Reset fuel trim?

[Delasangre]

I just replaced my O2 sensor, mostly an experiment/throwing parts at it to see if it fixed my horrific fuel mileage. Wondering if there is a way to clear fuel trim to start over and let it read the sensor fresh. I imagine re-flashing my tune would do it but wondering if there is a simple way with VCDS

 

[P2B]

There is no fuel trim to clear on a diesel.

 

[Delasangre]

There is no fuel trim to clear on a diesel.

Does the ECU not adjust the fueling based on AFR from the upstream o2?

 

[CanadianALH]

Does the ECU not adjust the fueling based on AFR from the upstream o2?

Don’t believe so. I think it used a fuel and boost map among other things. It looks at the amount of fuel added by you putting the pedal down then ads boost accordingly I believe( at least a really simple version of it all). Someone knows the CR better can chime in like maybe @lemoncurd

 

[lemoncurd]

You mention a tune, I’d wager that’s why you have bad fuel economy

you could also have a bad alignment

who tuned your car?

 

[lemoncurd]

Don’t believe so. I think it used a fuel and boost map among other things. It looks at the amount of fuel added by you putting the pedal down then ads boost accordingly I believe( at least a really simple version of it all). Someone knows the CR better can chime in like maybe @lemoncurd

Sadly I’m not versed in the nuances of how the ECU actually works. A real tuner like @Xtremefunky may have some thoughts though

 

[P2B]

Does the ECU not adjust the fueling based on AFR from the upstream o2?

No. Diesel AFR is variable during normal operation, approximately 30:1 at full load and maybe 100:1 at idle. Diesels never approach stoichiometric AFR, they always run "lean".

 

[CanadianALH]

You mention a tune, I’d wager that’s why you have bad fuel economy

you could also have a bad alignment

who tuned your car?

Ah good point didn’t see the tune mentioned. Yes… I wonder who did it. Maybe something got botched when the tune was sent, wrong stage etc.

 

[Delasangre]

You mention a tune, I’d wager that’s why you have bad fuel economy

you could also have a bad alignment

who tuned your car?

Its got an MRTune on it, I told him about my mileage issue and he asked for my last tune and while he said he didn't see anything wrong he send a slightly revised one which added maybe 1 mpg but still ****. I am running his Stage 2 +55 HP tune with a full delete.

 

[AverageAndy]

What is horrific? We had the same tune/stage/delete from MRTuning on our 2013 Jetta sedan and usually got low 40's for mpg (but part of that could have been how much fun it was to drive).

Maybe try loading the Stage 1 tune and see if that makes a significant difference?

 

[Delasangre]

What is horrific? We had the same tune/stage/delete from MRTuning on our 2013 Jetta sedan and usually got low 40's for mpg (but part of that could have been how much fun it was to drive).

Maybe try loading the Stage 1 tune and see if that makes a significant difference?

I have been averaging about 33 for the last 6 months, no matter how hard I try to get good mileage.

 

[lemoncurd]

I have been averaging about 33 for the last 6 months, no matter how hard I try to get good mileage.

thats about what i get, the CRTDI's just get less fuel eco than the older TDI's

 

[Delasangre]

thats about what i get, the CRTDI's just get less fuel eco than the older TDI's

I COULD be ok with that, except I WAS getting 40 going to work every day, and then suddenly about a month after delete and tune it dropped to 31-33. It USED to get good mileage I just can't figure out why since there are no codes. I already did all the usual suspects too. New calipers and parking brake cables, both coolant sensors, timing belt, all filters and fluids, ect.

 

[ZippyNH]

Just curious...ever inspected you intercooler?
Know on a HD truck...one if the first thing we would look at is the intercooler...loss of mpg is often the sign of a leak.
Sometimes it's obvious, and can be heard, but often it's more insidious...with cracks etc in the tubing

 

[Xtremefunky]

Does the ECU not adjust the fueling based on AFR from the upstream o2?

Not at all. Which doesnt mean it doesnt use it all, its more for error detection and redundancy.
From a "tuner" perspective. Its neat to be able to log it. Even tho the CJAA ECUs are super greedy with giving out useable logs,
compared to their other brothers.

Sadly I’m not versed in the nuances of how the ECU actually works.

Stay quiet peasant then. Joke. haha. Greetings to you

approximately 30:1 at full load and maybe 100:1 at idle

Kinda depends on the engine. 30:1 is a bit too optimistic at full throttle. Its more like Lambda 1.5 (~22:1).
But also depends on the engine. Some actually go this high, but then we are talking about heavily "de-tuned" factory cars.
Like VW Caddys or VW T5s with 84HP from factory with 2L and turbo.

Depending on the engine as well, they actually also run rich for a short amount of time when getting the turbo spooled up.
BMW on their 3L N57 run lambda ~0.95 to get the turbo spooled and VAG on their 3L CDUD goes down to like ~0.97.

So it really depends. In normal driving conditions the OEM wants to keep the lambda at around ~2. Not because this is
the most efficient, this is due to keeping NOx levels low. High lambda, causes high NOx and thats the stuff you dont wanna have.
By disabling the EGR you get up to lambda 4 in normal driving conditions, which gives you the slight efficiency advantages,
but from a chemical aspect, its a horrible trade off, because the NOx values will shoot thru the roof. Like 5-10 higher.

 

[oilhammer]

I'll happily take higher NOx out the tailpipe in return for less fuel going in the tank.

I like my diesels LEAN, LEAN, LEAN... .no smoke, clean oil, high MPGs.