Engine efficiency is often controlled in an indirect way through combustion timing control. This requires a priori knowledge of where to phase the combustion for different operating points and conditions. With cylinder individual efficiency estimation, control strategies aiming directly at fuel consumption optimization can be developed. It has previously been shown that indicated efficiency can be estimated using the cylinder pressure trace. This paper presents a method to use the estimated efficiency as a feedback variable in an extremum seeking control strategy for online steady state fuel consumption optimization. The experimental results show that the controller manages to find the maximum brake torque region at the given operating point both with and without external excitation.