You may have an ELM327 interface with the wrong chipset wich can not be set to a higher speed than 38400. The cheap Chinese ELM327 interfaces can be equipped with a few different chipsets. In the past I bought and modified about 20 ELM327 interfaces and found 3 different types of chipsets which also uses different drivers.
These different chipsets are from the following manufacturers:
* Silicon Labs
My experience is that only the ELM327 with FTDI chipset is capable to work at a speed of 500000. For the other chipsets it is not possible. Trying to change the speed in ELMconfig results in an error.
The type of chipset which is used for the ELM327 interface can be determined by the drivers that are used.