Problem with Chevrolet Luv

Acbo to buy a Chevrolet Luv Work 2.5 Turbo year 2004 with 109000 kms, I made it check whole by a mechanic I found it excellent, I changed oil with filter, air filter, there was no oil filter for which I did not change it, then travel, and I found the surprise that when climbing slopes loses fury even reaching up in third to the third to the third to third.
I need to know what can be, thank you very much in advance