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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.

Any ideas?

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2 timing belt is currently off 1-2 notches, or was of 1-2 notches and has since been corrected? – mac Nov 16 '12 at 15:29
It could be. As far as I could tell, I got it right, but it's the first time I've ever realigned it. – R.. Nov 16 '12 at 19:11
Hopefully there is a test port on the fuel rail. If so, you can rent a pressure gauge from a parts store. – Mark Johnson Nov 26 '12 at 20:13
Exhaust smell has gone away and fuel efficiency is up to about 22-32 (city vs highway) since replacing the O2 sensor. I've got a local shop I trust to check it out further and I ran by them a list of things I thought might be wrong, so I think I'll just get them to check it out. I wouldn't want to pay ~$60 for a single-point test, but it sounds like they'll be able to cover all bases for the standard diagnostic cost, which sounds pretty reasonable to me. – R.. Nov 26 '12 at 20:19
Could you update your post to include problems you are still having after replacing the O2 sensor? – Eric Fossum Dec 12 '12 at 0:09
up vote 3 down vote accepted

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.

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I had a timing belt slip by one notch and the car (130hp MR2) sounded like a lawnmower and could barely get to 30mph. I'm surprised yours ran as good as it did in that condition! :-) – Brian Knoblauch Jan 16 '13 at 12:46
Probably mine was off in the opposite direction. It wasn't just a slip but me re-aligning it after the slip and failing to get it exactly right. – R.. Jan 16 '13 at 21:23

This may sound stupid, but you changed the oil, right? It sounds like you haven't done much maintenance on the car, maybe this is what happens to a 20 year old engine when it is neglected.

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Yes, I'd last changed the oil about a year before the head gasket blew, and I changed it again immediately after putting it back together and cleaning everything up. Also used a cleaning oil additive (before changing the oil). – R.. Nov 20 '12 at 0:55

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