08'MustangDude

Dunno what to tell ya dude, it has to be a bad valve or something for it to
only have 15% compression in the effected cylinder. All you have are;
Rings, Valve issues, and the Head Gasket...

Derf00

Leak down test...... If you are not willing to or unable to get that performed you're at a standstill for further diagnosis aside from just throwing parts at it (up to and including head replacement).

08'MustangDude

With the correct tester, 2 gauge is the easiest:
The first step is to ensure the engine is warm, so the rings expand.
Remove the oil fill cap.
Prop open the throttle Body.

Remove the plug and turn the engine over until the piston is at top dead center (TDC)
on the effected cylinder. Use a breaker bar or ratchet, or whatever works for you to
get the piston at TDC, and it can be the compression or exhaust stroke, doesn't
matter, as long as the piston is at TDC.

Install the air-fitting adapter into the spark plug hole. Remember to double-check that your
gauges read zero before you start.

Put air to the cylinder and then gently rock the piston around TDC to ensure the rings are
seated. Keep in mind that cylinder pressure is used to help seal the rings to the cylinder
bore, so you want to give them every opportunity to do so. If you force the air in too fast,
the crank can move, which will open a valve.

Slowly put 50lbs of air in; the one gauge reads 50 (regulator), if the other gauge reads 40,
that's a 20% loss in compression, so you listen to where air is coming from.

From the oil fill, it's compression rings.
From the intake (throttle body), it's the intake valve (no coolant loss) or head gasket (coolant loss) or both.
From the exhaust, it's the exhaust valve or head gasket, same conditions as above, or both.
Check for bubbles in the coolant reservoir, head gasket.
If from the spark plug hole, then the threads or plug bore is damaged.
All that's left is a cracked block really, and that you'd hear somewhere else...

You are going to hear SOME air at every test point, especially if the motor is cold, but the
loudest one is the culprit. IF they're all evenly sounding, then you have a intermittent issue,
which could be rings or valves.

That must be done to determine the reason/where you're loosing 15% compression in that
cylinder, and honestly, 15% is supposed to be acceptable in most cases with a new motor; and
20% on a motor with some decent miles on it. So, in reality, 15% should not be an issue, you
have to be at 20% or more for concern. It depends on the motor and what FORD says is acceptable.