TSI: 3 imaginary boys

[Cyrus]

Upon reading TSI for the first time, I was surprised to find in the end 
that Carl was imaginary. Upon re-reading it immediately, it seemed to me 
that, until quite late in the story (in McAffee's room) it is not at all 
clear whether any among Tim, Grover, Etienne and Carl are real persons. 
I constantly got the impression that Grover, Etienne and Carl existed 
only in the imagination of Tim, with no hard evidence pointing to the 
contrary. That is, of course, until Tim picks up the phone in Mr. 
McAffee's room and hears Grover on the other end (pp.173-4). Now, is it 
just me, or is Pynchon creating such an ambiguity on purpose?

Cyrus