We detect and measure diffusion along resonances in a quasi-integrable symplectic map for different values of the perturbation parameter. As in the Hamiltonian case, results agree with the prediction of Nekhoroshev theorem. Moreover, for values of the perturbation parameter slightly below the critical value of the transition between Nekhoroshev and Chirikov regime we have also found a spread of some test orbits along macroscopic regions of the action space.