finished / unfinished progressive actions

"I have been washing my car" This sentence uses the tense known as present perfect progressive. It is used for actions that started in the past and may or may not be finished. Some other context is needed (or assumed) to ascertain whether the action has finished or not.

In the case of "I have been washing my car", I would assume the car washing is complete. If it was ongoing, present progressive "I am washing my car" would be used (and there would be evidence of car washing).

In the case of "I have been wondering...", I would assume that the wondering continues. If it was complete, the simple past tense "I wondered" would be used. Wondering is internal, there might be no outward evidence, so best to assume a steady state.

Both these examples are personal interpretation, though. It’s easier and clearer to use other tenses instead: "I just washed the car" (simple past); "I was washing the car" (past progressive); "I am washing the car" (present progressive).