We recently bought an older house (1950's or so) and there is a problem with the basement shower/sink & toilet. They are all side by side in the basement and the shower looks unused. Initially I noticed a strong sewer smell in the basement. Didn't know where it came from. But when I flushed the toilet or when I tried the sink faucet, water started coming up the shower drain. It doesn't make a noise or anything (so may not be a venting ptoblem). COuld it be plugged or maybe a bad DIY plumbing job?

Thank you

The initial sewer gas odor was noticed because the appliances had been unused for some time and the water simply evaporated from the shower and/or the sink trap allowing the smell to enter the basement. Flushing the toilet and running the sink faucets effectively filled the traps and eliminated the odor.
Your system is probably vented OK if there is no movement of the water in the basement toilet bowl when you flush an upstairs toilet.
Sounds to me like there is a stoppage in the basement sewer line possibly where it joins the main line. If there is no cleanout access in the basement, you may need to remove the toilet and run a snake into the line from there which should solve your problem.

I have cleanout access in the basement for the main sewer line, the main floor bathroom and the kitchen sink. Can I use any of these? Also there is a sewer bakup valve right next to the basement bathrooms/shower/sink. Can I use that to clean the line instead of removing the toilet?
Thanks

I'm not sure what a sewer bakup valve is-is it something like a backflow preventer? At any rate, if you can flush upstairs toilets and not cause basement shower backups, your problem isn't in the main sewer line and is isolated to the basement branch anywhere from where it ties into the mainline upstream to the basement.
Describe please the "bakup valve" so I can maybe figure out what it is.