I have replumbed my washing machine drain line to the drain line. When the washing machine goes to the rinse cycle water comes up the stack. All drain lines are an Inch and a half with a trap installed. I am suspecting that the T fitting in the drain line is causing the problem and should be replaced with a Y-Tee.

Does anyone have any suggestions to help solve my problem

Hey BUD... tell me how tall is the standpipe from your ptrap? The stand pipe is the pipe your washing machine hose connects into. That pipe, by code, needs to be between 18 and 36 inches... believe it or not. AND, if you have a maytag or another strong washing machine... you may even need to bump up to 2" drain pipe (still with 18-36 standpipe.. ok?). It is not a difference between tee fitting and wye fitting... for sure.

Online, most here will tell you to increase the drain size to 2" and to have standpipe at least 30 inches tall. I go shorter most of the time.

Anyway, how tall is your standpipe? Is this drain trapped/vented correctly? Get back to me.

Got to spread my complements around before I can give MASS another one but he is right on the money.