Re: 83 Grand Marquis Rough Running
83 huh, not sure, but check under the hood. You should know what an injector looks like. I'd start with the simple and obvious (yes I'm an optimist). When was the fuel filter last changed? A quick disconnect tool, a new fuel filter and a trip under the car will let you find the filter (most are located on the driver's side frame ,inside frame, and are snap in and out cannister types) and change it for little money. Word of caution though, get out the air hose and blow the road debris out of the quick disconnect fittings BEFORE using the tool, or you will have a fitting which is neither quick, nor will it disconnect.
If this fixes the problem, great. Throw in a bottle or two of injector cleaner in the tank and be done with it. If it doesn't fix it, then I'd look at the CAT also.
My job, basically is to think up Goofy Sh*t.