Not a plumber --- Request comment from professionals.
No vent was installed when house was built anywhere near the 2 in standpipe trap which is under the floor in a VERY low craw space.  The closest existing vent thru the roof is 1-1/4 in 20 feet away on a kitchen branch line.  Installing any new vent will require extensive horizontal under floor run to exit into garage and up to ceiling with a AAV or continue thru the roof. I do not want to install exposed vent pipe up through floor into utility room due to direction of drain from trap and - wall - window - dryer - refer etc locations.

Some newer Wash Machines have a Higher Pump Discharge overpowering the drain.
If after verifying drain pipe AND vent are clean and not blocked ----

1. Adapter, Standpipe - 36878 -- Rubber adapter will slide down into a standard size 2 in OR smaller standpipe which will block overflow after hose assembly connection is inserted. These adapters are made for and and sold by washing machine manufacturers.

2. Siphon Break -- 285320 -- Each PVC end is 1-1/16 OD for 1 in ID insert hose connections.  Install one end to wash machine discharge hose (With clamp and adapter if needed) with the vent end in the vertical position.  Connect other end from Siphon Break with 1 in ID hose and extend down into rubber adapter on standpipe.  Also install a PVC 90 degree elbow (Having "Insert ends" between Siphon Break and standpipe.  Discharge pressure will seal off the Siphon Break vent.  When discharge is complete the vent will allow the standpipe to empty out down to the trap.  I would NOT use a Adapter to block discharge overflow without also using a Siphon Break.

chances are it is not the lack of a vent but rather a partial stoppage.

I would pay a plumber for his/her time to evaluate and advise you on steps to correct all this.

The drain and vent pipe has been cleaned with both a snake and a expandable bladder from direction of the standpipe, from under house, a outside clean out and the vent.  A section under house even also had the bladder shoved down it while un-expanded the entire length of house while flushing on and off.  That straight section was in a sweep tee with end clean out that could be looked down with a light.  There is a section of the branch line of about 6 feet or so that is STILL CAST IRON which allowed cleaning with the snake etc.  That is the only horizontal cast iron remaining. A partial block there is a possibility.   All was previously replaced with PVC. 

 Packy, thanks for your suggestion.  I have wondered about that and would like to replace it.  It will be some time before I can do it as my health is not up to a lot of crawling around now and I am not having any wash machine discharge overflow problem after making the adapter - Siphon Break modification. That's why I have not jumped on adding a vent. 

Not being a experienced plumber I have no idea if such a remote vent that I have could be the real or only problem. 



Edited 1 times.

Packy you were right -- Although installing a rubber adapter and siphon break on my washing machine discharge standpipe DID stop all overflows, I finally got around to replacing the last few feet of cast iron 2 inch drain pipe with PVC anyway.  One of the cast iron fittings, a long radius - sweep tee was almost completely blocked.  After replacement of all the cast iron except for some vertical in wall vent connections, the washing machine discharge fully discharges as it should when tested without the previous listed adapters. Thanks again Packy.