Finally figured out why my sink started smelling like a sewer

I went through almost the exact same thing last year after I installed a new Kohler pedestal sink. I kept smelling that swampy odor and couldn't figure it out since all my PVC looked brand new. Like you, I checked the pitch with a torpedo level and found that my tailpiece was just a hair too long, causing the trap arm to angle back toward the basin. It only took about a quarter-inch of standing water in that horizontal run to start growing some really nasty biofilm. One thing I did differently was swap out the plastic strap I was using for a more rigid J-hook hanger. Those flexible plastic rolls can stretch over time, especially when hot water runs through the pipes and softens the material. Making sure that slope is exactly 1/4 inch per foot is such a small margin for error. I also ended up flushing the overflow hole with a mix of vinegar and baking soda because even after fixing the slope, some of that scent had permeated the gunk trapped in the sink's internal channel.

I had a similar stench in our guest bathroom, but my issue wasn't the slope at all. I spent an afternoon checking the pitch of the PVC with my level, and everthing was perfectly slanted toward the main stack. I eventually realized the smell was acutally coming from the overflow hole in my Delta faucet. Over about three years, a disgusting film of soap scum, hair, and toothpaste had built up inside that hidden chamber where the water drains if the bowl gets too full.

Every time I ran the tap, the air displaced by the water pushed those funky odors right out of the overflow vent and into my face. I ended up taking a small bottle brush and some diluted bleach to scrub inside that little opening. It was shocking how much black gunk came out. If your pipes are level and the trap is holding water correctly, I’d definitly recommed cleaning out that overflow channel before you start re-plumbing the under-sink assembly.

While you're auditing the drainage geometry, don't overlook the potential for capillary action caused by hair and coagulated soap accumulation specifically around the pop-up stopper’s pivot rod, wich can effectively wick water over the trap weir and lower the liquid seal enugh for sewer gases to infiltrate the room. I once spent days chasing a phantom pitch issue only to find that the internal mechanical linkage was coated in a particularly resilient biofilm that required physical agitation with a bottle brush to fully eradicate. If your slope is truly dialed in at that critical quarter-inch per foot, check if the stopper assembly itself has become a structural lattice for anaerobic bacteria.

I'm a bit skeptical that a tiny bit of back-pitch would cause a full-blown sewer smell unless the water was sitting there for weeks. Have you ruled out a failing AAV or a partial clog further down the line? Sometimes when you're manhandling the pipes to fix the slope, you inadvertently break loose a hairball that was actually holding onto the stench. If the odor returns, I'd check the venting. A partial vacuum can pull water out of the trap even if your pitch is perfect.