Abstract: The existence of an unacceptable vapor leak can be determined by monitoring the pressure within a fuel system of an internal combustion engine. Once the engine is switched off, a period is timed to allow pressure variations due to movement of the fuel to subside to an acceptable level. The fuel system is closed and a first rate of pressure increase is determined by measuring a first time taken for the pressure to rise by a predetermined amount. A leak of a predetermined size is then introduced and a second rate of pressure increase is determined by measuring a second time taken for the pressure to rise by a predetermined amount. The first and second times are then compared to detect whether or not an unacceptable leak exists.

Abstract: Engine speed variations following combustion events within the cylinders of a multi-cylinder internal combustion engine are compared with a threshold during a predetermined number of combustion events. A distribution of speed variations exceeding the threshold with respect to the cylinders is formed. The distribution is analyzed to determine whether misfire is occurring, as indicated by speed variation following combustion occurring in half or less than half of the cylinders, or whether the vehicle is driving over a rough surface, as indicated by speed variation following combustion occurring in more than half of the cylinders.