GxT V055-01 Diagnostic Analyzer - Manual (Page 14)

14
Power Contribution
This test measures each cylinder’s contribution to
the power output of the engine, without using spark
suppression. Power Contribution is not available when
Engine Setup "Ignition" is set to Diesel.
To run this test on distributor engines, the black Primary
Amps probe must be placed around the wire supplying
12 volt power to the ignition coil(s) with the label facing
toward the battery feed.
There are two screens available. Press SELECT to
change between screens. The bar graph shows relative
power contribution per cylinder. The other screen shows
the variations numerically:
This test relies upon steady timing, fixed air-fuel feed, and a constant amount of engine drag to make
cylinder comparisons. To put a steady drag on the engine, turn the heater fan on high and the headlights
on bright. Keep the engine at idle RPM. However, if idle is surging, use a throttle holder to bring it up to
1200. Low RPM gives better readings provided that RPM is stable. Putting an engine in open loop mode
may also help.
The graph suggests how cylinder power differences affect the RPM during an engine cycle.
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
1 2 3 4
Cylinder
Sequence
RPM
3 -10 8 -1
Power
Reading
POWER DIFFERENCE WAVEFORM