The 2006 Subaru Impreza is best known for its high-performance WRX and WRX STi models, but the standard Impreza is no slouch. Featuring all-wheel drive and a 173-horsepower 2.5-liter four-cylinder engine, the base level Impreza can hold its own. Because the 2.5-liter is a horizontally opposed engine, you will not see the spark plugs where you typically expect to see them. Despite their unorthodox positioning, installing spark plugs on the 2006 Impreza is not too difficult a task.
Check the gap on all four new NGK FR5AP-11 or equivalent spark plugs using a spark plug gap tool. The correct gap for the 2006 Impreza is between 0.039 and 0.043 inches. Exchange any incorrectly gapped spark plugs for new ones, as you cannot adjust the gap on these plugs.
Loosen the hose clamp on the engine side of the mass airflow sensor with a flathead screwdriver. Press and hold the unlocking button on the MAF sensor’s wiring harness and unplug the harness from the receptacle. Unclasp the two latches holding the top half of the air cleaner box to the lower half. Lift the top half of the air cleaner box and the MAF sensor from the engine compartment.
Pull the air cleaner element from the air cleaner box. Remove the two bolts securing the bottom half of the air cleaner box to the Impreza’s body using a ratchet and socket. Pull the bottom half of the air cleaner box from the engine compartment.
Trace one spark plug wire toward the side of the engine until you find the thick rubber boot that connects the spark plug wire to the spark plug. Grab the spark plug boot and pull it away from the engine with a twisting motion.
Loosen the spark plug from the engine using a ratchet, 6-inch extension and a spark plug socket. Pull the ratchet, extension and socket from the spark plug tube to remove the spark plug; the spark plug socket has a rubber insert that holds the spark plug inside it to ease removal.
Remove the old spark plug from the spark plug socket and press a new spark plug into the spark plug socket. Thread the new spark plug into the engine using the spark plug socket and 6-inch extension. If you feel any resistance as you thread the new spark plug, immediately remove it and thread it in again.
Tighten the spark plug to 15 foot-pounds with a torque wrench, 6-inch extension and spark plug socket.
Inspect the removed spark plug wire for any defects, including excessive aging, brittleness, cracks or breaks. If any defects exist, replace all four spark plug wires.
Apply a small amount of dielectric grease to the inside of the spark plug wire boot. Insert the spark plug wire boot into the spark plug tube and press the boot inward until you feel it click onto the spark plug.
Repeat steps 3 through 8 to replace the other right side spark plug.
Set the lower half of the air cleaner box into the engine compartment, aligning its bolt holes with those in the Impreza’s body. Hand-thread the air cleaner box’s bolts, then torque them to 25 foot-pounds with a torque wrench and socket. Set the air cleaner element back into the lower half of the air cleaner box.
Set the top half of the air cleaner box onto the lower half, guiding the MAF sensor into the air intake tube. Tighten the hose clamp on the air intake tube with a flat-head screwdriver to secure it onto the MAF sensor. Plug the wiring harness into the receptacle on the MAF sensor, then press the clasps over the top half of the air cleaner box until they snap into place.
Loosen the nut on the negative battery cable end’s pinch bolt using a combination wrench. Pull the negative battery cable end upward and off the battery. Lift the rubber cover off the positive battery cable end and loosen the nut on its pinch bolt with a combination wrench. Pull the positive battery cable from the battery.
Remove the two nuts securing the battery tie-down bar using a combination wrench. Lift the battery tie-down bar off the battery, then lift the battery off its tray and out of the engine compartment. Use caution, as batteries are deceptively heavy.
Remove and install the spark plugs by following steps 3 through 9 under “Right Side Spark Plugs.”
Set the battery onto its tray and lower the tie-down bar onto the battery, guiding the tie-down bar’s mounting studs through the holes in the bar. Hand-thread the nuts onto the tie-down bar’s mounting studs, then snug them with a combination wrench.
Reconnect the positive battery cable end to the positive battery terminal, then torque the pinch bolt’s nut to 3 foot-pounds with a torque wrench and socket. Press the negative battery cable end onto the negative battery terminal, then torque its nut to 3 foot-pounds.
