Here's the first try over the weekend, Joe Homeowner...
I checked all drains in the house and all drain normally except washer.
First I tried Drano twice in the washer drain and still too slow. Then connected a Y hose fitting to the drain pipe and ran both hot and cold at full pressure for 20 minutes. Max total flow would be less than the
15 gal/min of the washer pump. Normally cold alon is 6 gal/min and hot is 4 gal/min, so I'm guessing the total was 8-9 gal/min. I did not hear it backing up in the stand pipe.
Then snaked out from 1-1/2-inch cleanout plug from outside near the kitchen sink and it was clear for the 25-ft length of snake I have. Then I ran the hose directed into the cleanout and the most it would handle without backing up was 3-4 gal per minute.
While I was snaking the kitchen drain out, I could tell the drain took a sharp turn around 12 feet in, which is about where I would expect the main line to be. No resistance in the pipe in that 25-ft.
Then I pour a gallon of lye-based drain cleaner in the washer drain and let it sit overnight per instructions. This morning I flush it with hot water for 5-10 minutes per instructions. Then connected hot and cold back up to drain and ran another 20 minutes.
Then I snaked out the washer drain and cannot get past the bend just below the floor. When in and out with it and pulled out a very small amount of lint each time. Kept doing this until there was no more link coming out -- total was not enough to slow the flow. But never did get past that bent.
Then poured a 50/50 mix of lliquid soap and water in the drain and set for 1/2 hour, then flushed out with hot water again.
Still no difference. My bet now is that the blockage is in the main line which is too much of a job for me.
If I had the time and $$, I would seal up the drain standpipe, and run a new drain pipe out the wall to an outide sink plumbed back into the drain.
Any other ideas before I throw in the towel and call a plumber?