B3c mechanic in a bottle petrol test swabs pack of 5mechanic in a bottle petrol test swabs provide a quick and easy means to test your petrol, taking just a few minutes they provide a visual indication as to the condition, be it fresh, marginal, or bad.the mechanic in a bottle petrol test swabs are part of the multi award winning family of fuel solution products from b3c'. '.strtoupper('t')his product range is scientifically engineered to provide a quick, easy, and effective means to test fix prevent the problems associated with modern day fuels. application the mechanic in a bottle petrol test swab provides a quick and easy means to test your petrol, taking just a few minutes they provide a visual indication as to the condition, be it fresh, marginal, or bad. currently in the uk unleaded fuel can contain up to 15% ethanol'. '.strtoupper('e')thanol is hydroscopic, meaning that it attracts atmospheric moisture'. '.strtoupper('i')f ethanol and water are mixed together it becomes corrosive to rubber and metal components. it has been found that one of the side effects of ethanol fuel is that it can cause degradation of fuel filters, resulting in small particles of the filter being carried into the carburettor and causing running issues and the constant need to adjust the carburettor. this simple test allows you to check if your fuel is still fresh low moisture content from absorption of atmospheric moisture.directions step 1 submerge swab into the fuel for 5 seconds'. '.strtoupper('