I've posted about this before, but things are getting interesting...
Basil has a problem with a rough "sputtery" idle. Sounds (to me) for all the world like a stuck/cracked/ broken exhaust valve. Head replaced. Valves replaced. Still the same thing. My dealership has had 5 goes at fixing this. Attempt #6 was by the MINI/BMW Factory Field Engineer. Still no relief.
I know there are a few folks here with some serious engine experience.
Racers and the lot.
Anyone think that the cam could be the culprit? Since the rest of the valvetrain has been replaced, with no change, I'm leaning in that direction.
(I'm waiting for an answer to the leakdown rate; last checked, cyls 3 & 4 were leaking down 55%).
If the cam were mis-indexed, or twisted (no idea how that could happen), that would give the results I am getting.
I am relatively old-school as far as mech repairs go; hard parts are no problem. Computers/sensors, not so much.
Seems that if this were a DME problem, all cyls would be affected, not just 3 & 4.

Sorry for rambling....any help appreciated. Oh yeah, no CEL. Ever.

 

I wish I could help you with this problem, it's very hard to diagnose without seeing/hearing it on the spot.

If you would like to check the cam, find out the specs, get a dial indicator, and run each lobe to see what it measures. Obviously, this procedure would have been ten times easier with the head on the bench, but it can be done. You could always remove the cam and check it, but you would need journals like the ones in the head for the cam to rotate.

Are all the valve train components new? rocker arms/followers? springs?
Did anyone look at the tops of the pistons while the head was off? did #3 and #4 look any different? was there a lot of carbon build up?

Have you tried a Seafoam treatment? or any solution (GM Top Engine Cleaner)that you can dump in the spark plug holes and let soak?

EDIT: maybe a sticking or clogged injector(s) How does the car run off idle? Can you drive it?

 

Partsman,
Car runs OK* and is drivable. Rocker arms were not replaced. Injectors were swapped out for new, no change. Pistons, dunno. I would hope that they were checked, but I wasn't there. Upstream O2 sensor was also replaced, no change.

* OK being normal except for "flat spots" at 3200 and 3800 RPM, at part throttle. At WOT, it may be there but not noticable. And lack of power; with the mods that I have, car feels no better (and maybe less than) stock.

 

Well, cr@p! The dealer just called, and since they are stumped they are now saying it's normal. I really expected better, after them having the car for 6 visits and untold $thousands in diagnostic time as well as a new head and valves.
I'm bummed. The no-option, base MC I've been driving this week pulls harder than my modded S. And gets 10MPG better. <insert head banging smilie here>

 

When the cylinders leaked down that much, was it with the old head or the new one?

If your still getting the high number with the new head, then it has to be the rings, or a combination of pistons/rings. Either that, or your dealer bolted the head on with no gasket.

 

Leakdown was old head. They say it is no longer doing that, but now I no longer trust anything they say.