Do you have a scan tool by any chance? If not, then on ebay you can get them for about £15 or you can also get an OBD2 to bluetooth devices, under £10 and then use an app called Torque (free), the bluetooth adapter sends data via bluetooth to your module.
I mention the bluetooth device because it can pull codes but also show live data i.e. you can get it to show you what the ECU is seeing in terms of coolant temp etc. Scantool / bluetooth OBD2 will help you focus in pretty quickly on what it will be. My initial thought would be 02 sensor / MAF sensor / coolant sensor, as they affect (amongst others) how much fuel is added in. If 02 sensor for example is seeing a lean condition, it will be adding extra fuel in that case.
Hope that helps?
Cheers,
Ray
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