except that there is no need to provide the fill sensor 43 description is given using the reference symbols shown in fig2 the fault diagnosis routine of the fuel vapor treatment unit of the second embodiment will be explained following the flow chart of fig8 this routine is also started concurrently with the driver switching on the ignition switch to supply power to the engine control circuit in step 200 engine start judgment is made when based on the detection value of the rotational speed sensor 32 it is judged that the engine has started detonation control proceeds to step 201 where it is judged if a predetermined time has elapsed the function of step 201 corresponds to the start time vicinity detection means the function for permitting fault diagnosis when the engine start is judged corresponds to the fault diagnosis permit device or means after the predetermined time lapse in step 201 engine operating conditions are stabilized so that purging of the canister 21 can be adequately performed by performing fault diagnosis when the residual quantity of fuel vapor inside the canister 21 has been sufficiently reduced over richening due to the flow of fuel vapor to the inside of the intake passage 3 at the time of fault diagnosis does not occur and hence deterioration in driveability and emissions can be prevented when after the predetermined time lapse in step 201 control proceeds to step 202 the various operating conditions detected by the beforementioned sensors are read in in step 203 based on the various operating conditions which have been read in it is judged if predetermined fault diagnosis start conditions such as the below mentioned conditions have been satisfied fuel temperature is equal to.