I’ve seen a handful of posts inquiring about the difficulty of replacing spark plugs in the Gen 6 FA24 turbo engines, and since I tackled it last weekend, I thought I’d provide a brief writeup of my experience. I followed the instructions in the service manual on STIS but found a few necessary tricks that do not get mentioned. It’s a downright, certified pain in the rear, but it can be done in half a day. So come along for the ride, if you’re interested in learning how I did my spark plugs in a Sunday afternoon!
Tools (all wrenches I used were 3/8):
- slim ratcheting wrench
- torque wrench
- regular extension
- universal joint
- assorted metric sockets
- 14mm deep socket
- 10mm angled wrench
- #2 standard screwdriver
- magnetic pick-up tool – indispensable for retrieving sockets, adapters, etc, from the spark plug well, and for the inevitable bolt that falls down into the engine bay and disappears within the undercover
- speaker – indispensable if you, like me, cannot bear to be alone with your thoughts and need to have music on while you wrench
Materials:
- 4 New spark plugs (brand/UPC/spec used is NGK 96698 SILKFR8A6 Laser Iridium Spark Plug) – I bought these from NGK for much less than what they cost from Subaru, and they are the exact same plug
- Anti-seize lubricant
- Dielectric grease
- Electronics contact cleaner
Starting with the driver’s side, since that’s easily the most complicated, here’s what I did and how I did it:
- Per the service manual instructions, I completely removed the battery and the tray, hold-down, etc.
- Next, the manual says to disconnect the wiring harnesses for the fuse box and ECU. There’s a little notch just underneath the locking clip, push it in and then the clip slides up, pushing the harness out of the connector as it does. Very easy.
- Here’s the first trick. The service manual simply says to remove the harness from the clips (that part is easy, just be gentle with the screwdriver) and “move them aside so that they do not interfere with the work.” To help make this large, unwieldy harness more manageable, I disconnected the two circled leads from the positive terminal distribution block. The lead circled in red is attached underneath the terminal. With these two connectors removed, the giant harness can split into two slightly less unwieldy harnesses. I tucked the engine harness up behind the AC suction hose, and the other harness behind the intercooler. I had to disconnect a couple clips on the engine harness as well. You might want to find something, like a bungee cord, to hold the harness back so it’s not pressing on the AC hose. I didn’t do that, it didn’t seem to be flexing the hose too much, and I’m not going to lose any sleep over it. But this is the internet, so someone’s bound to complain about it 😉
- With the harnesses out of the way, there is just enough room to access the plugs. I also removed the brackets that the harness clips into, to give myself a little more room to work. The ignition coil has one 10mm bolt holding it in place, the positioning of some of them required an angled wrench to remove. Others I could reach with a regular 10mm socket on my slim ratchet. The coil plug is a little tricky to disconnect. I pressed on the metal locking spring a couple times to loosen it up before I tried pulling the connector out. Once you’ve got the screw and the connector removed, you can try to pop the coil off the plug. Don’t forget to mutter a few curse words of your choosing about the fact that factories don’t use any kind of dielectric grease on assembly and the rubber is sticking. This is an important step.
- To get the spark plug out, first, you must accept the fact that the frame rail that’s in your way will likely receive a scratch or two. I first dropped my 14mm deep socket down into the well, followed by the extension, and engaged them together and on to the plug by hand. Then, I slipped my slim ratchet into the gap, and after a little fiddling with the positioning, I was able to engage it onto the extension and crack the spark plug loose. Once the plug spun freely, I pulled the ratchet out and turned the extension by hand until the plug was completely clear of the threads. Then I pulled the extension out as far as I could, pushed the socket back into the well, removed the extension, then finally, used my magnetic pickup tool to grab the deep socket, then the plug itself, out of the well. Mission: halfway successful!
- At this point, I used some electronics cleaner spray to clean off the ignition coil and boot so that I’d have a decent electrical connection. I also put a thin coat of dielectric grease along the ceramic shaft of the new spark plug, and the connector on the coil pack.
- Installing the new plug is the reverse of removal:
- Drop the new plug into the well
- Drop the 14mm deep socket down after it
- Drop the extension in and engage it with the socket, and the socket with the plug
- Carefully spin the extension by hand and let the plug find its way into the threads, tighten by hand until it bottoms out
- Getting my torque wrench on the extension was the hardest part. For the driver’s side rear, I had to use a universal joint instead of the 3/8 extension for the wrench to fit in that tiny gap. Torque spec (as of 8/4/23 when I downloaded the procedure from STIS) is 13.3ft/lbs, but the closest my wrench could get was 13.5. I took it slow and made sure everything was engaged and turning freely the whole time – the last thing I would want is a cracked head.
- Once the plug was torqued, I fished out the socket and extension piece by piece with the help of my magnetic pickup tool. Then I slipped the coil pack back into the plug well and slid the connector on. I put a light coat of anti-seize on the coil pack retaining bolt, and threaded it back on. As I tightened the bolt, I made sure the coil pack was on square.
- The process is basically the same for the other 3 plugs! On the passenger’s side, all that needs to be removed is the air filter box and intake duct (remember to disconnect the sensor and unclip the harness from the airbox). It’s a lot easier to work on this side.
- Remember to replace both battery cables on the positive terminal in the correct locations, and reinstall all the clips for the engine harness.
Here are my old plugs, with about 62,000 miles on them. I didn’t see any signs of unusual wear, fouling, carbon deposits, etc.
This is my first attempt at a write-up, hopefully it is useful. If anyone else has done this job and they’ve found a way to make it even easier, I’d like to hear about it. Similarly, I’d like to hear about any potential mistakes or omissions I made here.
