iPhone 3G, 3.1.2, it is jailbreak/unlocked.

When I am in range of my WiFi the battery drains in a very normal way, as if should; so battery is 100% good.

Immediately I go outside (no WiFi) and the WiFi icon change to 3G (or E with 3G turned off, it doesn't matter) the battery start to drain 1% per minute, sometimes faster. Doing nothing at all.

If I enter again WiFi range, the battery stop the drain and everything is normal again.

So, what thing the jailbreak installed for this to happen? so I can remove it or deactivate it.