Ignition Lead Inspection; Spark Plug Inspection - Lycoming IO-390-D Series Maintenance Manual

F. Identify by tag or label each spark plug as it is removed, according to cylinder number and
top or bottom position.
G. Remove and discard the gasket from the spark plug (Figure 2).
Figure 2
Spark Plugs

3. Ignition Lead Inspection

NOTICE: The ignition leads are an all-weather, shielded wire constructed with over braid.
A. Examine spark plug ignition leads (Figure 3) and spark plug ceramics for corrosion and
deposits.
B. Examine each ignition lead for chafing, insulation breakdown, frayed wiring, deterioration,
heat damage, wear, and cracking. Replace the harness if any lead shows evidence of chafing,
cracks, or wear, frayed wiring, or damage.
C. Examine the ignition lead connections. Make sure the ignition lead connections are secure.
D. Make sure the ignition lead mounting clamps are tight.

4. Spark Plug Inspection

NOTICE: Corrosion and deposits are evidence of leaking spark plugs or incorrect cleaning of the
spark plugs or connector ends.
A. Remove the spark plug connector nuts.
B. Examine each spark plug for chafing, corrosion, wear, and cracking during every inspection.
Replace any worn, cracked or corroded spark plug with a new spark plug.
NOTICE: The general guidelines in Table 1 identify acceptable and unacceptable spark plugs.
The figures in Table 1 show the condition of the fine wire ground and center
electrodes as well as the level of wear indication and condition of the spark plug.
Refer to your spark plug manufacturer's manual for specific instructions.
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IO-390-D Series Engine Maintenance Manual
IO-390-D Series Engine Maintenance Manual
Figure 3
Ignition Lead
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March 2020