There are a few ways to dissolve engine sludge, but the most common and effective method is to use a solvent. Solvents can be found at most auto parts stores and are usually applied with a spray can. Simply spraying the solvent onto the affected area and allowing it to sit for a few minutes will typically dissolve the sludge. If the sludge is particularly stubborn, you may need to scrub it with a brush before rinsing it away with water.