I like to believe I have a minimal understanding of how the fuel trims work on our cars. I may be over-thinking this but, tell me if I am correct on this;
On a stock engine, with stock intake and exhaust, built to perfect specs , in a perfect environment would read an equal +~- or "0" fuel trim. To account for variables, the manufacturers program in a measure of "trim + or - to improve drivability and prevent the CEL from lighting every hour.
Now, if this were true... if one took this engine set up and fitted a free-flowing exhaust, a free-breathing cold air intake and, enlarged the intake ports and manifold would this not cause the ecu to add pos. fuel trim? I would think so... is there a limit to the amount of air (or mods) one can add so the fuel trim limits are safely avoided?
Also, could one determine the benefits of a modification by observing the before and after fuel trims? I would think showing a small pos. fuel trim after a modification would indicate an improvement in air flow through the engine. Now, if only I were smart enough to understand long term fuel trims!

 

Its not that the ECU keeps a fixed airflow value for your engine, if that would be the case, then indeed allowing more airflow would mean you go out of spec.

The ECU is checking the air/fuel ratio via the O2 sensors, and determines then pending on the mixture if more or less fuel is needed to keep it at lambda 1, which will be shown by the stft/ltft values.

Best way to measure if you have more airflow would be to check the MAF readings, but remember that on cold high pressure days you already get more air then on hot days, so you need to factor that in when you want to see if you have an improvement.

 

Letting more air in won't change the trims. The exception is if the MAF fails to see the air, such as when there's an air leak.

The reason why is that the basic amount of fuel to be added depends on the air (and sensors like IAT, ECT etc). So more air means more fuel but that is NOT what fuel trims are about. The calculation so far can be thought of as looking up the air (MAF reading) in a table (like a spreadsheet).

The fuel trims come into account next, mainly due to what the O2 sensors readings are. If they're already right then the trims will be zero. If the O2s aren't right (based on the exhaust from the fuel burnt instants ago) then the fuel trims are adjusted. Then the exhaust gases are rechecked and the trims readjusted, that's what CL (Closed Loop) is all about. It's doing this multiple times per second, all the time it's in CL.

Changing the exhaust and/or cats can change the back pressure. That's not so simple as allowing more air in. The back pressure has at least 2 effects: internal EGR and the EGR via the EGR valve (aka external EGR). Personally I wouldn't mess with them because it costs money, may sound horrible and can make things worse or cause codes but so long as you're OK that there might be trouble go for it. At all times bear in mind that EGR reduces in-cylinder temperatures from scarily hot down to less-scarily hot. So reduced EGR means hotter temps where arguably you do not want them. You'll be conducting an experiment on engine longevity at higher in-cyl temps I suppose. Someone like avos may have a feel for whether that's OK.

 

This forum has indeed a lot of brilliant people... THANKS for all the info. I considered sensors like the IAT factoring in to MAF, I did NOT consider the OA MAF factoring into the OA fuel trim... makes sense though. Nor did I consider the EGR factoring into the exhaust system... again perfect sense. Its a good system of checks and balances. One more question if I may; stft is whats taking place at the moment, just how long is the value in the meaning of LTFT?

 

Have a look of the excellent fuel trim primer in this thread: https://www.jaguarforums.com/forum/g...ed-quiz-49317/

 

Which way does LTFT go when O2 sensors get old or dirty?