Why would a 1987 Mercury Sable lose power and backfire and then stall out and now turn over but not start?

Answer 1

No the problem is caused by 1 of 4 things, but is most notably the first one.

Replace the Spark plug wires, and the spark plugs. Fouled or incorrectly gapped plugs will cause backfires like crazy. Also poor sparking wires will most certainly cause backfiring problems. I practically replaced and rebuilt the whole darn engine, before realizing that I hadn't replaced the wires ever.

Also the intake manifold and air cleaner may need to be cleaned out. Often the gunk built up inside the intake manifold can cause the system to be leaky via the vaccuum seals of the valves. This is not loads of fun to do, but can be done, and requires someone with knowledge of tools to remove the various components and get at the internal parts of the upper intake manifold to wash out the upper valves with carb cleaner spray, and paper towels.

Lastly you could have a vaccuum leak. Those are most fun to diagnose as you need to listen for a hissing sound. One or more of the hoses connected under the manifold may be disconnected. These hoses are not usually made out of the best material in the world, and may have become cracked, broken, or don't hold a good seal. Replacing them can be a bear, because often times there isn't a cheaply made replacement available. But use your best judgment and the people at Home Depot/Lowe's/Autozone may be able to help.