Question 23 UFIV02 - Assistant Engineer - UFIV

The Correct Answer is D **Explanation for Option D (Correct):** The sintered metal supply and return filters (often called "pencil filters" or "basket filters") are extremely small, fine filters located directly within the body of the unit injector, either at the fuel inlet port or inside the injector assembly itself. Their purpose is to provide final, critical protection against contamination entering the precision components (like the needle valve and plunger/barrel) just before injection. Because these filters are housed internally within the injector unit, they are not designed for routine, in-situ replacement while the engine is running or during simple maintenance checks. They are considered integral parts of the injector assembly and are accessed, inspected, and replaced only when the unit injector is completely removed from the cylinder head and disassembled (reconditioned) in a specialized clean workshop environment. **Explanation for Incorrect Options:** * **A) Whenever the fuel system primary suction strainer is cleaned.** The primary strainer is a coarse filter located early in the fuel line (often before the transfer pump) to protect the pumps. Its maintenance schedule is frequent and dictated by fuel flow, independent of the critical, internal protection provided by the unit injector filters. * **B) Whenever the fuel system secondary filter elements are replaced.** Secondary filters (e.g., spin-on or cartridge filters) are fine filters designed to protect the high-pressure fuel equipment. While their replacement is a regular maintenance task (often every 250–500 running hours), accessing and replacing the internal unit injector filters requires much more invasive work than simple external filter element changes. * **C) Whenever the coalescing fuel filters are drained of sludge and moisture.** Draining coalescing filters is a very frequent task, often performed daily or weekly, based on visual inspection or alarm levels. This routine action addresses bulk water and heavy contaminants and is far too frequent for the replacement of components that require complete engine shutdown and removal of highly specialized precision equipment (the injectors).