It's not actually reverting to the old map, it's just the fuelling is being trimmed out of the new map.
The fuel trims work by pretty much reading what the lambda is saying (i.e. running rich or lean by measuring the amount of Oxygen in the exhaust gasses) and then tweaking the fuel offset a bit to compensate in the attempt to get 'perfect mixture' of 14:7/1 air to fuel ratio (stoichiometric rate).
If your car is remapped to run a bit richer through the mid-range and high end of the rev range then the ECU will see that as running too rich and will take out 'x'% of the injector timing until the lambdas are happy.
So your new fancy remap that adds in fuel to provide better torque in the midrange will effectively be dialed out by the ECU over time.
You can do a few things to stop this from happening:
- Disconnect your lambda sensor. This will throw up a light on your dash and your car will proably fail MOTs unless plugged back in.
- Fit a lambda emulator (sold for the LPG market) which pretend they are lambdas but fake the rich/lean signals i.e. they always sent a 'perfect' signal. You need to make sure your map is bang on if using one of these as your car will not know if it's running dangerously lean.
- Get a device that resets your fuel trims to +/- 0%. This will only last a few days until the ECU has fully corrected the fuelling again.
Some tuners may be able to tweak the ECU code to ignore the trims though... worth asking them before you get a remap!
Rich