Chapter
B
: Ignition System Diagnosis and Repair
41
Even if the color of the insulator and deposits are nor-
mal, rounded and worn electrodes indicate that a plug
should be replaced. These plugs are simply worn out.
The voltage required to spark across the gap has in-
creased and continues to do so with additional use.
Misfiring under load is a clue to worn out plugs. Such
plugs also contribute to poor gas mileage, loss of
power, and increased emissions.
Also check spark plugs for physical damage caused by
a foreign object in the combustion chamber, a plug of
the wrong reach being hit by a piston or valve, or by
careless installation. Be careful to prevent dirt from
falling into spark plug holes during service.
Spark Plug Installation
Spark plugs, both new and used, must be correctly
gapped before they are installed. Although a wide vari-
ety of gapping tools are available, a round wire feeler
gauge is the most efficient for used plugs, figure 2-13.
Adjust the gap by carefully bending the ground elec-
trode, figure 2-14.
• Do not assume that new plugs are correctly gapped
• Do not make gap adjustments by tapping the elec-
trode on a workbench or other solid object
Cleaning the threaded plug holes in the cylinder head
with a thread chaser will ensure easy spark plug instal-
lation. With aluminum heads, use the tool carefully to
avoid damaging the threads.
Some manufacturers recommend using an antiseize
compound or thread lubricant on the plug threads. Use
thread lubricant only when specified by the manufac-
turer. Antiseize compound is commonly used when in-
stalling plugs in aluminum cylinder heads. Be sure to
use the specific compound recommended by the man-
ufacturer, as not all are compatible with aluminum.
Whenever thread lubricant or antiseize is used, reduce
the tightening torque slightly.
Once the plug gap has been properly set, install as fol-
lows:
1. Wipe any dirt and grease from the cylinder head
plug seats.
2. Check that gaskets used on the plugs are in good
condition and properly installed.
3. Install the plugs in the engine and run them in by
hand.
4. Tighten the plugs to specification with a torque
wrench following Service Manual specifications.
ROUND WIRE FEELER GAUGE
Fig. 2-13.
Checking spark plug electrode gap.
SPECIAL BENDING TOOL
SIDE ELECTRODE
CENTER ELECTRODE
Fig. 2-14.
Adjusting spark plug electrode gap.
Summary of Contents for ASE-A8
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