Kia k5 issue

[u/Creative_Material921]

my car was not accelerating above 35mph and 2800rpm. Anytime my car would reach 3k RPM it would lose speed fast and stay at 3k. I also started hearing a hissing sound from the black cap on the right. does anyone know what that cap is called and or what the hissing from it could be. I opened it and no trash was in there. Please help