Grout Cracking and Water Leaking into shower pan

I have a 6 year old home that I purchased 2 years ago. The shower is separate from the tub and may have been completed by the previous homeowner. The grout on the bottom corner on the opposite wall from the shower head has started to deteriorate and split from the wall on both sides of the corner. The corner itself has a small hole developing. I noticed a terrible smell over the winter and had two plumbers come and tell me it was the shower drain. I didn't think so and kept checking where I thought the odor was coming from. The odor is only there after the shower is used, not when the area is dry. Today, I took a sharp, thin blade and ran in across the tile where the grout had already separated. I got some gunky material on the blade and the smell is horrible. Now I am certain the odor is from the shower pan under the tile, I considered removing the grout, pouring some bleach in to kill whatever is growing in there, let it dry thoroughly (not sure how I would know this) and then re-grout. But then I became concerned about water and mold in the walls.
If anyone has any experience with this or advice/suggestions, I would sincerely appreciate it.

Your wall assembly and shower base way well be saturated completely with moisture and shower goo. A properly constructed shower base will drain water and no standing water will be left in your assembly. A poorly built shower will maintain 1/8" - 1/2" if not more standing water that will wick up the backer board and breath through the entire floor and the bottom 4-18" of your backer board.

It is common practice to lap the backer board over the shower's membrane. If drywall or mastic was used durning construction it could be covered with mold.

If the grout is failing it could be a sign that the bottom plates are swelling and the shower is getting worse. I would see if you can inspect the method of construction and perhaps remove a tile carefully to inspect. Remove a tile not from the bottom but rather one or two courses above the weak area. If you can upload a picture I can draw an arrow on which one or maybe two tiles we would remove for an inspection.

Use a mask when working around the shower. I often get sick shortly after a demo involving large amounts of mold.

Air movement will help dry the shower faster. Consider using a fan to help and dry the shower out.

We use a heavy duty tile and grout cleaner here on nasty showers. Some elbow grease and careful scrubbing can clean a shower up nicely. Try this approach first and if the smell or mold comes back quickly consider inspecting further like I described above.

JW

If the shower floor was not propery prepared with a subfloor sloping to the drain, the water, which will always get through the grout, will sit there and stagnate.

jimbo said:

If the shower floor was not propery prepared with a subfloor sloping to the drain, the water, which will always get through the grout, will sit there and stagnate.

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Can I safely dry out the subfloor and re-grout or is that not the wisest option?

The grout cracked because of movement between the tile, wallboard, and shower base. It was apparently a poor installation and anything you do will probably just be a temporary bandage. It is very likely that redoing the shower stall is the only good option.

Whenever there is a change in plane of tiles surfaces such as a corner or bottom of wall to floor, grout is not the best option to use on that joint. It should be caulked with a good quality silicone caulk. Walls and floors always move with expansion and contraction and the corners are where this is evidenced. Grout does not allow the proper amount of flex and will repeatedly crack and deteriorate.

Yes you can. It very time consuming but it can be done. My concern is how long it has been this way and mold. Before you fix the sub floor and the grout be sure you don't have any mold. If so treat that first then fix your sub floor and shower wall.

A mixture of Mule Team Borax in a garden sprayer and a wet/dry shop vac should be able to rinse the crud out. As was said, where there is movement from a change in plane, silicone and not grout should be used.

If water has gotten to the wall surface behind the tile, it would be a rare installation that the tile is not coming loose.

It's hard to tell for absolute certainty, but it is likely that your shower was not built to industry standards, and anything you do to it short of tear-out and replace will fail. There are a few accepted ways to build a shower and lots of ways to mess up.