Windshield washer fluid is stinky - what to do?

If you are worried about running the windshield wipers by using the existing fluid, how about buying some sort of syringe, like a turkey baster, to remove it?

Pull the hose at bottom of tank, let it drain ( if pump is on tank run pump, be careful not to burn-out pump ).
Refill with 100 % windshield washer fluid, do not add any water. Run washer to clear all lines to sprayers.
Should help the stink, maybe to low a concentration of fluid caused a “Sewer smell” .

Did something get into the reservoir and die? There should be no organic material in that fluid, and for that reason no “rotten” smell.

Drain, flush and then refill the reservoir.

If it were me, I’d assume that it’s caused by mold growth in the reservoir, possibly nourished by the low concentration of alcohol in the washer fluid (the alcohol is only toxic to he mold a high levels), and I’d try the following:

Drain the reservoir.

Empty a few of bottles of 3% hydrogen peroxide into it (H2O2 is both a good glass cleaner and an effective mold killer). Add enough that the motion of the car will slosh it into all inside surfaces.

Drive around with that for a few days, frequently running the washers to move the peroxide through the system.

Like people here said, you need to drain your washer system and flush all of the lines. I suspect that you filled the reservoir which molded after several months. I have seen similar situations in post holes that were dug then filled with water. After you flush the system use washer fluid that was designed to de-ice because it has chemicals that will clean the bacteria from your system. I suspect that using the contaminated water in you reservoir is dangerous to your health because some of the bacteria enters the passenger compartment and you and your passengers breath them.

Calif prohibits the sale of the poisonous de-ice washer fluids. I have some “imported” from Nevada, but it won’t be enough. Maybe I can mix up my own using methyl alcohol, antifreeze, and Windex.

I once had a windshield fluid tank that grew some kind of slimy stuff that clogged the filter and pump inlet. That might explain why adding bleach,vinegar, & solvents didn’t eradicate your problem. You might have to actually replace tank, hoses, or other portions of the system – or at least disassemble & hand-scrub.