Scoped it with a camera–found heavy carbon build up on the intake valve in cylinder 3, and on the piston itself.
Heavy carbon build up on the intake valves? if it’s in the combustion chamber side, is not ideal, but it can happen. Now if it’s on the intake manifold side, that’s odd…… these are port fuel injected engines, the fuel will constantly wash down the valves, keeping them squeaky clean. Do you have any photos of those valves?
If the injector #3 was working fine, then I would suspect something going on with the PCV, because the vacuum source for it is mainly between cylinders 3 and 4. Maybe some crap got dislodged from the PCV and made its way to cylinder 3.
EDIT: I read it again, i don’t know why it got it wrong.
First of all: Was this car parked/not ran for quite a while until now?
You mentioned static and dynamic compression tests, those are two completely different readings.
I think most people (me included) go only with the static (at cranking speed) compression test. 145-150psi across the board shows all the cylinders are relatively even. I assume you floored the accelerator pedal while cranking or removed the throttle body to make sure all cylinders can fill up all the way.
I agree it’s weird for cyl 3 have twice the reading value as the other cylinders during dynamic test. 60-70 for the other cylinders doesn’t seem bad, considering at idle, there is vacuum at the intake manifold, so I wouldn’t expect to see a reading of a cylinder near 100% full of air.
The only thing i can think of is something going on at the gasket between intake manifold and cylinder head on the other cylinders except #3. There are reed (flapper) valves at each cylinder port that under vacuum conditions should open and allow all cylinders evenly to pull an vacuum at the PCV chamber built in the lower intake manifold. And when there is boost present, they will be shut (to not pressurize the PCV system), and the gasses would go instead through the aluminum tube around the engine to the turbo check valve (at the compressor intake of the turbo), which then should get shut with the vacuum of the intake manifold once it’s back to idle conditions.
I’ve never read about this happening on a T5 (as the passages are decently sized), but read it happened on a 2.4i, which has very small holes instead of reed valves, and 4 of them were clogged with carbon). So, let’s assume for some reason, the reed valves on cyl 1, 2, 4 and 5 are shut. Then all the PCV gasses (during idle and low load) would be vented only through cylinder #3. All that excess blowby (and maybe add a leaky turbo check valve) might be enough to throw off the fuel mixture and consequently cause the misfire.
The gasket in question:
Flapper valves (viewed from lower intake manifold side)
Intake manifold:
If the car didn’t run for an extended time period (hence why I asked), I’m thinking maybe somehow the flappers could gotten stuck shut the intake manifold.
Either way, at this point I would pull the upper manifold and inspect all valves and confirm there isn’t any traces of carbon (they should be very clean), if not then also pull the lower manifold and confirm the stuck flapper theory.
A 2012 car with 75k miles suggest lots of short trips. If it’s the case, I would also inspect the PCV system, either clean it up (cleaning writeup here) or replace it.