This is true of all modern cars if you change the mixture to eek out more power, the ecu will trim the fuel back until the lambda sensors are happy. The only way to stop this from happening is to fit a lambda emulator which tells the ecu the mixture is right at all revs/throttle positions. This will impact on your cruising mpg though unless very carefully mapped and might give you MOT test problems (unless you set it up on a switch).
My current car runs on lpg and I have the same problem. I want it to run leaner on lpg, but the petrol ecu keeps trimming the fuel to richen it up if I tell the lpg ecu to take fuel out. All I can do is get the lpg fuelling as bang on as possible and make do with it, otherwise it'll just trim out the fuel which impacts the petrol mixture when I switch over.
Short term trims are wiped when you turn off the ignition and ignored at full throttle but long term trim stay unless you reset the ecu and do impact full throttle mixtures. This is designed like this so as the car ages and injectors become less then 100%, or you get a slight air leak... the ECU adapts the trims to keep the fuelling/emissions within spec.
The only way the ECU knows what direction to trim the fuel to is by reading the lambda sensors in the exhaust, so if you remove or emulate the signal then your remap data should remain intact.. removing the lambda will throw a fault code though and put a service light on your dash. As far as I understand, any timing changes made during the remap will remain, this will only be dialled out if there's a knock sensor in place which detects knock - which will retard the ignition to protect the engine.
Rich
