Abstract: A measurement of the quantity of gas entering or within the coolant of an internal combustion engine cooling system, is made with the engine running, preferably at high idle speed, and a test is performed to ensure that the thermostat is open, whereby it is known that coolant is circulating through the entire cooling system, by providing a radiator cap connected to an orifice of a known size that will allow gas to escape from the cooling system and measuring substantially the pressure differential across the orifice, such as by measuring the pressure on the radiator side of the orifice with respect to atmospheric pressure.

Abstract: The average of two horsepower measurements utilizing an empirically-determined horsepower constant, made with electronic diagnostic equipment employing engine transducers, during three successive acceleration bursts from a low engine speed to a high engine speed with the air/fuel ratio regulator defeated so as to provide full fuel, is compared against the average of two additional such horsepower measurements made in three additional acceleration bursts which are underfueled due to feeding the air/fuel regulator with atmospheric air; the horsepower difference is a measure of the action of the air/fuel regulator; one burst of each set simply purges air accumulations in the fuel lines, if any. If desired, indications of horsepower may be calculated without an inertia constant, since the measurements herein are comparative.

Abstract: A single pressure transducer tapped into the fuel system of an engine, at a point to be responsive to the pressure of the fuel operating against the pressure regulating bypass valve of the fuel system, is used to measure fuel pressure at low idle speed and at high idle speed of the engine; the two pressures when compared against normal pressure can determine bypass valve condition since lower than normal pressure at low idle speed in combination with normal pressure at high idle speed is an indication of a bypass valve which is stuck in the open position, and higher than normal pressure at high idle speed is indication of a bypass valve which is stuck in the closed position. The test can also discern other faults since a lower than normal pressure at high idle speed in combination with a lower than normal pressure at low idle speed is indicative of a bad pump.

Abstract: Electronic processing means are utilized in a composite measurement of performance of an engine cooling system thermostat, with diagnostic procedures to ensure viable measurement. The speed of the engine is initially tested to ensure the engine is at high idle speed, so that temperature in the cooling system will increase to that at which the thermostat will normally operate; and during the testing, the speed is continuously monitored to be sure it remains close to the initial test speed (near high idle) to minimize coolant pump pressure variations due to engine speed. Three temperature diagnostics are performed to ensure a viable test: first, if the temperature ever exceeds 15.degree. over the rated thermostat temperature, the test is terminated in a fault since the thermostat is presumed to be stuck in the closed position; second, if the temperature fails to increase by at least 2.degree.