"Explaining" is a good choice of verbs there. (You could also say "saying", which is the direct replacement of "telling" in this case.)"After the incident, Tim excused himself explaining that he needed to went to the bathroom."
Is the sentence grammatically correct?
I wonder whether the following is true in this case.but there should be a comma after "himself".
but in many uses, commas are a matter of individual preference.
No. The comma would still be necessary if you re-phrased the statement: After the incident, Tim said "Excuse me", explaining that he needed to [do something].Is this because 'excuse oneself' is an idiom?
I'm waiting impatiently.We're still waiting patiently for Nikitus to twig what's wrong with '... he needed to went to the bathroom'.
We do not broadcast our need to use the toilet.