HX Lean Burn
 

Hey guys, havent been on here in awhile, I just picked up a 2005 honda civic HX. I have been driving a 95' VX for the last 3 years and I am having troubles trying to figure out if the engine is going into lean burn. In the VX it was very noticeable, the car would jerk and then I would know. With the HX I do not feel this movement, I have not been through a complete tank of gas yet but my scan gauge is reading 33-36 mpg while doing about 60mph. Any suggestions? Is there anything on the scan gauge that would tell me?

Re: HX Lean Burn
 

Car needs a tuneup.. I mean if you're cruising 60mph, the scan gauge's engine liters is appropriately set at 1.7L and the scan gauge is reading like 36mpg, then something is up. While I'm not aware that you're suppose to feel the lean-burn kick in on the newer cars, I would imagine however you fuel economy to be much better than 36. Now again, I have to ask, the 33-36mpg reading, is that your AVERAGE fuel economy of 50/50 city highway driving or is this the average fuel economy of 60mph highway driving according to the scan gauge? At 55mph on my civic, the scan gauge reads from 45-55mpg depending on if I'm climbing a slope or descending one and I have a Civic LX with a shorter geared transmission, no vtec or lean-burn.. I also test drove a 2005 Civic EX and when I was cruising at 55/60mph, the scan gauge read about 33-36mpg which kind of put me off since I would have expected much better highway fuel economy.

Re: HX Lean Burn
 

It is very difficult to notice when my lean burn kicks in. As far as I understand the scan gauge doesn't calculate lean burn mode. The scan gauge thinks you have a standard motor with a 14.7 a/f ratio. My scan gauge reads in that same region as yours and I get 45+ mpg.

Re: HX Lean Burn
 

Um, scan gauge doesn't go by the A/F ratio to calculate fuel economy.. All it does it take the MPH you're going, the injector pulse width, a vague estimation of fuel pressure, the displacement of the motor and figure out the fuel economy. Even though 14.7 is stoichiometric, in my experience, according to smog reports cars are running at an A/F ratio of 14.1 not 14.7 and I think that's so that the catalytic converter continue to work since if there is no fuel but lots of nox, it won't work at all. The car going into lean-burn should be completely irrelevant to calculating fuel economy since all it's doing is reducing the injector pulse width.

Re: HX Lean Burn
 

The Scangauge doesn't have access to injector pulse width. On many vehicles it also doesn't have access to fuel pressure.

I believe it uses MAF and O2 readings to calculate fuel rate.

Re: HX Lean Burn
 

it doesn't have access to o2 sensor readings on any of my cars.. They should definitely have access to injector pulse width readings.. O2 readings are too imprecise and they wouldn't be useful for figuring out how much fuel was used...

Re: HX Lean Burn
 

O2 sensor data is part of the most basic subset of OBDII data. I'm amazed that it can't get O2 from your car. Perhaps it uses fuel trim instead.

MAF is the main data used to calculate fuel rate. O2 or fuel trim, if used at all, is just used for minor adjustments to accuracy.

Here's a list of OBDII data accessible by standardized means.
https://en.wikipedia.org/wiki/OBD-II_PIDs
Not all is available in all vehicles, but anything outside of it is likely to be a manufacturer-specific extension to OBDII. Injector pulse width/duty cycle is not part of basic OBDII; it is part of manufacturer-specific extensions.

Re: HX Lean Burn
 

Here is the thing though... if the o2 sensor is required for calculation of fuel economy, then how and why is it that fuel economy can be calculated with the o2 sensors disconnected?

Re: HX Lean Burn
 

The O2 sensor is only for minor adjustments. MAF is the main data used to calculate fuel rate.

It can be approximated from other data with reduced accuracy. For example, RPM and displacement can be used to guess. I'm not sure which other variables could be used to calculate it, off the top of my head.

Re: HX Lean Burn
 

Not all cars have a maf sensor, some use a map sensor which is a completely different way of calculating how much air is flowing in.