The presence of trace elements in fuels with high vapor pressure, such as diesel oil, can cause several problems, such as the poisoning of automotive catalysts and environmental pollution; thus strict quality control is required. Despite its high digestion efficiency for some organic samples, microwave-induced combustion (MIC) is still not applied to flammable liquids because unpredictable fast reactions can result in the risk of explosions. In this study, the use of quartz wool as a flame-retardant to control the combustion rate of volatile fuels in closed systems for the further determination of trace elements via inductively coupled plasma mass spectrometry (ICP-MS) is proposed. Diesel oil is used as an example of application in this approach, and Cd, Co, Cr, Cu, Mn, Ni, Pb and V are determined in the digests via ICP-MS. The proposed system allows a safe burn of up to 400 mg of diesel oil using a pressurized oxygen atmosphere (20 bar) without a dangerous pressure increase or damage risk to the microwave system. Nitric acid (2, 4, 7 or 14.4 mol L−1) is evaluated as an absorbing solution, and quantitative results for all the analytes are obtained using the HNO3 concentration of 4 mol L−1. Using the proposed method, a very high digestion efficiency is obtained (>99%) and the accuracy is evaluated using a certified reference material, which presents an agreement higher than 97% with the certified values. In addition, the possibility of using diluted acid solutions allows negligible blank values and consequently low limit of quantification values in the range of 0.001 to 0.20 μg g−1 for all the analytes.