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Subaru WRX CEL Light on - Help!

I have a 2004 Subaru WRX Wagon with 138,000 miles whose CEL light is on. I have had a private mechanic replace the plugs and wires and tune the car, light went out but came back on. He said the misfire is at 1,3. He swapped the coils and determined that was not the problem. Told us to take it to the dealer and check the codes. Brought it to a Subaru dealer and they ran the diagnostics and it came up Code P0301. Compression test and leakdown test were fine. They removed spark plug for cylinder 1 and inspected with camera. Found exhaust value darker than other but not sure that is the problem. They were unable to determine the cause of the check engine light. They claim the next step is to drop exhaust and inspect valve guides or remove heads to inspect valves. I have read many stories on line about how engines were pulled when in many cases it was not an engine problem at all. Car is running fine but does idle hard and will hiccup on highway. Can anyone help with this problem? I don’t want to spend lots of money needlessly? Thank you in advance.

Their first instinct is to say it is related to the valves? That is an expensive job, should they decide it needs service which is probably why it is their first instinct…
If you’ve replaced or moved the coil packs, wires and plugs and it stays with that cylinder, I’d check the injector first.

I’d be very reluctant to bring it to the dealer. Do you have a shop nearby that deals a lot with Subies?

The private mechanic we used does a lot of subies. Don’t think he checked the injector. Is this a relatively easy thing to do?

Yes. I doubt it is fuel pump related but it could be related to fuel delivery to cylinder 1. An injector is a lot easier to check or swap than a head.

You could run some Techron through the gas tank. It is a gas additive, costing about 7-10 bucks. Since your car runs well, it is certainly worth a try because the need to repair does not seem immediate, in your case. It cleans the fuel system, including the injectors.
It could be that the one for cylinder 1 is sluggish or not quite opening correctly.

Thank you for the suggestions. Will give it a try.

If the Subaru has an adjustable valve lash, I would start there. You don’t need to pull the head for that, just the valve cover. Then look at the injector.

Just curious, but what kind of numbers did they provide you on the compression test? These should have been written down and you would be surprised at how many mechanics and shop manuals are dead wrong on this issue.
The shop manual specifies an incorrect pressure and many mechanics take that erroneous info as the gospel truth.

A good engine should have 190 PSI, give or take a bit, on each cylinder.

Cylinder head valve problems do not involve guesswork and I might add that inspecting and adjusting valve lash every 30k miles is a process that is generally not done.

There’s more I could add to this but knowing compression readings would help greatly.

They didn’t give us the readings. They should said it was fine.

If the shop didn’t use NGK spark plugs I suggest you purchase one that is rated for your car and install it in #1 cylinder, which is at the front of the engine on the passenger side. If that solves the issue then replace the other plugs also with the same. Soob engines like to run with NGK plugs. Solving this kind of trouble can be tricky at times. Various engine sensors can come into play also for causing the trouble.

I like @RemcoW idea of using some Techron also. Try the cheap things first.

Tried the Techron and ran the car for a few days. Light is still on. The spark plugs are Subaru plugs. Going to have them check the injector next. Does anyone have any other suggestions I should talk with my mechanic about?

If using the Techron is going to work then it may take longer for it to get things cleaned up. You may have run a couple of tanks of gas of the additive for it to do the job and even then it might not get the light turned off. It is just a thing to try out. You might try having the code cleared if you don’t have an IM inspection coming up soon to see if the code will stay away now. Clearing the codes also clears the monitors and they will take a while to set again. The IM testing requires that some of the monitors at least be set before a test can be done. A place like Autozone can clear the code or better yet, you may want to think about getting your own tester.

Well they don’t think it is the injector and we cleaned the MAF sensor which was really really dirty. Worked great for 4 days, no light, but then the light came back on. They suggested we run the codes again and this time replace the MAF sensor. Anything else they say will involved engine work and be very expensive and this car has 150,000 miles on it so they suggest it is not worth it. If the MAF sensor does not work they said we might want to think about getting another car since the repairs could start to cost more than the car is worth. Please any thoughts would be appreciated.

Swap injectors.
Check/clean EGR valve and ports.

I agree. See if the problem follows the injectors.

Is it expensive to swap injectors and if that is the problem, what do they need to do next and is that costly? I am told that with a turbo on the WRX, anything like this is labor intensive because all engine parts are “on to of each other” and will costs lots of money. Need to find the answer or get another car because of ongoing medical treatment.

Sorry to hear about your troubles.

I’ve never touched a WRX but Subarus are pretty much all the same when it comes to lay out.
On them, you can pretty easily get to the injectors. To get some elbow room, the following will need to be done:
On the passenger side, one would need to take the air box off.
On the driver’s side, one will likely need to take the battery out.
Both those are easily removed. The engine heads and thus the injectors are readily accessible with them removed.

What did they think about checking that valve lash?

Bringing it tomorrow for the MAF sensor. I will update after I talk with the mechanic.

Here are some other ideas before you assume it’s the valves:

Smoke test (Intake boot leak, Vacuum line leak, Intake Manifold Gasket)
Spark Plug
Spark Plug Boot
Coil (swap coil to test)
Camshaft position sensor
Crank position sensor
Fuel Pump
Idle Control Valve (ICV)
Oil in spark plug sockets from leaking VCG

Try to buy the MAF sensor yourself. Get some cheapo mechanic to install it. It takes 5 mins to swap it.