Does this sound like a head gasket? Or is something more sinister going on?

I am getting an overheating issue. Fan is verified working, both slow and fast speeds. Radiator has been flushed, and filled according to spec (Turn up the heat, open the bleeders, fill, etc).
The car then runs for a couple hundred miles with no overheating, but slowly starts to run the high speed fan more often. Then it overheats seemingly out of nowhere.
Letting it cool down, and checking the fluid, seems to be a small amount of missing fluid. Top it up, bleed it, and it runs fine again for a few days, and then it happens again.

There is no white cloud coming from the exhaust.
There seems to be a small amount of discolored oil on the inside of the valve cover, near the crank case breather hose. But the oil itself seems fine.

I measured compression on my cylinders the other day, and got appx 140 on all cylinders except for cylinder 4, which was like 116.

When the engine is completely cool, I can let the pressure out of the coolant (why is there still pressure when cool?).
Crank it a couple of times, shut it off, and the coolant is pressurized again. I don't remember this being the case before.
Indeed, the pressure will remain in the system overnight.

I don't see any coolant leaks anywhere (except when I open the radiator cap, and water pushes out).

Is it worth a head gasket replacement at this point?

 

If the high speed fan runs more often, it's not overheating out of nowhere. That's your cue the temperature is going up. You could see it more clearly with an OBD2 scanner that gives precise coolant temperature. The dash temperature indicator is notoriously imprecise.

That's your first clue. Where is that coolant going? That's what you need to find out. Is it going in the cylinders? Or somewhere else? Keep in mind you could have a leak that doesn't make it to the floor. For example a leaky radiator cap. Or small enough leak that stays trapped in the skirt.

Good, but doesn't rule out a heat gasket.

No clue what to make out of this. Could be a clue too. At least oil doesn't have coolant.

Not good. That leads to a very important question: is that cylinder steamed clean (no carbon deposit at all)? You can inspect that with a borescope. If that cylinder is totally clean and others are not, that means coolant is getting in and you need a head gasket or new head (if it's wrapped).

What do you mean by "letting pressure out of the coolant"? On non-supercharged, the thermostat cap is pressurized, and the coolant expansion tank is not. What you describe is weird indeed, I don't remember pressure building up that fast. I don't remember if the system still had some pressure after letting it sit overnight though.

I'd say further diagnostic is required to positively ensure it's the issue. Either a borescope showing a steam-cleaned cylinder, or exhaust gases in the coolant would confirm a head gasket replacement is needed. At this point, it's "strong suspicion", as several clues point toward that (coolant slowly disappearing, overheating, and lower compression on cylinder 4).

 

Thanks for the response - I used a boroscope, and this is what I see. The car hasn't been ran for about 3 days, and the temperature just dropped drastically, so I'm kind of assuming the drops on the sides of the cylinders are condensation.
But cylinder 4 looks somewhat different. (I couldn't get a good pic of cylinder 3 this time around, but it looked similar to 1 & 2 yesterday).



Cylinder 1


Cylinder2


Cylinder 4

 

I can't really tell either way just from the picture, and keep in mind I'm not a professional mechanic anyway. If cylinder 4 looks different and has a lower compression ratio, along with other clues and absent any indication the coolant is going somewhere else, yeah I'm tempted to say it's a head gasket.
If you can, try to get another opinion, preferably from someone that's more experienced and that can visually inspect the engine.

 

Ended up doing a partial rebuild - Bought a remanufactured cylinder head off ebay (from starfox_cylinder_heads). Replaced the head and the head gasket (plus new belt, and new injectors I had sitting around). No more fluid loss/overheating, more power, the engine sounds fantastic compared to before. I'm back to liking my mini.