Hello all. this is going to be my first post here! Recently picked up a 2009 Mini Cooper S with 139000 miles owned by a mini master tech! It has been super great to me and ive taken it on 500+ mile road trips a few times since owning it. However around a week or two ago I took it to a town near me, about an hour into the drive I got a blinking CEL and a very apparent misfire/lack of power. Against my better judgement and other circumstances, I drove it home staying out of boost (it took forever i must add). Tore it down a bit and did some basic diagnosis, new plugs, moved coils around to see if misfire moved. Had a code for a random misfire, cylinder 1,3,4 misfire, and p115c. cleared and only cyl 3 misfire code came back, with an obvious misfire coming from 3. compression test revealed 3 was toast, tore head off, found hole in an exhaust valve! I am decently mechanically inclined (or naive) and someone confident I can get it back together with some guidance. What I am not confident on: (this is where the questions start)
cylinders look great from what ive seen, no odd scoring, no obvious wear, cannot feel anything. pistons look good with some buildup. headgasket looked pristine, plugs looked good, and really nothing looked super bad other than a lot of carbon buildup on valves.

1. What should I replace while im here? This car was maintained GREAT with reciepts before i bought it. gaskets looked good, no leaking from anything.
Do I replace just the valve, its keeper, and guide, with an oem replacement and slap it back together with a headgasket kit?
2. Are there any better valves I can buy that can prevent this from happening again (why did it happen in the first place?) that arent crazy expensive?

3. I'm finding full sets of intake and exhaust valves for 50$ and up to 500$ ? what are the differences and should I just buy the cheaper ones.

4. following number 3, should I replace ALL the valves, ALL the guides, ALL the seals, or just the problem ones? Would it be cheaper in the longrun than having it walnut blasted or trying to clean the buildup myself?

5. Are there any "UPGRADE THIS GASKET, UPGRADE THIS BOLT, BUY THIS BILLET PART, DO THIS CERTAIN HEADGASKET, DO THESE VALVES WITH THIS SEAL!!!!" type recommendations that arent break the bank type that are definitely a worthwhile peace of mind investment?

ideally i want to get this back together and know it will be okay, but also not spend a whole bunch of money going down the rabbit hole of "while im down here", because that only stops when the money runs out.

From what i can tell the head doesnt need machining, but from my understanding new valves will need to be lapped to seal properly?

 

If it were me, I would replace all valves and stem seals, at the least. Also clean everything up really good - get all the carbon out of there. Get a new timing chain kit, too. Perfect time to make that all right and good. If you’re feeling ambitious, maybe smooth out the intake ports a little bit too. A little efficiency improvements won’t hurt.

 

That is such a common failure, mine failed at 110k. Make sure and add a catch can if one isn't currently installed. Might as well change the vaccum pump if original.

 

If it was mine
(oh, wait, I have one in exactly the same condition waiting for me to get time to work on it)
I'd strip the head, clean all the valves, and crack check them.
I'll bet you find there are a few more exhaust valves with cracks.
Measure all the springs for height and pressure at installed height, to be safe.
You can reuse all the rest of the hardware if it's not obviously damaged.

As to what to do for valves, if you can find an aftermarket exhaust valve you like better, may as well try that.
With BMW engines, the stock valves are very very good. But these are not exactly BMW engines,
so I gots no experience with them, long term.

As to what does it- heat. The exhaust valves take the brunt of the exhaust stream, and
going even a bit lean (how's your HPFP?) under full boost adds a TON of heat.
As do transient conditions where mixture's allowed to go lean to save on emissions,
or when the fuel system is under- delivering,
and it takes the DME some number of combustion cycles to compensate.
Not enough to throw a code, but enough to really stress the valves. Again and again.

The N14 in the car I have is trashed- the cylinder walls are heavily scored- so I'm guessing this car
overheated at some point, AND the injectors look as if they're leaking. All sorts of good stuff.
But if your shortblock looks OK, yeah, try lapping a new valve on #3 to make sure the seat's ok,
then replace all of the exhausts, make darned sure your fuelling is good, and off you go.
The more fastidious and well- heeled would spring for a full valve job- me, I'd base it on
how much I wanted to spend on the car.

My long- term experience with failures like this, though, makes me think there's a fuel- related issue going on...

t