Misfire When Hot Only - 2015 Subaru STi - P0302, P0352

Working on a car that has a persistent misfire code for cylinder #2 but it only happens when the car gets fully hot and has been driven for awhile. It will not misfire when cold and drives fine until it gets hot and then it will sometimes misfire randomly and other times just completely drop the cylinder (or at least that's what it appears like).

Stuff Tested So Far
-LeakDown on Cylinder #2 - 8%
-Compression on All Cylinders - 140-145PSI
-Replaced Spark Plug on Cylinder #2
-Replaced Coil Pack on Cylinder #2
-Swapped injectors Betwee Cylinder #2 and #4 - Done Twice to confirm
-Tested Power to Coil connector - 12V
-Tested Ground to Coil Connector - 0.9ohm resistance
-Tested Trigger fromECU Harness to Coil Connector - 0.9ohn
-Swapped in Known Good MAF Sensor - No Change

At this point the only thing we haven't tested is the ECU itself and I don't have a scanner to check the injector or coils while driving.

Do you always get the P0352 along with the P0302? Because that points directly to a primary ignition circuit issue.

If not, then theres other ways we can explain the P0302. A valve sticking in its guide comes to mind immediately. These Subaru engines are known for it, and the misfire happening hot matches a valve guide with a clearance issue.

Prooving it can be tough. You can try hooking up a vacuum gauge to the intake during the misfire, looking for a twitching needle. That'd be a smoking gun for a valvetrain issue.

The other option I've had better luck with is pulling the exhaust manifolds down and visually inspecting the exhaust valves. The ones that are working normally won't have witness marks around the stem, but the one that's been acting up will. Its tough to describe, but you'll know it when you see it. Rotate the engine as needed to look at the valves when closed.

Another possibility is a failed head gasket. Have you noted any coolant loss, or overheating? The gasket failure may or may not show on the spark plug, depending on how long its been going on. The smoking gun for this would be a hydrocarbon test, OR a leak down tester producing bubbles in the radiator. I saw you did a leak down test already, but I'm not sure if it was done hot or cold.

Tyler wrote: Do you always get the P0352 along with the P0302? Because that points directly to a primary ignition circuit issue.

If not, then theres other ways we can explain the P0302. A valve sticking in its guide comes to mind immediately. These Subaru engines are known for it, and the misfire happening hot matches a valve guide with a clearance issue.

Prooving it can be tough. You can try hooking up a vacuum gauge to the intake during the misfire, looking for a twitching needle. That'd be a smoking gun for a valvetrain issue.

The other option I've had better luck with is pulling the exhaust manifolds down and visually inspecting the exhaust valves. The ones that are working normally won't have witness marks around the stem, but the one that's been acting up will. Its tough to describe, but you'll know it when you see it. Rotate the engine as needed to look at the valves when closed.

Another possibility is a failed head gasket. Have you noted any coolant loss, or overheating? The gasket failure may or may not show on the spark plug, depending on how long its been going on. The smoking gun for this would be a hydrocarbon test, OR a leak down tester producing bubbles in the radiator. I saw you did a leak down test already, but I'm not sure if it was done hot or cold.

Thanks for the response Tyler. The P0302 comes up first and then the P0352 second, in most cases they pop up at the same time. It does kind of seem like an ignition break-up when it happens and it can happen under any load but definitely most noticeable under heavier load.

The leakdown was done hot, didn't check for bubbles in the coolant as was a bit rushed trying to diagnose. One thing worth noting is the car had coolant on the overflow and when the customer was asked he stated that he spilled it on the overflow. So it's possible there's an issue there.

I've had coil connectors fail on quad cam subies before causing all sorts of headaches. Normally older than this, but it would explain the p0352 too.
Aftermarket I'm pretty sure you can get the replacement connectors.
Are the little latch clips still all there on the connectors? The pins look perfect?


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