An acidic fluid has a pH level below 7.0. In cars, acidic conditions are a warning sign of fluid failure. Engine coolant is formulated to be basic (typically a pH of 8.5 to 10.0) to prevent rust. Over time, heat and engine gases turn the coolant acidic, causing it to attack the solder in copper radiators or pit the aluminum in modern engine blocks.
Engine oil also becomes acidic as it ages. The buildup of combustion blow-by creates carbonic, sulfuric, and nitric acids. If the oil is not changed, these acids will eat away at copper-lead bearings, leading to oil pressure loss and engine knock.
Mechanics use pH test strips or digital meters to check if fluids have turned acidic. If a test shows a low pH, the fluid must be flushed and replaced. This simple check protects expensive cooling and engine components from chemical damage.