Sewage ejector air coming up shower drain

[Qwertyjjj]

I'm getting sewer smells coming up the shower drain from the ejector pump during the night. I can only think that gas is building up but not sure.
The ejector pit has a vent to outside but is it possible the gas is rising up the main waste pipe instead and getting caught in there? You'd think pressure would not build up with the ejector pit vent.
The shower drain is not vented by the way (installed many years back probably a bad plumber). This is all on a septic system.

 

[Sylvan]

The ejector could be causing back pressure when discharging causing trap seal loss and this causes sewer smells

 

[Sponsor]

 

[Smooky]

I'm getting sewer smells coming up the shower drain.........
The shower drain is not vented............

Does the shower have a trap? If the shower is not vented the trap may be getting sucked dry when water is being pumped out of the basin. I would check the vent on the basin and make sure it is not clogged up or has not dropped below the water level in the basin etc. Make sure the float on the pump is working correctly. What water goes into the basin, is it from the shower where you notice the odor? Where is the water pumped up too ?

 

[Qwertyjjj]

The ejector could be causing back pressure when discharging causing trap seal loss and this causes sewer smells

It's smells overnight thought when the pump isn't running.
However, the above might occur during the day. How can I ease that? Do I need to vent the shower somehow? There are no other vents on that level of the house. The sink and toilet connect directly to the septic tank by gravity and the sink has a cheater vent.
The shower connects to the ejector pump, separate waste pipe.

 

[Qwertyjjj]

Does the shower have a trap? If the shower is not vented the trap may be getting sucked dry when water is being pumped out of the basin. I would check the vent on the basin and make sure it is not clogged up or has not dropped below the water level in the basin etc. Make sure the float on the pump is working correctly. What water goes into the basin, is it from the shower where you notice the odor? Where is the water pumped up too ?

The shower is on level 2 but rather than go directly to the septic tank by gravity, the plumber connected it to the sewage pit in the basement. Water from there is also connected to kitchen and basement bathroom. However, only the shower on level 2 goes into the pit.
The pump in pit pumps it up 8ft to the septic tank outside. Only the shower drain smells...Daytime but more at night.