The following are engine problems
that can be readily diagnosed by interpretation of the GEM. Frequently
the same symptom may result from different engine faults. In some
instances, with the aid of the GEM, it is possible to pinpoint
the exact cause even while airborne. You may also experience changes
in engine performance (reduced power, roughness, etc.) simultaneously
with the GEMs annunciation of a problem. Due to the Graphic Engine
Monitor's unique ability to display both EGT and CHT for all cylinders
at once, it is usually apparent which cylinder is responsible
for the malfunction. This pinpoints the problem for the mechanic
and facilitates making the necessary repairs.
Determination of a problem depends
upon your knowledge of the engine. As you accumulate flying time
with the instrument, you will observe and recognize the pattern
of bars that is normal for different phases of flight. When the
pattern is not normal you will have reason to investigate. The
GEM can assist in finding a malfunction before it becomes serious.
Generally, the symptoms described
in this diagnostic guide will occur in cruise. Finding engine
problems during start-up is covered under Using the GEM on the
1. GEM shows: a
gradual or sudden rise in EGT of one cylinder. When temperatures
are 50°F above normal the display will blink to warn you of
the rise in temperature.
Probable cause: A) A fouled or
defective spark plug or ignition wire. A plug with a cracked insulator
may misfire at high altitude but function normally on the ground.
What to do: Switching to left
and right mags momentarily will determine which plug or lead is
at fault. Switching off the good plug will cause the EGT in affected
cylinder to drop while all others will rise. A fouled plug may
clear itself when mags are switched.
Probable cause: B) A reduction
in the fuel supply to one cylinder. A partially plugged injector
will cause EGT to rise. A leak in the fuel supply tube between
the fuel distributor and injector will have the same effect. A
completely plugged injector or fuel supply will result in no combustion.
What to do: Switching mags will
cause EGTs to rise in all cylinders. There will be no drop in
temperature as with the fouled plug.
2. GEM shows: above normal CHT in one or more cylinders.
Probable cause: A) Broken ring(s)
will cause higher CHT because of additional cylinder wall friction.
What to do: Perform a compression
test. Low compression with leaks past the rings will indicate
ring problems. Leaky valves will tend to mask the problem. Depending
upon the location and number of breaks, the effect on a compression
test will vary.
Probable cause: B) Misplaced or
damaged engine cooling baffles can adversely affect the direction
of cooling air to certain cylinders. The temperature may vary
with aircraft altitude and phase of flight (climb, cruise, descent).
What to do: Open cowl flaps and
reduce power. If the temperature is dangerously high, cool the
engine with enriched fuel flow and land as soon as possible.
Probable cause: C) Obstruction
in the cooling system such as a bird's nest under the cowling.
High CHTs resulting from this problem may be apparent during run-up
What to do: Inspect under the
cowling and remove any obstructions.
3. GEM shows: high CHT readings on one side of the
engine (cylinders 1, 3 and 5, or 2, 4 and 6).
Probable cause: A) Restricted
cooling air for one side of engine caused by either a misaligned
cowl flap or restriction at the air intake. On some aircraft one
inch of cowl flap misalignment will cause a 50°F difference
4. GEM shows: all EGTs rise uniformly and the entire
Probable cause: A) One magneto
has failed with only one spark plug firing in each cylinder. The
temperatures rise uniformly in all cylinders.
What to do: Check to make sure
the mag switch is correctly positioned. Do not switch to single
mag operation in flight (unless you want a total power loss).
Enrich the mixture to reduce EGT. Land as soon as possible.
5. GEM shows: abnormally low EGT in one cylinder. (Two
or more bars lower than the normal indication for your engine).
Probable cause: A) An exhaust
leak between probe and cylinder, a cracked pipe, a loose or warped
flange or a blown gasket.
Probable cause: B) An intake valve
is not opening completely, resulting in a partial change of the
fuel-air mixture, and consequently a lower combustion temperature.
Probable cause: C) An intake or
exhaust valve is not closing completely, a burned valve, or poor
compression. Reduced compression results in a lower combustion
Probable cause: D) In turbocharged,
fuel injected engines an induction leak will enrich the mixture
and cause a drop in EGT.
What to do: Perform the appropriate
tests and inspections.
6. GEM shows: less uniform EGTs (most visible at cruise
Probable cause: A) Dirty injection
nozzles. Fuel nozzles may accumulate residue which will restrict
fuel flow slightly. The resultant rise in EGT is usually one or
two bars in several cylinders.
What to do: Clean fuel nozzles.
Regular service will prevent recurrences.
Probable cause: B) Induction system
leaks, bad seals or bad gaskets.
What to do: Inspect and repair
the induction system as required.
7. GEM shows: high CHT on one or more cylinders under
all conditions after engine overhaul.
Probable cause: One aircraft owner
discovered a piston connecting rod of the wrong type (too long)
installed during an engine overhaul. This increased the compression
ratio for this cylinder and caused the high CHT. Suspect this
unusual problem only as a last resort.
8. GEM shows: all columns move up and down rapidly
several bars. The action is random and can be described as bars
dancing up and down.
Probable cause: A faulty ignition
system, harness, ground or magneto. GEM will pick up an impending
ignition harness failure before it is serious enough to materially
affect engine performance. This may also be caused by a loose,
dislocated, or damaged spark plug cap or a fault within the magneto.
What to do: Run-up the engine
and check operation on each magneto. If the symptoms diminish
or change on one mag then the problem is conclusively ignition
related. This type of interference can frequently be heard on
a Com radio with the squelch open or on the ADF. It is not only
an indication of impending engine problems, but will reduce Com,
Nav, ADF, and Loran performance.
The GEM will frequently identify
problems in an ignition harness that tests OK on a high tension
What to do: If the problem shows
on only one mag then replace that harness otherwise replace both.
9. GEM shows: extremely high CHT reading on one or
Probable cause: Exhaust leak at
flange which causes hot exhaust gases to strike the CHT probe.
What to do: Look for a blown exhaust
gasket, a loose or cracked exhaust manifold, or a loose or missing
exhaust manifold stud.
10. GEMINI shows: Left engine normal, right engine has
a slight drop in EGT and rising CHT indications in one or more
cylinders possibly accompanied by engine roughness.
Probable cause: Detonation.
What to do: Reduce power and enrich
the mixture. On the ground, Inspect the cylinders for signs of
detonation, look for fuel contamination, clogged injectors, or
11. GEM shows: Extremely high or full scale EGT indications
followed by high CHTs in one or more cylinders.
Probable cause: Pre-ignition. What
to do: Reduce power and land as soon as possible. Inspect the
cylinder for damage.
12. GEM shows: An increase or decrease in EGT for all
cylinders, especially after engine maintenance.
Probable cause: Retarded timing
will increase EGT and advanced timing will decrease EGT. Note
that cold, dense air with its higher oxygen content produces more
power and higher EGT.
13. GEM shows: a drop in EGT for all cylinders in carbureted
Probable cause: If accompanied
by a drop in manifold pressure in aircraft with constant speed
props (or a drop in rpm for aircraft with fixed pitch props) this
is an indication of carburetor ice.
14. GEMINI shows: Black CHT bar missing in one or more
columns on the left engine, entire EGT columns missing on the
right (CHT bar automatically reverses color to remain visible).
Probable cause: Thermocouple probes
may eventually fail after many thermal cycles. Loose or dirty
connections in the wiring harness between the probes and the display
may have the same symptoms.
What to do: Inspect and test probes
and wiring harness, continuity test will reveal burnt-out probes.
Burnt probes may appear good at low temperatures but fail when
hot. Performing a run-up after swapping suspected faulty probes
with known good ones is the most reliable test.