the check engine light in my 1998 rodeo is on. the code is evaporative emissions malfunction. what causes this code?

hey have you checked the gas cap if mine is not on tight my check engine light comes on.

You need the exact code. There are different levels of EVAP leaks. If the code os P0440, gross EVAP leak, it may be just the gas cap. If it is a mediuma or small leak, there is probably a cracked EVAP hose. FYI : The EVAP system monitors pressure inside the fuel tank. The reason the CEL comes on when you leave the gas cap loose is because it senses a huge pressure drop inside the tank. Hence, the term "gross" leak. Hope this helps.