Can anyone tell me if I can add a vent only behind the Toilet and use a 3" ABS Sweeping 90 TEE to do so? The actual drain is 9 feet away across the room. See my attached Drawing.
Your question is confusing. The pipe going up the wall behind the toilet is its vent. The other pipe will only be a legal vent if your inspector would approve it. Some would, but many would not.