Consistently negative LTFT

[seppstefano]

Hi all from a long time lurker, but only with a few posts. Now I beg your pardon for a long one:

I've an issue on my 2007 Honda Jazz GD (the European version of the Fit, equipped with a 1246cc i-DSI engine). The car has two spark plugs and no VTEC.

Since new the car was fuel thirstier than I expected/hoped.

Several years ago I bought a scangauge and the LTFT value keeps displaying negative numbers.

After a recent 200Km journey on highway it tends to be at -6 LTFT, but while driving around the city I get a typical -10 LTFT, and up to -13 LTFT

Today I checked STFT and while idling it displayed positive values (+2, +3, +1) i.e. it didn't oscillate around 0.

I figured that for some reason the ECU detects too few fuel (a dirty or defective oxy sensor before the cat?), so keeps increasing STFT (and this might explain the high fuel consumption). But - for some reason - another sensor (the sensor after the cat?) senses there is too much fuel (and says LTFT -10).

My issue is that this LTFT doesn't succeed in correcting the general consumption. I.e., the STFT keeps being positive and the consumption is high (while I have no error codes).

What could I do?

I thought I might clean the lambda sensor before the cat. Sometimes ago I installed a non standard header and I thought it should improve the things, but it didn't...

Any idea is welcome.

Thanks for your patience,

Stefano

[seppstefano]

Bump for any feedback ???

Thanks in advance,

Stefano

[Daox]

Long term and short term fuel trim are all dictated by the first oxygen sensor. The second sensor is just there to verify that the catalytic converter is working properly.

Just because your short term trim is positive while idling doesn't mean it'll be positive when accelerating or cruising. There are tons of variables that control how much fuel is injected. Unless your LTFT or STFT are being maxed out, I would doubt there is an issue. If your LTFT is ALWAYS negative, and STFT is ALWAYS positive, then you might have an issue. I don't see how this could happen as its all just programming in the ECU. I don't see how a defective sensor could cause this to happen.

[seppstefano]

Thank you Daox,
I seem I recall that I've read somewhere that the post cat sensor output is evaluated by the ECU of some models (e.g. http://www.audiworld.com/tech/eng53.shtml) , but I don't know if it is the case with the Honda.

Anyway while my LTFT is always negative, STFT varies. But perhaps I miss something. If LTFT is negative I understand the ECU should try to compensate by injecting more fuel. Yet it seems the sensor doesn't sense the added fuel...

But perhaps it's too late and I should go and sleep...

Stefano

[Daox]

If you really want to know more about O2 sensor, her is a very through run through of fuel trims and how the O2 sensor works with the ECU.

https://www.youtube.com/watch?v=oRX2V6_a3dc

[seppstefano]

Thanks for sharing. I'll report anyway my findings after cleaning both of the oxygen sensors.

Stefano

[redpoint5]

OT- but how much better fuel economy could someone get if they were able to delete the cat and keep the STFT at 0 instead of constantly going lean and rich?

[seppstefano]

Personally I tried also this route. Consumption was lower and response was better until the damned closed loop started adapting, so killing both mileage and responsiveness.

I can't but think that my ECU wants to burn more fuel than needed.

[Daox]

It does for reasons explained in the video, emissions / catalytic converter.

[redpoint5]

Originally Posted by seppstefano

I can't but think that my ECU wants to burn more fuel than needed.

I wonder what improvement in fuel economy we could see if cars were designed with MPG in mind; without a catalytic converter and the ECU tuned for economy? Would 10% through cat delete and ECU tuning alone be possible?