The testing of diesel engines are normally done on no load. But the bus are started at full load. Why??

Answer 1

Ya know, I really don't understand your question/remark.
If you are referring to the "Snap-Test" that DOT performs on the trucks, this is only to determine whether or not the vehicle has been modified or has been properly serviced and maintained.
Actual testing of the engines are conducted under load and measured very precisely according to FTP (Federal Test Procedures) Protocol, as are the engines for the buses.
Most localities are exempt from the roadside DOT testing, but must be conducted at least every 2 years and results reported to the governing authority.

Hope this clears things up.