Since you asked...

The OP does not have bent valves. He can not have bent valves. The engine wouldn't be running like it is with bent valves. It's rare that heads with bent valves run much, if at all. And, engines with bent valves sure as heck don't idle worth a damn.


Imho, you're right that on the M271, the valves very likely have build up on the valve seats.
Imho, the bigger question is how are the compression rings, and the rest of the engine/drivetrain?
At that mileage, the supercharger is garbage and so are the cats and O2 - if there are original.




But, imho, the more interesting question is:
"How do cylinder heads get bent valves, if the timing chain/belt doesn't jump a tooth?"



That's easy and somewhat involved.

For an older over-head valve push-rod one-intake valve and one-exhaust valve setup:
o) The better flowing values get big and heavy.
o) The lifters are much bigger, and hold more oil.
o) The cam has a lot more physical lift.

The above all makes big valve and big lift overhead valve engine subject to valve float.
That's where the lifter doesn't bleed down fast enough, the valve springs don't push the valves close fast/hard enough, and the inertia of the big valves prevents them from moving very quickly.




However, on the M271....
o) The lifters are very small. So, they can bleed down quickly.
o) There are two smaller intake valves and two smaller exhaust valves.
o) The physical lift of the cam is much less, since it's an overhead cam design, and a newer engine design.
o) Plus, the M271 isn't a high reving 7,000+ RPM beast.

So, imho, there's a basically zero chance, or concern, for an M271 engine with a stock tune having a valve float issue.


If the valves hit the pistons and got bent, then an inspection scope would very likely show physical damage to the top of the pistons. Also, as I mentioned, bent valves do not like to move inside a cylinder head.



Going back to the older overhead valve push-rod big V8 engines....
Getting enough air/fuel into and out of a high power big engine 6+ Liter older overhead valve engine, often means big valves and high lift. Add in high RPMS, and valve float can become a significant problem.
To prevent valve float on my high-lift cam, I used to run heavy valve springs in my old car when I was in college. However, that then also causes more wear on the cam. Yea, whatever. I had a two-piece timing cover to allow for easier cam changes anyway.


I hope the above helps.
And yes, some of use still have and modify V8 overhead valve push-rod engines.

 

Well I've had a couple cars that had bent valves and ran just like my benz. Rough idle, limp mode, still drivable. But anywho, did a seafoam treatment tonight and it did absolutely nothing. Let it soak and everything. So back to pulling the head I go! Thanks for the input thus far. I honestly don't think anyone has ever had my same issue and found a solution or even a reason as to why. Just going to do it the old fashioned way and pull the engine apart and find out myself.