s 710 a coolant temperature signal may be obtained by ecu 100 from the cooling system thermostat not shown if the above said conditions are not met a determination is made whether engine revolution is less than 470 rpm and the engine idle switch is turned on s 720 successively if conditions of the previous step are not satisfied a determination is made whether a difference between a desired and a measured output value of the throttle position sensor 400 is greater than 0 5 v and whether a coolant temperature is greater than 70 degrees centigrade s 730 then if conditions of the previous step are not satisfied it is determined whether the throttle valve has a problem relating to its complete closure s 740 as shown in fig2 if any one of the conditions in steps s 710 through s 740 is met the ecu forcibly operates the limp home valve 600 to take in a sufficient amount of the air necessary for running the engine thereby preventing the engine from stopping s 750 next while the limp home valve 600 is forcibly operated to prevent the engine from stopping it is determined whether the idle switch is turned off whether a difference between a desired and a measured output.