This past summer I've been noticing that my truck is not making the same power as it used too and I've noticed a pronounced increase in engine knock (detonation). The detonation usually occurs when I am driving on the highway on my way to work in the morning with me just adding a little gas to maintain my speed. Interestingly enough the detonation doesn't occur when I floor the acceleration pedal but only when applying medium power with engine RPM over 2000 on the highway.
From my research these symptoms could be either from a bad oxygen sensor or bad gas. Recently I've been using Walmart (Murphy's) gas since they built a new walmart close to my house which is cheap and convenient. I am using 86 octane gas which should be good for my Ranger.
Another possibility is the oxygen sensor which could be leaning the mixture too lean. However the check engine light is not on and the other symptoms of a bad oxygen sensor like running rough or having trouble starting the engine are not there.
So my question to all of you is what do you think could be causing the loss of power and the increased engine knock?
Edit:
By the way the engine on my Ranger is the 4 Liter V6 engine not the V4 engine that comes with the baseline Rangers.