We recently replaced the valve in a guest room toilet. Now, when we flush, the water in the bowl doesn't look "clean." After a few days, the water in the bowl turns yellowish and cloudy with a greasy-looking film on the top. Needless to say, if we don't flush that away, it really begins to stink! What could be causing this problem? Thank you for your help!

 

I can't imagine how you'd get foul water supplied to the toilet, but not everywhere else in the house, unless maybe a grey water system you'd know about if you had. So I doubt the new valve has anything to do with this.

Check the water in the tank. Should be crystal clear, besides some mildew around the tank walls.

If tank's OK, then you have a blockage in the toilet drain, possibly a dropped part from the old valve, which allows a decent flush to pass but always traps some matter.

 

I checked the tank this morning. We haven't flushed the toilet since Thursday night and now it's Sunday morning. Water is clear (but not as clear as the toilets) but if I look really closely, there is some greasy-looking scum floating on the top.

To get really graphic about what's going on in the toilet bowl, it looks like someone spit in it a few times...it has some areas with that look slimy and a little bubbly. The water is slightly cloudy.

If it sits like this for a week, it turns yellow, greasy and stinky--like a latrine. By the way, the three other toilets in the house are crystal clear.

 

This is perfectly normal bacterial action in a tiolet that is not used often (for days at a time). The "slime" and bubbles on top of the water is a bacterial film. There is always bacteria in the toilet, when you add fresh water, the bacteria says "wow" and blooms creating the mess. Best thing to do is to flush the toilet more often or use an antibacterial block in the bowl.

 

Liz2, it may just be bacteria acting on it's own out of the blue, so to speak. This certainly doesn't happen where I live, with mountain water and copper pipes, but maybe kans_flatlander's water supply isn't too pure?

Kans_flatlander, could you confirm
means that just after flushing, the water isn't as clear as your tapwater? If that's what you're saying then I'm pretty sure there's an obstruction in the toilet trap. You can test for this by flushing a generous ball of toilet paper (not the roll ) and then using a plunger. If material routinely gets snagged inside the trap, plunging then will fill the toilet bowl with whisps of paper.

Toilets with partial obstructions will flush tolerably well. I know because I've snaked some after they finally became so full of items they wouldn't flush at all, and found objects that were lost years prior. The symptom is just as you describe. Don't live with this.