97 DX Startup Idle and Low MPG?
 

Hey everybody,

I have a 1997 Civic DX Hatchback 130,7XX, and I was throwing a CEL P0421 or P0420 code back in July. I checked the exhaust manifold and it was cracked in the center, so I had that replaced along with the catalytic converter. I also replaced the oxygen sensor on the manifold because the light came back on. I also changed a valve near the air box which I heard can throw codes like that. I had been getting about 35-40MPG before the issue. While I had the CEL on, I would get about 22, and now I get about 30 driving either highway or city with no CEL.

I'm wondering what the issue could be that I don't get the mileage I used to (35-40). I don't have anything heavy in the car. I replaced the tires about 2K miles ago, but that shouldn't cause that much of a difference in MPG. Could it be the lower oxygen sensor? I also notice at startup that the car idles high but then lowers to the normal level after about a minute. I work at AutoZone so I could get discounted parts. I'm thinking it could be dirty injectors, the fuel filter, or that other oxygen sensor. Any suggestions? All help is appreciated.

Re: 97 DX Startup Idle and Low MPG?
 

Wow.

What brand was the O2 sensor you installed?

Any particular code for that, or just because the light came back on?
"a valve"?? WTF is that?
Codes like what?
Does the engine lack power now?

Check cam timing first.

If that's ok, then unplug the first O2 sensor and drive it a few days, see if there is any improvement.

Re: 97 DX Startup Idle and Low MPG?
 

I don't know much about Honda engines as this is my first civic and I've had it for less than a year. I replaced it with a Bosch sensor.