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S401 Spark Plug Replacement

The job everyone hates, and the one that so many people wished me luck for. Subaru spark plugs.

If anyone ever tells you that doing the spark plugs on a Subaru is fun, they are lying. Although, if anyone tells you they are super duper impossible to do, they just haven’t done them before.

I’ve done a few sets now, and know the drill.

The willing victim, ready to begin.

I ripped straight into it. Pulling the airbox and piping out to gain enough space to access the coils and plugs. Even then, it’s tight.

You can just make out the two coils in the head. Removing them was bit of a faff too. When STI made the S401 they added reinforcing plates to the front crossmember mounting points, in the form of big steel plates. As seen here, #3.

So when you are working with piss all space to start with, having a thick steel plate there just makes it that little bit harder.

I had to use a ratcheting spanner to remove the bolt on the rear coil, there just wasn’t enough space for a socket. Ratcheting spanners are one of the best inventions, ever. You can also see the STI reinforcement plate on the chassis rail, beneath the brake lines.

Once I got the two coils out, I made sure to label them as to whether it was front or rear, so I didn’t mix them up. I also labelled the front coil wiring, so I could identify where that went.

Once they were out, a spark plug socket, short extension and a ratchet, and the plug was also out.

This is where I had a moment of clarity. This explains a lot.

Does look great. A quick check, followed by another two checks, confirmed that the plug removed is the same part as the new ones (NGK PFR6G)…. Not sure i’m convinced of that.

All of the plugs were the same, HUGE gaps, severe wear on the electrode. They had obviously been in there a long time, there is no way this car was running as well as the previous owner claims. They all stank of petrol and the FR LH plug was wet with fuel.

Getting back to actually removing the damn things, once I had done the two RH plugs I reassembled the airbox and installed it again. I took this chance, whilst it was out, to give the Airflow meter a quick clean. Easy to do, just two screws and it pops out. Spray it with some Contact Cleaner, and air dry.

I moved to the other side, which has a little bit more to pull out to access the plugs. Out came the battery, and the washer bottle was unbolted and moved out of the way.

With them gone, access is actually easy on this side. It’s only let down, once again, by the STI plate on the chassis rail making it bit of a fight to remove the coil and get the spark plug out. Its doable though. I was getting pretty pissed off when I got my socket stuck when removing the rear plug. Ended up having to lever the engine slightly to get it back out.

With the new plugs in, coils back on and the car reassembled, I noticed that the hose between the intercooler and throttle body had some residue around it. Sure enough the hose clamp was loose. Gave that a tweak and moved on.

The car starts and runs much better now. It’s not perfect, it still hesitates down low and the idle is still lumpy, it also seems to be down on power, but both turbos are working as they should and it can happily rev to redline.

I forgot to do the idle relearn, so had to do that today. It fixed some of the idle roughness, and it idles up where it should, instead of down at about 500rpm it’s now about 750rpm. Idle relearn details are here.

The next step is to run some Upper Engine Cleaner through it, and hope that helps. If not i’ll need to start digging a bit deeper, maybe into coils (since the plugs must have been stressing them out HARD).

So, all in all… It’s a win.


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