Gasoline of low existent gum determined by ASTM Method D 381 can give objectionable induction-system deposits in service. As a result of leaching by liquid gasoline, the deposits that accumulate in ordinary service contain relatively little naphtha-soluble material. In contrast, naphtha-soluble material is a variable and often a large part of the deposits obtained in laboratory tests. The Union Oil induction system deposit (ISD) test, in which the naphtha-soluble gum is extracted and rejected, is described. It is shown to give reproducible results that follow regular trends in gum development not detectable by ASTM Method D 381. ISD gums are found to correlate with engine test results provided a correction is applied for the effect of gasoline volatility, based on the 90 per cent recovered temperature as determined by ASTM Method D 86.
The influence of several variables on gum formation in the ISD test is discussed. Inhibitors may either increase or decrease gum deposits. Oxygen, oxidants (ozone or nitrogen oxides), smog, higher manifold temperature, and longer manifold residence time all increase the ISD gum. Oily additives decrease it. Air-fuel ratio, road dust, or exhaust gas in the air, and several common gasoline additives showed no significant effect.