K20/K24 Spark Plug Replacement (Acura RSX DC5, Civic Si EP3/FA5/FG2, Acura TSX CL9)

The K20 and K24 use coil-on-plug ignition, so changing the plugs means removing the coil cover, unplugging and lifting out the four coils, and swapping the plugs with a long extension. The two things to get right are the gap and the torque. The cylinder head is aluminium, so a plug that is started crooked or over-tightened can strip the threads. Hybrid Racing's K-series iridium plug listing (https://www.hybrid-racing.com/products/ngk-iridium-spark-plugs-honda-k-series) lists fitment for the 2002-2006 RSX, 2002-2005 and 2006-2011 Civic Si and 2004-2008 TSX, and quotes a 0.044 in gap and 13 lb-ft. However, it is a retailer page, and its own note says the factory gap may not match every engine. Confirm both figures in the factory service manual or the plug maker's data for your engine. Adapted from a community writeup by General's Automotives on iFixit (https://www.ifixit.com/Guide/2004-2008+Acura+TSX+(CL9)+Spark+Plug+Replacement/100952).

1. 01

   Step 1: Work on a cold engine

   Let the engine cool completely. Plugs are easier to remove and the aluminium threads are less likely to be damaged when the head is cold.

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2. 02

   Step 2: Remove the coil cover

   Open the hood and remove the plastic cover over the coils. On the TSX it is held by four 10mm fasteners; keep them safe, as one iFixit commenter found the cover screws can break.

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3. 03

   Step 3: Unplug the coils

   Press the locking tab on each coil connector and pull it straight off. Do not pull on the wires.

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4. 04

   Step 4: Remove the coils

   Remove the 10mm bolt holding each coil and pull the coils straight up out of the plug wells. Lay them out in cylinder order so each goes back in the same hole.

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5. 05

   Step 5: Clean the wells

   Before removing the plugs, blow or vacuum any dirt out of the plug wells so it cannot fall into the cylinders.

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6. 06

   Step 6: Remove the old plugs

   Fit the 5/8-inch plug socket to the 10-inch extension and ratchet, and turn each plug counterclockwise until it is loose. Lift it out with the socket's rubber insert. If you do not have a plug socket, a 16mm socket with a magnetic pickup can be used.

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7. 07

   Step 7: Read the old plugs

   Look at the old plugs for heavy carbon, oil fouling, wet deposits or a badly worn electrode. Anything unusual on one cylinder is worth investigating.

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8. 08

   Step 8: Check the gap

   Check the gap even on pre-gapped plugs. Set it to the factory specification for your engine (Hybrid Racing quotes 0.044 in for stock K-series engines, but confirm this in the service manual). Adjust only the ground strap, never the fine iridium centre electrode.

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9. 09

   Step 9: Thread the new plugs by hand

   Drop each plug into the well in the socket and turn it by hand, or with the extension alone, until it is fully seated. If it binds early, back it out and start again; it is probably cross-threaded.

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10. 10

    Step 10: Torque the plugs

    Tighten each plug to the torque given in the factory service manual for your engine. Hybrid Racing quotes 13 lb-ft, while some owners quote higher figures for Honda coil-on-plug engines, so do not guess. Never over-tighten a plug in the aluminium head.

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11. 11

    Step 11: Reinstall coils and cover

    Push each coil firmly down onto its plug, refit the 10mm coil bolts, reconnect the connectors until they click, and reinstall the cover.

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12. 12

    Step 12: Start and check

    Start the engine and check for a smooth idle. A misfire or check engine light usually means a coil connector is not fully seated.

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