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Discussion Starter #1
My 06 type S that I bought a year ago is getting me very bad gas mileage (15-16L/100km) and I have no idea why.
My mods are Injen CAI with K&N filter, chipped ECU and a custom exhaust line with headers, cat delete and a dynomax muffler, so technically I should be doing pretty good. Still, my engine runs rich for some reason (black residue on the muffler tip). At idle, the absolute throttle position sensor reads 50% and the upstream O2 sensor reads 0.000V. I just put on new tires and replaced the flex pipe of the exhaust because it was leaking, and my mechanic thought that removing the cat and downstream sensor is what's causing it, but I'm not satisfied by this answer.
What sensors should I check? Could it be another exhaust leak? I'd like to daily it, but it's just not economical as of right now

Thanks guys
 

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Sounds like you should diagnose the O2. It corrects for fuel and if it isn't you get what you describe. Check the connections and wiring as you should have a constantly changing voltage as it adjust F/A ratio. Anything after the first O2 isn't a big deal as the second sensor just monitors cat performance and you'll get a code, but it won't trim the fuel. How does it idle, and does it get worse warm. A leak before the first O2 will richen the mix as it thinks it's too lean--again the sensor is working telling you that you have a problem, not that the O2 sensor is bad. No voltage isn't good as that goes back to the ECU to change the amount of fuel to get it to the correct A/F mixture.
 

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Past Driver’s Ed
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Your mods shouldn't be the problem and hopefully the car ran correctly after the tune. There are multiple things that can lead to poor gas mileage. Start with checking the primary sensor, and the MAP sensor. The MAP sensor can cause a drop in gas mileage. If your car’s computer inaccurately reads the pressure in the intake manifold as high, the engine will inject more fuel to meet the heightened engine load. This can reduce fuel economy and possibly lead to detonation.
 
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