High Fuel Trim at Cruise

[GT08]

Hi,

I drive a Mk2 Jetta with a 2.0l ADY converted to EFI with a Perfect Power Xms.

Recently I started feeling a stubborn sluggishness at cruising speed (like going from 105 to 120km/h with part throttle). This is around the 2500 - 3500 rpm range.
This is combined with the management showing a really lean AFR (Target 14.5, Actual 15.7). This is combated with a really high fuel trim (Long term and short term).

Idle and WOT does not really seem to be affected. Idle AFR seems exactly the same as always at 13.7. This is with fuel trim disabled for Idle.
WOT AFR seems fine with some STFT of +6 to +9 with great response at WOT changes.

This happened before some time ago after I gave in the Jetta at a place to have the lifters in the head replaced. The mechanic there for some reason adjusted the vernier pulley on the cam by like 3 degrees and the management was compensating. After correcting the pulley all was well.

This time it happened out of the blue with no changes.

I can see in the Long term fuel trim map there is at least 5 areas with +10 and the STFT is at +12 at times.

I checked the cam pulley and it seems to be where it should (on 0 deg change), but I cannot say for certain that the key slipped and it somehow adjusted.
I checked for vacuum leaks and found only 1 at the airbox connection pipe, so that should not really change anything major (or so I think). I re-seated the clamp to try and correct it.

All I can think of now is the Wideband sensor is going bad, or a ground of the management is bad, or there is not really anything wrong just the cooler air of winter.

What do you guys think? Anything else I can check to find the reason for the leaner AFR?

Any help would be appreciated.

[panic-mechanic]

Well add some fuel in the main map on the areas where it seems lean. Then clear the long term trim and see what it does. How much wriggle room are you giving the lambda to correct in F7?.

[GT08]

Well add some fuel in the main map on the areas where it seems lean. Then clear the long term trim and see what it does. How much wriggle room are you giving the lambda to correct in F7?.

Thanks for the reply. It just seems somewhat out of character to lean out so suddenly by so much. It is almost a first to have +11 on any block so in more than 4 blocks seem too much.
If I remember correctly the max adjustment is like 50 in the F7 screen.

I will make some changes and monitor it in the coming week. This is the first full winter after you helped me change some settings on my Jetta.

The fact that idle and WOT seems stable is a good sign I guess as it does make a clear vacuum leak less likely.

[panic-mechanic]

Colder air is more dense which requires more fuel.

[GT08]

Colder air is more dense which requires more fuel.

Thanks Panic, just wanted to update this thread, you were right. It has stabilized now with a few +10s in the long term map.
The sluggishness is gone and all seems to be good again. So no real vacuum leak it seems.

[panic-mechanic]

The beauty of cold air.

[VAG Fan]

I have dangerously little knowledge of electronic air-fuel management systems, but I like to read anyway.

I was wondering: does the management not monitor air temperature (i.e. air density)? Does it not compensate for these fluctuations by itself?

To put it naively: does the OP have to make this adjustment each winter or summer?

[panic-mechanic]

You 'can' let the management look at ambient air temp and it does have compensation to do it. It was also busy fixing it incrementally due to the constant adding of the closed loop system and it would eventually have done so. Whether one chooses to connect an ambient temp sensor and make use of it on that system is a user's choice , it has the ability.

[GT08]

I have retro fitted an air temp sensor onto the intake manifold of my Jetta, but the place that did the initial wiring for the management decided not to wire it in due to my car being naturally aspirated at the moment. The benefits of tuning for air temp are a lot more prevalent on turbo cars (or so I have been told ).
That being said I would have liked to at least be able to monitor intake air temp even if it was not used for tuning, but my lack of wiring experience means I should rather not try to wire it in myself.

Luckily most of the adjustment this time with the Long term map was quick enough that I only noticed a big difference for like 2 days. Not bad compared to what it was before the long term adjustments.

[panic-mechanic]

Yeah I have never bothered myself.