THAT is criminal.
Selling proper maintenance is not.

There is a reason that Mercedes shops charge a lot for a major service. It
takes most of the day, and there are many parts designed to be serviced at
specific intervals. Fail to do so and it will cost you big. Do it on
schedule and the car will last. My 1963 190Dc went for 774,000 miles with no
engine work. Never had the head or oil pan off. It finally needed an
overhaul and got scrapped. My 82 300TD is at 250,000 and runs great. My 82
VW Jetta diesel is at about the same, getting tired though. Carbureted cars
didnt usually make it to 200,000 miles.

Chrysler got into big trouble when brake systems failed and there was no
interval recommended to change the fluid. For some reason American car
makers tend to recommend under-maintenance. I don't understand why BMW
thinks steering fluid (Dexron ATF) should be replaced at 30,000 miles and
Ford doesn't think you need to change it at all.

A BMW will idle bad if the dipstick isn't seated in the tube. A spark plug
with 36,000 miles on it should be bad. A rat's nest in the intake doesn't
filter air very well.
This is a machining NG, but there are some guys here who know a lot.
Answering a question online like the OP's is usually futile, unless he gives
full symptoms, maintenance history, and tells us what he has checked. Even
then it's a crapshoot, I cant hear it, so my wild guess is likely to be
wrong.

--
Stupendous Man,
Defender of Freedom, Advocate of Liberty