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Diagnosing Engine Problems with GEM

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 Ground.

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 or taxi.

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 in CHT.

4. GEM shows: all EGTs rise uniformly and the entire display blinks.

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 temperature.

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 power settings).

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 lead tester.

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 more cylinders.

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 timing problems.

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 engines.

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.

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