Hello Readers!
I will apologize for all words and explanations now...Sorry.

I did let people with supposedly more experience & know-how talk me into using 1 1/2" PVC instead of 2" PVC for a drain line from the clothing washer to a septic vault(gets pumped every 3 yrs. by the Village)...my mistake.

So, let's assume I get 2" PVC run to my junction from the kitchen for purposes of this question, which will end up a lot more questions...each building on the other.

Let's start with what I have that functions fine ---

1) Washer dumps out a hole in the wall on the ground...and then runs under an enclosed porch causing unseen problems which I will not worry about addressing for now. I want to fix the washer draining improperly.

2) Kitchen sink drains out a 1 1/4" PVC (black...old-school code color) which is a branch drain line from the main trunk drain(?- is that what it is called) that drains into the septic vault.

3) the tub & toilet drain into a large (3 or 4") PVC into the septic, with a 'Y' for the branch for the kitchen drain line.

All this works fine and is vented and has the traps that are fine.......back to my washer draining on the ground...

So, I let others talk me into using 1 1/2" pipe instead of 2" pipe. I used a 'Y' into the kitchen branch. The problem is that it did not work - When I let the washer exhaust water, pressure built up and the water reversed course. I sloped it plenty to promote the draining process. While making sure it would not work, the kitchen sink clogged! My guess is that the pressure that built forced the gunk into the 1 1/4" drain for the kitchen sink. The clog was easily removed.

Now the kitchen sink is fine and I have a 'Y' with a 1 1/2" PVC pipe in the ground (easily removed) that drains nothing and the bathroom fixtures operate just fine.
Will using 2" PVC for my washer instead of the 1 1/2" PVC fix the pressure problem? Should I get something bigger? Should I not be adding a branch for the washer to the branch for the kitchen sink? Should I have spent the $1200 for a plumbing company to do it?(I have spent 30 hours and $50, which is a LOT less than $1200 which I cannot get) Are any other plumbing issues recognizable in this dissertation?

Sorry for all the words again.........