Re: 1989 Grand Marquis Won't Idle
These older beasts are simple to work on. As already mentioned, you need to read the trouble codes from the computer. Replacing parts at random will only drain your wallet, and make you severely frustrated.
There are many things that can affect the idle, and one common problem is a vacuum leak. Considering the age of your car, and taking into consideration that many people won't touch the vacuum hoses until something goes wrong, I would check out each & every vacuum hose. I'm not saying this is definitely your problem, but if vacuum hoses have never been replaced, you are almost guaranteed you'll find some that are worn, brittle, or pourous. This is good preventative maintenance even if you discover all of the hoses are fine.
Also, I wouldn't touch the idle adjustment - this won't resolve your problem.