can an officer search your vehicle without asking your consent if he
gives you no probable cause for the search?

As with all things legal - I am not a lawyer - my only legal advice is
to see a lawyer if you have any specific questions.

With that said, it really depends...

1) You say "he gives you no probably cause"  I don't believe he has to
tell you if he has probable cause... He just has to be able to back it
up in court.

2) He can visually check anything that is in plain view for his own
safety and if something there gives him probably cause, he can
probably go further.

Assuming that he had no probable cause and you did not consent to a
search, you might have a case...