A Core deposit is a form of deposit you pay until returning your old part. If you don't have the core at the time of purchase, you must pay the core charge. That charge is refunded to you when you return the core.
Microbes in the form of bacteria and fungus are present in all diesel fuels. Long periods of fuel storage can create ideal opportunities for microbes to grow in fuel tanks. The first indication of microbial contamination is mucous-like accumulations on fuel-filters and increased requirements for fuel-filter replacement. Microbes can only be removed from the fuel system by use of a diesel fuel biocide.