When washing machine drains the water is overflowing from the drain pipe. Have already checked that drain line is not clogged. Vents are clear. Could it be something with the machine?

Last week I went on a service call for a sweet old lady that initially said that her washer was over flowing with water. Naturally I beleived it to be a valve (this was a kenmore 80s series) so I loaded up a few valves incase I had a dud or two. I replaced the valve and let it run and it seemed to do well as far as filling up. As I was loading the tool box into the service truck she comes outside saying it was flooding her floor yet again. After cleaning the mess for her and snakeing out get drain pipe to find a gumbo mud clump I just charged her a few cookies fresh from the oven she had pulled out. I received a return customer and some amazing raisin cookies.

We have put the garden hose in the drain pipe and run full water and it doesn't back up. Only when the washer drains during the cycle. Have had someone come and check lines and vent and they didn't find any clog. Had machine about 3 years and had no problems the drain overflowed one day. Not sure where to go from here.

The drain line has to be clear to its full diameter. Rust, corrosion, etc., or anything that constricts the full 2" drain line opening can cause the drain to overflow. A water hose is only half the diameter of the drain, and therefore is not a valid test. When the pump dumps the entire 1.75" volume of water in the drain, any constriction will cause the line to overflow. Cleaning out the restriction will open up the line and no more problems.