret 54 ENGINE DIAGNOSTIC CENTER - Manual (Page 45)

The graph suggests how cylinder power differences affect the RPM during an engine cycle.
POWER DIFFERENCE WAVEFORM
A low power cylinder will show a negative power difference reading. Smooth running engines
have power difference readings less than 5. Dead cylinders usually read more than -15.
Low power cylinder problems are associated with worn or broken piston rings, valves and valve
springs, broken head gaskets, vacuum leaks, fuel imbalance, spark plugs and wires and EGR
problems. If a cylinder test produces a higher RPM, it may be from crossed spark plug wires,
an open intake valve, leaking EGR, or an engine computer effect.
The results of this test, if compared to the Electronic Compression and Ignition Secondary
tests can provide additional clues about the cause of poor cylinder performance.
To gain experience with typical engines, we suggest that, while you are testing a good engine,
short out a spark plug to see the effect of a dead cylinder.
The readings are taken from the RPM during each cylinder’s power stroke, and adjusted to
give somewhat similar readings for most engines. Look for consistency among cylinders.
Engines with eight or more cylinders may show a weak cylinder in a later position than actual.
For example, a weak #4 cylinder could appear as #5. Engines with particularly heavy flywheels,
for their displacement, may show the effects of a weak cylinder spread across two or three
subsequent cylinders. Engines made with staggered timing patterns will show large difference
readings when they are working correctly. For example: six cylinder engines built from an eight
cylinder block.
1 2 3 4
Cylinder
Sequence
RPM
3 -10 8 -1
Power
Reading
4-16