Help is what your after I assume?

You are getting this result since your base catalyst (Caustic Soda / NaOH / Sodium Hydroxide) is neutralising acidic free fatty acids (FFA / ester chains broken from the triglyceride oil molecule) present inthe oil as they are produced when the oil is heated and used for cooking.

The base catalyst takes them out of the reaction. So, the more used the oil is the more you loose, hence the acid base two step process, google for FATTA.

It uses methanol and an acid to dry up water produced when methanol bonds with a FFA to form methyl-ester ie biodiesel, then a second stage uses minimal base to just crack the remaining oil and make biodiesel out of that too. At the end you may get close but never get 100%, close.

I hope you are not confused, but you need to read up a bit in the chemistry of the process.

Matt

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Biodiesel Bandit

Landcruiser 98 B100
Peugeot 306 stdt B100
Peugeot 2004 2L 307 HDI Does not like B100