Detects Combustion Leaks on Diesel Engines. Use the diesel leak detector fluid with the Lisle 75500 Combustion Leak Detector. Indicating fluid changes from red to yellow when exposed to diesel combustion gases in the radiator. IMPORTANT: You must also order a Lisle 75700 Hand Vacuum Pump Adapter too *Fluid can be re-used as long as it hasn't been contaminated fluid needs to set out in the air to change back to original color