1997 Honda Civic Gas mileage

Tiny
MAXIMUMCRAZY
  • MEMBER
  • 1997 HONDA CIVIC
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 154,000 MILES
Hi,

We have a '97 Civic with 154K miles that recently went through a number of upgrades. We swapped in a low-mileage engine from a salvage yard (32k miles). We did a pretty thorough tune-up at the same time (new oil seals, new plugs, wires, new O2 sensors, new air filter, new fuel filter, new belts, new H2O pump, new engine mounts). We also recently purchased a new set of tires. This last fill-up, I noticed a significant decrease in fuel efficiency, which dropped to 24 mpg from about 28-29 mpg. My car also recently recorded an engine trouble code P0420 which appreas to be related to the efficiency of the catalytic converter. Any idea what would cause the drop in fuel efficiency? Plugs and tires can affect fuel, but will it make that much of a difference in fuel economy if, say the spark plugs were incorrectly gapped? Is it possible one of the new O2 sensors is bad?
Thursday, October 2nd, 2008 AT 10:13 PM

2 Replies

Tiny
KHLOW2008
  • MECHANIC
  • 41,814 POSTS
Hi maximumcrazy,

When the Check Engine Lamp illuminates, it means an error has been detected and the ECU would set some of the sensors to a preset value and this can cause a substantial difference in fuel economy.

You should get the error code resolved first.
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Wednesday, October 15th, 2008 AT 1:44 PM
Tiny
MAXIMUMCRAZY
  • MEMBER
  • 2 POSTS
Hi,

Thanks for the reply. I think we've got everything resolved. The car has 2 oxygen sensors - one above and a 2nd below the catalytic converter. Turns out they were plugged in backwards. So, the computer was actually getting info from the wrong sensors. Everything works better now, and the fuel economy has improved.
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Thursday, October 16th, 2008 AT 10:28 PM

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