The throttle body has a path for air other than the throttle that is regulated by an electrically-actuated plunger valve. I can see this valve getting stuck open, closed, or just gummed up in a dirty engine. I don't know if you can buy the valve itself as a new one comes with a new TB.
The valve & air path is in the upper right corner of this pic of an upside-down 3.5L TB:
When I last looked at the throttle plate on my 300M it was clear that some dark stuff does get deposited on it.