Is it at all possible to do a time series on economic data RICs like USPMI=ECI or is it not possible when the CF_* and Close Fields aren't mapped on TR's side? To be honest economic indicators aren't that great as they don't seem to be consistently populated.
That RIC works on GetInterdayTimeSeries_4 operation if you choose a Monthly interval and comes back with a CLOSE field value only, returning values like 59.8 for september.
... I do not know what that ric is in particular but I presume monthly makes sense for it.
Different rics will have different intervals available, unfortunately via TRKD API there is no "discovery" api that would let you query whats available without making a real actual data request.