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Tom, ASE Master Certified
Category: Dodge
Satisfied Customers: 864
Experience:  Chrysler Level 3 Technician, Chrysler Training Instructor
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I had my oil changed fuel mileage dropped from 19 Hwy to 14

Customer Question

I had my oil changed fuel mileage dropped from 19 Hwy to 14 hwy, 15 city to 9 city drove 700 miles and my engine blew up cylinders 6 & 7 bent Valves. the engine prior to that ran so smooth you would think the engine was off at stop lights
new rebuilt 5.7 hemi lobes at idle and has a harmonic vibration at 50 to 55 Mph and higher. No engine Codes, oil barke in oil changed at 250. miles engine broke in over 600 miles at 55 Mph over 1000 miles on engine. getting 19 Hwy and 10 to 12 city not good enough, rebuilt engine at first got 10 to 12 HWy and 6 to 8 city I was told that was normal until engine broke in, HWY got better city not much better suspect valve stuck open high speed vibration suspect harmonic balancer. Please give me you opinion
Submitted: 1 year ago.
Category: Dodge
Expert:  Tom replied 1 year ago.

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.

Expert:  Tom replied 1 year ago.

Your fuel mileage sounds pretty normal for a Jeep Commander. They were never known for good fuel mileage.

Customer: replied 1 year ago.
I bought the engine from O'Reilly's it came with a 3 year unlimited mileage warranty looks like I'll probably need it. This morning the car at warm up sounded like the rods or lifters on the drivers side were knocking (wasn't the engine clicking like most 5.7 Hemi's have) the knocking sped up with the engine RPM and in about a mile went away. The idle actually bounces you up and down like riding a tractor and the smooths out when it warms up but never go's away
Expert:  Tom replied 1 year ago.

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.

Expert:  Tom replied 1 year ago.

Checking in on this question.