Just installed sink, shower, and toilet in basement. Have grinder pump to lift waste to septic. Toilet overflows when I connect the p-trap to the sink. When connected and I fill sink and drain the sink the toilet water is moving. When the p-trap is disconnected toilet works normal. Drain for the sink is 20" above finished floor, bottom of p-trap is about 15" above floor. Top of vent pipe is about 6.5' above the drain pipe. What is my problem and how do I fix it?

Thank You

“Toilet overflows when I connect the p-trap to the sink.”

This sentence means your venting is not laid out properly

You should have 3 vents

1 ½ vent is needed for the sink
2” vent for the shower
2” vent for the toilet

All theses vents goes above the fixtures and ties into a main vent that needs to go through the roof and or outside

Normally you should have a Tee fitting for the sink and that’s vented above that tee. The shower has a 2” vent that goes above the ceiling. The toilet 2” vent needs to be within 3’ of the toilet to vent properly this all goes above the ceiling and either or tied into a main vent through roof or run separately out to the outside. As long as your grinder pump is set right and working properly and no clogs to that pump when venting is fixed all should be fine.

21boat is right that you have a venting issue, but not all bathrooms are required to have 3 vents as he presents (many states have different venting requirements)...some bathroom groups are WET VENTED and therefore only have ONE vent (usually a 2" vent from the sink). Here, you say you have the sink vent at 6.5 feet above the drain pipe...is this a mechanical vent (an AAV)? If this is an AAV, remove it and with the trap attached to the sink, flush the toilet and see if things are fixed. If removing the AAV solves the issue, purchase a new AAV and install...should resolve this for you.

Otherwise, tell us how the venting has been set up for this bathroom group and also let us know if there is a separate vent for the grinder pump and if that vent is an AAV or if it is a vent that is piped up and through the roof of the house, OK?

Back to you...

Mark

Final note: Toilet vents can connect to within 3-10 feet of the toilet drain depending on size of drain pipe and what state you are working in.

Here is a typical bathroom rough in in my area. Good luck, Tom