At flex fuel technology introduction in the Brazilian market, new technical issues came-up due to ethanol chemical reaction with component materials developed only for gasoline aging. One of these items was the gel formation at fuel system. This gel flows through the fuel systems and clogs the fuel filters. And, as final result, the fuel pump is lost due to overpressure working condition.To implement solutions against this way of failure, it was necessary to understand gel formation mechanism in laboratory level. In this investigation, many boundary conditions were changed and analyzed in order to identify contribution of each factor during gel formation process.As result, it was possible to implement a laboratory gel formation test for fuel filters, not depending any more from field tests and allowing test repeatability to develop an improved fuel filter.