Should the caps on the sewage drainage pipes outside be airtight? We've had a little back-up (vent related, it looks like) and I loosened the caps in case water couldn't drain fast enough, so overflow would go into the yard instead of the shower stall. I'm thinking we have a vent issue, but until we can get them cleaned next week, I'd like to leave the caps loose when doing laundry. Just wondering if loose caps contribute to drainage problems or if it matters.
The risk is that you have sewer gas escaping at ground level and this could easily enter your house. This gas is potentially flammable or explosive.
Venting issues are not going to cause a backup. Slow draining or gurgling are common symptoms of venting problems. If you have a backup it is very likely your sewer is clogged.
There are also environmental concerns with discharging sewage on your lawn in that it can enter the drinking water supply. I imagine you could be fined for this.