Fuel trims what makes them max out at idle?

[Davezj]

What i am after is a bit of discussion as to what could possibly make the fuel trims max out.

a bit of back ground into fuel trims.
fuel trims add and remove fuel based on the O2 sensor going rich or lean.
fuel trims only work in closed loop operation with the O2 sensor.
you can think of long term fuel trim and short term fuel trim as 2 columns of numbers, Tens and Units LTFT are the tens and STFT are the units.
STFT goes from -16.8 to +16.8
LTFT goes from -4.7 to +4.7
when STFT climbs higher and high the LTFT will increase a bit to say 0.4 So as the LTFT has add more fuel to the required level the STFT does not have to add more fuel so it will reduce to about zero and fuel trims will stablise until the load on the engine changes.

my car has the following fuel trim at idle while running petrol.
fuel trim low (LTFT) 4.7
fuel trim in use (LTFT) 4.7
oxygen feedback trim (STFT) 16.8 then after its maxed out for a while it defaults 5.1 constant.

the long term fuel trim should sit at zero 0, then the short term fuel trim should wobble up and down above and below zero depending on driving conditions, but generally it should average out to zero.

my car is adding as much fuel as possible when the car is sat at idle.

if you disconnect the battery it resets all the fuel trim to zero. if i reset the fuel trim to zero and go for a drive immediately keeping the car in closed loop fuel operation. The long term fuel trim stays at zero and the short term fuel trim fluctuates around the zero point which is as it should be.

So my question is why do the trims max out at idle. this maxing out at idle is a real issue, because once the fuel trims have maxed out they will not reduce when driving the car in closed loop operation. so the car is running very rich when pootling about road town which wastes so much more fuel than i actually need to waste by driving a VR4.

i know this is a lot to get your head round from my description but it is worth knowing about.
all help very welcome.

this video is long but does explain what fuel trims do and how O2 sensors influence the fuel trim it is very good.

https://www.youtube.com/watch?v=oRX2V6_a3dc


can some of you do me a favour and check your fuel trims at idle to see what yours are doing. just as a comparison so i can gauge if my car is really bad or just all VR4 max out the fuel trim for some reason.