O2 sensors,
are only capable being accurate around 450Mv (equates to 14.7:1) and a little either side.
As such they are known as narrow-band sensors.
They can be used as a
guide for fuel mixture outside the narrow bands, but that takes a bit of explaining and you usually need some kind of scan and logging software to be able to store and read it.
When an engine for example reaches it's Closed Loop Temp Enable setting (referenced from ECT or Engine Coolant Temp) ..the O2's start working and the engine goes into Closed Loop.
As long as you stay generally gentle with the TP and avoid sudden acceleration, or higher rpm (usually above 4,000) and/or high MAP, the O2's will continue to operate and keep the engine in Closed Loop mode. All the time continually sending info to the PCM to keep the fuel trimmed
** at 14.7:1.
It's possible to complete a full journey in CL mode by being gentle with your right foot and avoid going into PE (Power Enrichment) mode at all.
**These are known as Short Term Fuel Trims (STFT's) and are capable of adjusting instantly by either increasing or decreasing injector pulse-width to either lessen or add the amount of injected fuel to keep the engine at 14.7:1
In addition....every 20 seconds or so the Long Term Fuel trims adjust to the STFT's average and adjust to try and even them out.
i.e. STFT's have been taking out fuel because of a slightly rich condition, the LTFT's adjust PCM parameters to compensate.
Only the initial start-up and the time taken to get to CLTE is the car
not in CL mode. It's in Speed Density mode where it uses a Commanded Fuel While In Open Loop table that has ECT and MAP as reference and at each different ECT and MAP point there will be an AFR setting which the PCM will read from and use until the O2's come online when the correct engine coolant temp is reached.
If the engine subsequently does a re-start and the ECT is above Closed Loop Enable Temp, it still initially uses SD mode for a set amount of time regardless, and then goes into CL mode once again.
(For example if you've driven the car for 30 mins or so then stop for 10 mins and then re-start).
All the while, from engine start the Maf-sensor is sending initial air-mass information to the PCM which uses another seperate fuel mixture table as a base fuel mixture reference, the VE (Volumetric Efficiency) Table.
The VE table is the main table for SD mode and also when in CL mode.
If the VE table for example, is set too much on the rich side everywhere (this table usually uses RPM and MAP as a reference) then the engine LTFT's
*** will stay permanently rich (i.e. be taking fuel away because too much is being commanded from the VE Table.
Conversely if the whole VE table is set too weak, then the LTFT's will be
adding fuel permanently. (This is how a lot of factory tunes are set, for economy/emissions reasons but it makes the car more prone to KR or detonation).
Therefore when a car is running in what is ideally considered to be 0 to -5% LTFT range and the car goes into PE mode. It commands what is set in the PE versus RPM table quite faithfully (i.e. if 12.7:1 is set there, then that is what you get).
Whereas if the car has been running with the LTFT's on the plus side (weak) and you go into PE mode it adds the fuel based on the Fuel Trim average (which was weak in this instance) and you do not get a true 12.7:1 AFR it would be 12.7 plus the amount it had been running weak as a percentage so the car then
accelerates with a weaker AFR then commanded.
***LTFT's are normally up-dated every 20 seconds and this LTFT info is stored in the PCM/ECM. Whereas STFT info
isn't stored in memory.
LTFT's and STFT's have something like a plus or minus 25% range to work with.
The maf-sensor will also have an acceptable range to work with also.
Go outside of these without doing a maf-recalibration or VE table adjustment and you are heading for a problem.
>>> This post not poached or pasted from any source. It is in my own words relying on memory.
If anyone can see anything they'd like to raise questions or doubts about, please feel free to either post or PM me and I will happily check, correct if necessary, or back this up with my tuning info/files which I don't have with me at the moment.
Furthermore, anyone interested in seeing actual examples of Fuelling Maps, VE Tables, Timing Tables and anything else they might be interested in, both for petrol and where applicable Turbo-Diesels, again let me know & I'll be happy to do so, after Jun 14.
<<<
-- Sid447 --