Kitchen sink vent capped off and no longer open to the roof.We have noticed fumes from sink. What is this from?

Answer 1

You should not cap off any Plumbing vent as these are in place to prevent siphoning of the trap water that prevents sewer gasses (which could be noxious or even toxic) from backing up through the plumbing.

When you flush a toilet, the enormous rush of water creates a large suction which, if not vented to the atmosphere, can suck all the water out of nearby traps, such as those under a sink, bathtub or shower stall; you will often hear a gurgling sound when this happens. This leaves the drain pipes completely open to gases from the sewer system and, therefore, to anything that might have been dumped into the system including your neighbors bowel movement and toxic chemicals from your nearby illegal factory.

There is no justifiable reason for capping off a plumbing vent (often called a stack) - don't do it.