I just connected my new sump pump (1 1/2” line) to my vertical stack (2”). This is connected to the stack with a tee approximately 6-8” below where my washing machine tee. When my sump pump turned on, it back flowed out of my washing machine drain. What can I do to keep the water from coming out of my washing machine drain?

Any help would be greatly appreciated!

Are you saying the sump pump discharge rose up the washer stand pipe and overflowed? And the washer's still draining? That doesn't make sense.
What am I missing here? Regards, tom

Didn't we talk about not hooking up the sump pump to the sanitary sewer a day or two ago.

Its is always a bad idea.
Now that you have done it anyway, from what you describe I would guess the sewer is partially blocked and can not take the flow from the sump.

After reading your post again I see you are pumping into a 2 inch vertical stack. The capacity may not be there to handle the discharge from the sump.
Can you reduce the dicharge hose from the sump to a 1 inch? This should cut down the flow and maybe the 2 inch vertical pipe can handle it.
Another options would be to Pump to a laundry tub then let gravity take its course.

Thanks for your responses Ken and Tom. First and foremost, I can't run it outside as I live in a VERY cold climate and I don't need ice to form at the end of the pipe when I'm trying to get the water out of my house.

Tom, you are correct, the water ran UP the stack then BACK and out through the washer drain and stand pipe (the washer is not currently hooked up). I talked to a local plumber who said I should have run the sump drain ABOVE the washer drain (for a few different reasons) but that the 2” line should be able to handle whatever the sump pump pushes through the 1 ½” line (with the same conclusion,…there must be some blockage). SO,…I ran a garden hose as far down the stack as I could (I would guess it went about 3 foot down then about 12 inches across/under the floor before it stopped) and turned it on high (to dislodge anything that could be blocking the line). The drain took the flow fine. I then ran hot water directly from my hot water valve for my washer into the stack and about emptied the water heater (to try to break up any sludge in the line). I capped off the “Y” where the sump went in and ran a new line with a p-trap to the vertical stack ABOVE the washer drain line (its approx. 3 foot above where the washer discharge meets the stack). I will finish these connections tonight as I also had to move the hot and cold lines to fit the drain line in next to them. If it backs up again I’ll have to get a camera down the stack to see what could be blocking it,…but I’m hoping the hose dislodged any blockage so that the sump can flow freely now. Keep your fingers crossed!! Thanks for your help!

Ok, I did everything I mentioned in my previous post and connected the sump pump drain to the 2” stack about 2 feet above where the washer drain connects with it. Unfortunately, the water came up through the washer drain again. One thing I noticed was that it was splashing quite a bit (not just a rush of water) and it got me thinking that it could be the rush of air from the sump water discharge that is forcing the water in the washer p-trap up toward the drain opening. Is this possible? If so, what can I do? I thought about connecting the sump to the washer stand pipe with a Y,…would that fix it?

Water, like electricity, takes the path of least resistance. The reason it's backing up into the washer stand pipe is because the pipe it's supposed to drain into is partially blocked or the sump's so powerful it's raising up the standpipe before it can drain out the pipe.. I'm surprised your plumber didn't tell you this. One other thing I bet he didn't tell you. By connecting the sump to the sewer, (and by the way , this is against code in most areas) without first trapping it you have opened up your home for sewer gas to enter. Sewer gas is dangerous to your families health and the methane content makes it explosive.
Repipe the sump discharge to the outside or run it to a indirect waste in the floor drain and your problem's solved. Good luck, Tom