My 92 Civic has had low power (need to use at least one gear lower than usual to get the same power) and has been getting bad gas mileage (15-25mpg vs 32-42 mpg usual) since I replaced the head gasket. Exhaust also smells bad. Engine is otherwise running very smooth and quiet, though. I'm looking for some help diagnosing what the source of the problem might be. Here's some additional information:
Timing belt slipped and might be off by 1-2 notches, but seemed to be right when I lined it back up (I aligned the UP marking on the cam sprocket with TDC as measured by sticking a rod down through the #1 spark plug hole).
Oxygen sensor was shot (completely covered in soot and outputting -0.1v instead of 0.1v-0.9v) but has been replaced. That improved the situation somewhat, but not fully. I measured the voltage from the new O2 sensor and it's never getting above 0.1v. Could this result from incomplete combustion (oxygen left after combustion) and if so, what might the cause be?
Fuel filter was also replaced and fuel delivery system seems to be fine, but I don't know how to measure fuel pressure or pressure in the intake system.
Tried a lot of things, some of which helped incrementally but didn't fully fix the problem:
Torquing the exhaust manifold nuts to spec.
Replacing the muffler.
Finally took it to a shop for diagnosis and they found that the timing was off and fixed it for barely more than the diagnostic work. Now it seems to be running great. I think it must have just been off by one notch because I can't imagine it even running if it were off by more than that.