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APR Tech Videos

Spark Plug Gapping

STEP 1. Use the correct spark plug for your application. Pay particular attention to material type, heat range, and thread length.

STEP 2. Use the proper tools. The video shows which tool for adjusting the ground strap and a feeler gauge to measure the air gap. Do not use the wire loops. It is not recommended to tap the spark plug on a flat surface to lower the air gap. This could potentially damage the plug if the center electrode and ground strap make contact.

NOTE: Refer to your installation manual for the proper spark plug gap to be used. Although the spark plugs are pre-gapped from the factory to a certain standardized specification, your application may require a smaller gap for optimal performance and to prevent misfires.

STEP 3. Carefully bend the ground strap to open or close the air gap of your spark plug. Avoid contacting the center pin or galling or scratching the ground strap as this could lead to pre-ignition of the air/fuel mixture due to hot spots and premature wear.

STEP 4. Using the side of the feeler gauge, carefully measure the gap. Do not force the gauge between the ground strap and center pin. The gap is correct when you feel a slide drag and the next size up feeler gauge will not fit in the gap, generally 0.001” or .025mm larger than the desired gap.

STEP 5. Install your spark plug. Unless directed to do so via the factory repair manual, refrain from using pastes or sealants on the threads of the spark plug as this can weaken the ground connection between the plug and cylinder head. If at any time during the procedure, a spark is dropped, DO NOT USE IT, dispose of it immediately. The insulator could have a hairline fracture that is not visiable and could break off in the combustion chamber potentially causing internal engine damage.

Finally, when tightening the plug, use a torque wrench and tighten to the factory specification. Ideally, do this when the engine is at room temperature. Over-tightened plugs can damage the threads in the cylinder head and be a costly repair or require a new cylinder head. Under-tightened plugs will cause a poor electrical ground connection and can cause premature ignition coil failures and/or allow hot cylinder pressure gases to impede performance or melt the ignition coil.

Leakdown Test

A leakdown test is a static type procedure that can assist in isolating a mechanical component failure.

STEP 1. With a warm engine, remove all four ignition coils and spark plugs.

STEP 2. Place a long, straight piece of light weight wire into the spark plug hole of the number 1 cylinder. Refrain from using heavy or sharp items as this may dent or scratch the top of the piston.

STEP 3. Using a socket and extension rotate the crankshaft clockwise until the number 1 piston is at top dead center (TDC)

STEP 4. Prior to installing your leakdown tester, verify that the gauge is reading zero and the hose you are about to install DOES NOT contain a one way schrader valve.

STEP 5. Install the hose into the number one cylinder spark plug hole and connect the gauge. Note, if the piston was not at perfect TDC, the crankshaft may rotate when pressurized. If this happens, releave the air pressure and reset the number 1 piston to TDC.

STEP 6. Read the gauge set. Compare the supply pressure on the left gauge with the amount of air escaping on the right gauge. The deveiation is the percentage of leakdown. The kit your gauge came with should provide a method for determining this amount. Record the data for each cylinder. Less than 3-5% leakdown is acceptable.

STEP 7. If leakage is present, probe the intake and exhaust runners with a stethoscope. Remove the coolant resevoir cap to check for air bubbles caused from a leaking headgasket and remove oil filler cap to listen for air leaking past the piston rings.

Compression Test

A compression test is a dynamic type procedure that can tell you the overall health of an engines mechanical components.

STEP 1. With a warm engine, remove all four ignition coils and spark plugs. Lift up the rear seat cushion and disconnect the electrical connection to the low pressure fuel pump.

STEP 2. Connect a battery charger. This will keep the vehicles electrical system at a constant voltage during the cranking portion of the test.

STEP 3. Prior to installing your compression tester, verify that the gauge is reading zero and the hose you are about to install contains a one way schrader valve.

STEP 4. Install the hose into the number one cylinder spark plug hole and connect the gauge. Face the gauge towards the person that will be cranking the engine over. Prior to cranking, insure that the crankshaft area is free of tools, hoses etc.

STEP 5. Crank the engine over until the needle on the gauge face stops moving. The goal here is see what the highest amount of compression the cylinder can acheive. Do not worry about counting the amount of revolutions or how long its taking for the needle to stop moving.

STEP 6. Record and compare all of your data from each of your cylinders with the manufactures specifications pertaining to your specific engine code. Note the wear limit and acceptable variance between cylinders.

If one or more cylinders deviate from the manufactures specifications a mechanical malfunction may be present. A leak down test should be performed to diagnose specific components such as valves, rings, head gasket etc.

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