why always got Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)) in SharePoint 2013 timer job

I have simple custom timer job that add item to simple list (only Title column) , when debugging OWSTIMER ,I got this error [ Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)) ] when I was trying to access any thing from SPWeb object I spent approx. 3 days googling this issue but I got no solution ,and I checked the following :
1- check the timer service user have permission (administrator)
2- grant permission to the timer service user to all SharePoint databases (admin, configuration, content)
3- administrator have permission on the list
4- tried to add this job to another web application on the farm
5- impersonate user (failed to retrieve any user)
6- using elevated privilege

any help?