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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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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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Discussion Starter #4
Right, good points guys. Turns out my primary O2 sensor was bad so I changed it, but my fuel trims (short and long term) are still stuck at 0% and the primary indicates rich condition (varies between 0.6 and 0.8V approximately). What's more, my fuel system status is fixed on OL-Drive at all times, which from what I know happens normally at WOT and delivers the maximum amount of fuel. Perhaps the car would need a retune?
 

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You mentioned that your TPS is showing 50% at idle in your first post. Is that still the case? Open loop operation is indeed for WOT, but the ECU can still shift to open loop operation if certain parameters are met even if you don’t floor it. Your TPS can very well be the reason you are running in open loop at all times, and that would explain why you’re running rich, with 0% fuel trims. Your tune is not the problem if it was running correctly previously.
 

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Discussion Starter #6
Yes you are right, my TPS does read 50% still. So you're saying I should change the TPS to (hopefully) solve the problem? Is there a way to test it with a scanner? I know you can test it with a multimeter on a cable activated throttle, but maybe there's a simpler way I'm not aware of.

thanks a lot guys btw
 

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You’d have to use the multimeter. You can try calibrating with Kpro as well. If both those don’t work, I would replace it. Here’s a DIY on calibrating it via kpro

 

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Riiiiiight so update on the situation: I recalibrated the TPS using k-pro, did marvels instantly to the throttle response, the car definetly felt quicker and the gas consumption went down over the next 300 km. The TPS still read 20% @ idle tho so I ended up replacing it with an Acuity Instruments one and calibrated it same way. I also cleaned the IACV while the TB was out of the car, figure I might as well. Now, new sensor and a clean IACV later, the car feels great, idles at 1300 cold and not 2400 etc etc. Thing is, when I coast in neutral now, RPMs go to 1k, then creep up to 2k and then fall abruptly to 1k and back again. Did I mess something up?
 

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Riiiiiight so update on the situation: I recalibrated the TPS using k-pro, did marvels instantly to the throttle response, the car definetly felt quicker and the gas consumption went down over the next 300 km. The TPS still read 20% @ idle tho so I ended up replacing it with an Acuity Instruments one and calibrated it same way. I also cleaned the IACV while the TB was out of the car, figure I might as well. Now, new sensor and a clean IACV later, the car feels great, idles at 1300 cold and not 2400 etc etc. Thing is, when I coast in neutral now, RPMs go to 1k, then creep up to 2k and then fall abruptly to 1k and back again. Did I mess something up?

Sounds like you might have a vacuum leak possibly. Tough to say. TPS could be a likely culprit also. Check your TPS Readings?
 

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the downstream sensor is necessary for fuel trims, so without one you won't have any fuel trim. ECU will run the engine a bit rich to compensate. not sure about TPS issue. high idle and revs moving up and down are likely due to vacuum leaks, carbon build-up in the throttle body, or both
 

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Riiiiiight so update on the situation: I recalibrated the TPS using k-pro, did marvels instantly to the throttle response, the car definetly felt quicker and the gas consumption went down over the next 300 km. The TPS still read 20% @ idle tho so I ended up replacing it with an Acuity Instruments one and calibrated it same way. I also cleaned the IACV while the TB was out of the car, figure I might as well. Now, new sensor and a clean IACV later, the car feels great, idles at 1300 cold and not 2400 etc etc. Thing is, when I coast in neutral now, RPMs go to 1k, then creep up to 2k and then fall abruptly to 1k and back again. Did I mess something up?
I’m guessing vacuum leak as well. Did you replace all of the gaskets? And did you lubricate the IACV?
 
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