In the event that the fuel system is K-Lambda, the "ECU" is the Lambda Control Unit, which delivers a varying frequency pulse to the Frequency Valve. That valve controls flow of return fuel in response to exhaust oxygen content, and hence is a mixture control. Operating without the "ECU" will put the system in an open loop condition, and the mixture will be on the rich side of stochiometric.

I gather that you are outside the US; the US part# is 0 280 800 013.