Washing machine waste confusion

[Dark Eyes]

When my washing machine pumps out, after about 20 seconds, water starts overflowing out of the top of the downpipe behind the cupboard. I have removed that waste pipe, the ubend and the 90' connector to the off white connection to the down pipe from the sink waste pipework, and there is no sludge or obvious blockage. The sink drains freely, so I uncoupled the ubend, rotated the 90' bend upwards, and put the washing machine's waste hose into it, and ran the washing machine.; as long as I held the waste hose hard into the 90' bend it didn't overflow, if it wasn't pushed hard into the 90' bend, it overflowed.
All I can assume from this, is that the waste hose is sitting in the downpipe, and when it pumps out, the water is hitting the ubend and it can't get around it and through the 90' bend quick enough, and is back-filling up the downpipe - does this seem likely, or is there another issue I have missed?

 

[rpm]

Need another photo, is there another U bend just out of sight at the bottom of that photo?

 

[townfanjon]

Need another photo, is there another U bend just out of sight at the bottom of that photo?

I think its another trap , hard to tell

 

[Ric2013]

What I see is the sink waste trap combined with washing machine and another appliance trap (behind that wood) teeing into the pipe running down from the sink.
The blockage/obstruction must therefore be downstream of the tee where the two flows meet.

 

[rpm]

I think its another trap , hard to tell

That is what I`m thinking but I want to see the outlet even if it disappears through a wall.
(seems my posts are being held back again, what have I done now?)

 

[Darren Jackson]

Try swapping the outlet hose with the dishwasher one. In other words put the dishwasher hose into the hockey stick and the washing machine hose onto the P trap spigot from the sink. And see if that works.

 

[Dark Eyes]

Need another photo, is there another U bend just out of sight at the bottom of that photo?

Yes there is a U bend just below the bottom of the cupboard that joins onto the 90' elbow that goes onto the off white pipe that joins the down pipe from the sink waste.
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That is what I`m thinking but I want to see the outlet even if it disappears through a wall.
(seems my posts are being held back again, what have I done now?)

Unfortunately, the off white downpipe on the left, goes under the kitchen floor, where I suspect it goes into a 90' elbow that leads into a pipe run that goes to what was the outside wall, and is now an extension. It could be that it there is a constriction in that pipe or just not enough of an angle on it; however, I am confused as to why it seems to be okay when I connected the washing machine outlet to the sink waste, as it ends up down the same pipe.
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Try swapping the outlet hose with the dishwasher one. In other words put the dishwasher hose into the hockey stick and the washing machine hose onto the P trap spigot from the sink. And see if that works.

A good suggestion, I will give it a go next weekend - making sure that the jubilee clip is on tightly!