J.P. Instruments EDM 930 Primary Pilots Guide - Manual (Page 15)

For Your Safe Flight Page 10
Engine Diagnosis Chart
The following chart will help you diagnose engine problems in your aircraft.
Display Symptom Probable Cause Recommended
Action
TIT ~100°
higher than
EGTs This is normal
75° to 100°
EGT rise for
one cylinder
during flight Spark plug not firing
due to fouling, faulty
plug, wire or
distributor. Enrich mixture to
return EGT to
normal. Have plugs
checked.
EGT Increase
or decrease
after ignition
maintenance Improper timing:
high EGT
 retarded
ignition; low EGT

advanced ignition.
Check EGT for each
magneto to
determine any
uneven timing.
Loss of EGT
for one
cylinder.
Engine rough Stuck valve. Other
cylinders are okay. Have valve train
checked.
Loss of EGT
for one
cylinder; no
digital EGT Failed probe
or
failed wire harness. Swap probes to
determine if probe
or wire harness is
bad.
Decrease in
EGT for one
cylinder Intake valve not
opening fully; faulty
valve lifter. Have valve lifter or
rocker arm checked.
Increase in DIF
at low RPM Low compression
(blow by) in cylinder Check compression.
EGT and CHT
not uniform Normal for
carbureted engines.
Dirty fuel injectors or
fouled plugs. Check injectors and
plugs.
Decrease in
EGT for all
cylinders Decrease in airflow
into the induction
system. Carb or
induction ice. Check for change in
manifold pressure.