Slightly Elevated LTFT input
Thread Starter
Member
Joined: Jul 2010
Posts: 134
Likes: 8
From: near Rochester, NY
2019 GLE400
Slightly Elevated LTFT input
I've been using an "UltraGuage" in my 2008 ML350 to gain a little more understanding of how a "modern" vehicle processes data and how the systems interact. I've noticed that the LTFT runs 6 to 8% high on the fuel trims regardless of throttle. STFT hovers around zero. There are no codes. I'm at 60k miles. The car runs fine at all speeds. Economy is a bit over 22 mpg on the highway. (Yes, this is a first world "problem" 
.)
My understanding is that the LTFT is "multiplicative", so I don't think there is any significant vacuum leak or the LTFT should decrease on the highway. I have access to the MAP and MAF sensor readings, but no idea where to find "normal" under some defined conditions. O2 readings are about 0.65 volts.
Can anyone tell me where to look to find standard MAP or MAF readings?
I'm going to clean the MAF this weekend because it's cheap to do. If the MAF is dirty, I believe it could read "lower" than actual which would lead to a lean condition which the LTFT would correct for by adding fuel. This model uses a combination sensor in the short piece of 4" pipe with a screen at the back of the engine. I suspect that the visible bits are actually dthe IAT sensor and that the MAF wires are rather protected inside the rectangular bit.
Is there a good way to visibly see what I'm doing?
Last question. Does LTFT provide any subtle indication of fuel quality? i.e. if there's 10% alcohol in the fuel then it probably has less energy than 0% alcohol and LTFT may increase to meet the need. Most of the fuel around here has ethanol added.
Thanks for any input.
.)My understanding is that the LTFT is "multiplicative", so I don't think there is any significant vacuum leak or the LTFT should decrease on the highway. I have access to the MAP and MAF sensor readings, but no idea where to find "normal" under some defined conditions. O2 readings are about 0.65 volts.
Can anyone tell me where to look to find standard MAP or MAF readings?
I'm going to clean the MAF this weekend because it's cheap to do. If the MAF is dirty, I believe it could read "lower" than actual which would lead to a lean condition which the LTFT would correct for by adding fuel. This model uses a combination sensor in the short piece of 4" pipe with a screen at the back of the engine. I suspect that the visible bits are actually dthe IAT sensor and that the MAF wires are rather protected inside the rectangular bit.
Is there a good way to visibly see what I'm doing?
Last question. Does LTFT provide any subtle indication of fuel quality? i.e. if there's 10% alcohol in the fuel then it probably has less energy than 0% alcohol and LTFT may increase to meet the need. Most of the fuel around here has ethanol added.
Thanks for any input.
Last edited by thunderbirl; Apr 7, 2016 at 09:37 PM.
Thread Starter
Member
Joined: Jul 2010
Posts: 134
Likes: 8
From: near Rochester, NY
2019 GLE400
On my way to the station, the trims were as described above. As i pulled away from the pump, the trims started dropping and the LTFT ended up about 1/2 to 1/3 of what they were before the fill up. They stayed down for the rest of my 400+ mile trip at about 1.5 to 3%.
Mileage didn't seem to change significantly, but it's only a couple percent so I wouldn't expect to see that in one trip.
FWIW, it seems like the digitized increments for both the long and short term trims are reported out as +/- 0.78%. I think this corresponds to the smallest increments that can be adjusted by the computer (i.e., +/- 1 control unit).
I'm going to try filling up with this fuel for a while and make sure this effect persists.
