I think you've got it solved.  Looks HIGHLY probable!

If it were me, I'd have it outside the wall.  It'll basically be invisible anyway (behind the washer), so why seal something in that you might have trouble with in the future?

I had a drain problem with a washing machine that was similar to yours. Old house drain pipe outside of wall would overflow. The last thing I tried worked. I ended up cleaning the vent pipe from the roof down with a snake. It worked fine after that.

That was definitely a concern.  If a drain can't "breath", it can't function.  

I was on the roof yesterday.  You could see straight down thru the vent pipe from the roof down into the house where it had been cut.  It was as good as new, not even a hint of an obstruction.

Here's an update -

The new drain lines for the washer (foreground) and the floor drain (background with p-trap), as seen from beneath the house.



The new standpipe and floor drain, as seen from the washing area.  It's in rough condition - patching, painting, flooring and tray are not yet installed).  The hoses and extension cord go away when the appliances come back inside.

I am happy I have a raised foundation.  I can't imagine how this maintenance would be done if my house was built on a slab with embedded drains.

WOW!  This took me a month.  I am a sluggard!

I still have three pieces of baseboard molding to install around the stove but I did finally complete everything else on this project.  The appliances are back in place and are being used successfully (so far).  The floor drain pan is in and has been tested using buckets of water (this also filled the p-trap).  

The wife and I are both feeling a little "gun shy" after that last flood and the need to completely re-do the flooring we'd just recently completed.  Still, it's a nice floor and we are happy everything is back to normal in this very important room, the kitchen.

Well, well, last night it happened again - the washer leaked!  I noticed some water standing in the new drain pan.  At first, I was freaked out because I just finished repairing the flooring.  Then, I realized this was a double victory.  

Victory #1 - the new drain pan and floor drain worked!  It contained and drained any water that leaked from the washer.  

Victory #2 - we now know what caused the previous flooding - it's the washer (not the drains).

Why the machine periodically spills its guts is still unknown to us but when it does, we are prepared.  During the flooring and drain repairs, the washer was in the back yard and did not leak a drop, not even one, not even once.  We need to repair this one or buy new.


Can someone resurrect the old thread about washing machine recommendations?  I searched but did not find it.  I found it - Speed Queen top loader about to be ordered.