05/29/2026
The Helpline conducts a test called Thermal Stability (ASTM D6468), which exposes fuel to heat to evaluate its resistance to thermal degradation. The fuel sample is filtered and put in a test tube, then heated to 300F for 3 hours. The fuel is cooled and filtered again. The amount of light that can pass through the filter pad is measured, resulting in a “reflectance rating.” A reflectance rating of 70% or higher indicates clean, stable fuel. Lower ratings suggest the fuel has degraded, forming deposits that can clog filters and fuel injectors.
The difference in these samples is dramatic: the fuel on the left measured just 6% reflectance—the worst result the Helpline has ever seen—indicating severely degraded fuel unlikely to perform well in an engine. The sample on the right measured 96%, showing excellent fuel stability.
