Abstract: Monitoring equipment comprising a part which defines a cavity, and input device to input fluid under pressure into the cavity. Mounting devices are provided on the part to enable an engine fuel injector valve or other fluid release device to be connected to the part and to release fluid from the cavity intermittently when the system is in use. Controls are connected to issue a triggering signal to the fluid release device to cause the latter to release fluid from the cavity. A pressure sensor is coupled to the cavity to provide a measure of the pressure within the cavity. The equipment further comprises pressure signal modifying device connected to receive signals from the pressure sensor and the controls and constructed to provide an output signal which is a measure of the pressure of fluid within the cavity at the time the controls issues such a triggering signal, while the effect of signals from the pressure sensor received by the modifying device on the output signal therefrom at other times is obviated.