avgas contaminated by jet fuel. Fuel additives, such as lead, boost the
octane rating and slow down the combustion process, producing an even
pressure to the piston.
Pre-ignition
is caused by hot spots in the cylinder. Ignition occurs prior to
the spark plug firing. The EDM-760/790 depicts pre-ignition as a sudden
red line of the EGT on the analog display. This may occur in one or more
cylinders. The affected cylinder column(s) will flash while the digital
display will show an EGT higher than 2000°F.
At this temperature pre-
ignition will destroy your engines in less than a minute unless you take
immediate corrective action.
Data Logging
On monthly intervals, you may choose to jot down peak EGT,
measurement sequence values, and cruise engine settings on a
data
logging worksheet
in the back of this book. Look for trends as well as
absolute values. DIF is a good indicator of the overall health of the
engines. Typical values are less than 80°F for a factory new fuel injected
engine, and less than 150°F for a carbureted engine. If you discover a DIF
spread greater than this, identify the effected cylinder and initiate
preventive maintenance.
Trend data for EGT and CHT is also of value. Any departure from a
cylinder’s baseline requires investigation. Refer to the “Engine Diagnosis
Chart” on page 26. Data logging as a means of identifying trends is of
considerable value in preventative engine maintenance.
Section 8 -
Fuel Flow Option Operation
Select Switch
The select switch is a three-position toggle switch mounted on your
instrument panel near the display of the EDM-760/790. Any alarm
warning will appear regardless of the select switch setting.
In the
EGT
(
T
emperature)
position only the installed
temperature (and battery voltage) measurements are displayed
Page 30
Engine Data Management