Hey guys recently my 2001 Lexus IS300 started acting up, it won't go past 15mph uphill and sounds like it's about to die the whole way. The noise it makes sounds as if my muffler has been taken off. At first I thought it was my head gasket so I replaced the head gasket and other gaskets and seals it came with including the ones for the entire air intake and exhaust system and while I was at it, the water pump, the radiator, the thermostat, and the air intake. Along with brand new fluids, oil, atf fluid, and coolant. I'm stuck now. There's other theories such as maybe some of the piston connecting rods are bad but this time I need to know for sure before I take apart the engine again.