I have an '06 Civic with 175000 miles, auto tranny. Till lately, it got about 40 mpg highway… Then seemingly overnight, the mileage dropped to 25 mpg. Oddly enough, the car still was running fine, no misses or hesitation in the engine, plenty of power, same acceleration and smooth operation.
After months of investigating possible causes (I move slow), I replaced the obvious: plugs, wires, both O2 sensors… to no effect. Eventually I took it to a pro for a scan diagnosis that revealed no error codes or abnormalities EXCEPT he thought the converter was running “a little cold, could be plugging off”. Compression was “excellent”. SOOO… when I gathered the necessary cash (weeks later) I replaced the converter along with a new muffler (it was time). Still no change… good operation, terrible mileage.
Flash forward a couple more months to this week. Now I notice a definite “wanky-ness” in performance. I’m experiencing a definite loss in power, the engine is reving higher than it used to before shifting, and when crusing @ steady speeds the tranny seems to range around, upshifting and downshifting more than it used to do.
Casual diagnosis would imply that the tranny is shot, and perhaps the cause of my grief. But if so, how would one explain all the previous months of flawless operation?