This International Standard specifies a method for the determination of the existent gum content of aviation fuels, and the gum content of motor gasolines or other volatile distillates in their finished form, and at the time of test. For non-aviation fuels, a procedure for the determination of the heptane-insoluble portion of the residue is also described. CAUTION — This method is not intended for the testing of gasoline components, particularly those with a high percentage of low-boiling unsaturated compounds, as they may cause explosions during evaporation. NOTE 1 The true significance of this test method for determining gum in motor gasoline is not firmly established. It has been proved that high gum content can cause induction-system deposits and sticking of intake valves, and in most instances it can be assumed that low gum content will ensure absence of induction-system difficulties. The user should, however, realize that the test is not of itself correlative to induction-system deposits. The primary purpose of the test, as applied to motor gasoline, is the measurement of the oxidation products formed in the sample prior to or during the comparatively mild conditions of the test procedure. Since many motor gasolines are purposely blended with non-volatile oils or additives, the heptane extraction step is necessary to remove these from the evaporation residue so that the deleterious material, gum, may be determined. With respect to aviation turbine fuels, large quantities of gum are indicative of contamination of fuel by higher boiling oils or particulate matter and generally reflect poor handling practices in distribution downstream of the refinery.