Ultraguage Fuel Trim

[cchase]

I know a few people on here are pretty well versed with this unit, although my question in general is not specific to Ultragauge.

I've been watching short and long term fuel trims for a few weeks now. What I've observed at several RPM's is that at idle, LTFT runs about -3 to -4%, at 1500 RPM's LTFT sits at 0%, and at 2000 RPM it is about +4%. I'm not sure whether those numbers are acceptable or they should be telling me something. I understand what they represent and how they are calculated but not whether leaning out at idle and enriching at 2000 RPM's by that margin means anything. EDIT: I don't mean to say I completely understand fuel trim tables and how many there are/what speeds/etc but rather that it takes O2, MAF, etc. sensor data to determine fuel trim.

Under long deceleration (say down a hill coasting) the LTFT will go to -7 to sometimes as much as -10%. The question is, is Ultragauge only pulling raw fuel trim data from the ECU or is it interpreting the data in some way? I ask because I'm never quite sure whether the fuel cut-off threshold has been properly set, and curious if Ultragauge does not think that fuel cut-off has been reached but the ECU has cut off delivery, would Ultragauge report an erroneous negative LTFT, thinking that fuel cut-off WAS actually an overly rich event being compensated for?

Thanks in advance and hopefully I am being clear.

 

[Colt45ws]

Its pulling it directly from the ECU. The way it works is using a MAP or MAF the computer can calculate a engine load based on RPM and throttle position. It then looks up the load and rpm in the fuel tables and it is given a fueling. The O2 sensor is then used to correct the tables for that specific operating environment, and specific tolerances in the various sensors and operating characteristics of the engine.
Tolerance for the fuel trim is manufacturer specific, but those numbers sound within normal.

 

[punisher]

Originally Posted By: Colt45ws
............. but those numbers sound within normal.


Ditto. Not a problem with those numbers.

 

[cchase]

Thanks for the information. I was thinking that it was probably pulled directly.

Any idea why fuel trim swings so far negative in a deceleration situation?

 

[punisher]

Originally Posted By: cchase

Any idea why fuel trim swings so far negative in a deceleration situation?


Ignore it, the PCM does. During decel the PCM often cuts fuel completely, restarts injector flow etc. There is stuff going on during decel to save the last drop of gas, and reduce emissions. O2 readings, hence trims, essentially mean nothing at these times.

 

[Vikas]

Unless those numbers swing in 20% range, you can ignore them. It is the job of the ECM to monitor it, so if it is not normal, ECM will turn on the check engine light! Let ECM worry about it.

If your fuel trims are under 5% most of the time, it implies your car is running as good as it can. Congratulate yourself for taking care of it

By the way, the only calculations that UltraGauge does is for the fuel consumption. All other parameters are shown as reported by the ECM.

 

[cchase]

Originally Posted By: Vikas
Unless those numbers swing in 20% range, you can ignore them. It is the job of the ECM to monitor it, so if it is not normal, ECM will turn on the check engine light! Let ECM worry about it.

If your fuel trims are under 5% most of the time, it implies your car is running as good as it can. Congratulate yourself for taking care of it

By the way, the only calculations that UltraGauge does is for the fuel consumption. All other parameters are shown as reported by the ECM.


Thanks for the information. I wasn't worried about the fuel trim, just trying to understand how it was interpreted since of course a fuel cut-off event could also be interpreted as a -100% fuel trim. What you are saying makes sense to me.

 

[Vikas]

A good mental exercise is to figure out where the percentage load is calculated from. This parameter is calculated from other parameters. When I looked deep in to, I thought there was some chicken and egg going on as the load computation involved the load values itself.

Two most important parameters to watch on UltraGauge are ignition advanced timings and MAF grams/second values. You should get an idea of the range for those values after watching them for a while. Your seat of pants engine performance will very closely relate to those two parameters.