Dodge Repair Questions? Ask a Mechanic for Answers ASAP
Hi my name is ***** ***** I'll try to help with the Hemi question. The oil that is required for that engine is 5w-20. If it had the wrong oil viscosity put in it would normally set a trouble code for it, but drop in fuel mileage would happen with thicker oil.Bent valves sound more like a valve spring failure that hung them open and collided with a piston. That could be defective valve springs or some seriously high rpm.My biggest question about this is where is the "new" Rebuilt engine from? The harmonic balancer on this engine is neutral balance so it is unlikely that it is causing the vibration. My suspicion is that the rebuilder may not have assembled a balanced bottom end (crankshaft, rods, pistons) that are normally balanced as an assembly in a quality rebuild. Many cheap rebuild shops just assemble engines with combinations of parts that are not weight matched as an assembly.
Your fuel mileage sounds pretty normal for a Jeep Commander. They were never known for good fuel mileage.
Based on your info about the rough running and noises with no check engine light or codes, I would say that engine has an internal fault and would pursue the warranty replacement.
Checking in on this question.