Hi,
This is a tough one. That is a big change in MPG. Did this happen immediately after the new engine was installed? Also, has the check engine light ever come on and stayed on while the engine is running?
The idea that it hesitates on a steep hill leads me to think something is moving. For example, the insides of the catalytic converter can become loose and move on a steep hill. That would restrict the exhaust flow and cause a hesitation or loss of power.
Let me know if there is anything you can think of that stands out. For example, are there any noises or smells you notice from the vehicle?
Let me know.
Take care and God Bless,
Joe
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Monday, March 15th, 2021 AT 7:43 PM