GxT Ferret 54 ENGINE DIAGNOSTIC CENTER - Manual (Page 47)

4-18
Operating the engine above the controlled
idle RPM will minimize the effects of idle
compensators and put the engine at a
smoother operating point. RPMs of 1000 to
2500 are suitable for the test. Listen to the
sound of the engine as suppression is
applied. A large initial RPM drop is a good
indicator of a strong cylinder.
On non-computerized engines the timing,
fuel mix and idle setting do not vary with
time. However, active control systems may
have to be put in open-loop mode to keep
the timing and fuel mixture fixed, so that the
test results do not end with minimal % Drops.
Follow the manufacturer's procedure for
putting the engine controller in "open loop"
mode. Some common ways are:
• The idle speed motor may have to be
unplugged if the test is done at idle.
• Disconnecting the coolant temperature
sensor will usually cause the computer
to operate in open-loop fuel control as if
the engine were cold.
• Where the oxygen sensor is easily
reached it can be unplugged to lockup
the fuel control.
• Timing can usually be fixed by
disconnecting the MAP sensor hose, or
as on Fords, pulling the timing test plug.
• EGR valves should be disabled by
removing and plugging the vacuum hose
to the valve.
• On some engines there is a running test
mode, set through the diagnostic
connector, that lets the test run without
interaction effects.
After the test, carefully restore the
disengaged parts. Service codes may be set
during the test, but they will usually clear
automatically after 50 to 100 starts.
Low power cylinder problems are associated
with spark plugs and wires, piston rings,
valves and valve springs, vacuum leaks, fuel
unbalance, head gaskets, 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.
Manual Mode
Manual suppression can be used to listen for
noise changes in an individual cylinder that
may give clues about the problem.
To suppress ignition in an individual cylinder,
choose
Manual Suppression from the
POWER menu and SELECT the cylinder you
want to suppress. Press NEXT to suppress
ignition to that cylinder.
T
HIS TEST CAUSES UNBURNED FUEL TO BE
PASSED INTO THE EXHAUST
. C
ATALYTIC
C
ONVERTERS BURNING THIS FUEL WILL
BECOME HOTTER
. R
EPEATING THE TEST
FREQUENTLY MAY CAUSE EXCESSIVE HEAT
UNDER THE CAR
.