Experimental evaluation of cyclic variability or combustion instabilities of waste cooking oil biodiesel–diesel blends powered compression ignition engine is presented in this article. An advanced in-vehicle combustion analyzer armed with a piezoelectric pressure sensor was used for accurate measurements eradicating the experimental uncertainty. Cyclic variation in the combustion was investigated using the statistical and wavelet analysis method. Results of statistical methods and wavelet analysis were agreeing with each other toward self-validation. Statistical methods were used to calculate the mean and coefficient of variations, while the wavelet method has the potential to analyze the cyclic variation topography together with the intensity of variations in the engine combustion cycle, especially at low engine load conditions. Overall combustion analysis including wavelet analysis and statistical method indicates a more silent and smoother engine operation with biodiesel blending as it enhances combustion stability in unmodified diesel engines in comparison with conventional diesel fuel.