Dear all,
I have a strange experience that there are some sample of diesel engine fuel have a high TAN a bout 10 mgKOH/gram, what cause is it?

Assuming a high concentration of lubricity additive (carboxylic acids) or other acidic contaminant/additive did not get into fuel, where did you get the fuel sample from?
In diesel engines with recirculation or after low storage above ground - fuels with minimal antioxidants (sulfur, phenols, amines) such as those in Indonesia could oxidize to form hydroperoxides/acids in fuel/storage tank.
Any evidence of gums on valves/tight clearances or O-ring failures?

Thank you Mr.BOB k..i'm hydro-yayan..i'm forget my username, then i'm using the new user..BTW i'm got the sample from after low storage above ground, maybe you are right, the fuels with less antioxidants oxidize to form hydroperoxides/acid in fuel storage..and about the gums on valves, by investigate there's not much.