What could cause a 1985 Ford LTD to lose idle and develop engine shaking?

My 85 ltd is doing the same thing, I checked the cylinder pressure and found it to be 40 lbs.The sprk plug was black from fuel and oil deposits, and still wet looking.I'm going to pull the valve cover and watch the roker action,maybe the intake isn't closing completely. I think that the answer is simply "lots of mileage" and probably needs a valve job. But don't rule out PVC valve,egr valve,vacuum leak or fuel pump. Ill let you know what the answer to mine is if you let me know what yours is. respectfully, TOM KLINE (GOOD LUCK)

I have a 1985 midsize LTD 5.0L HO Police/Taxi with Center Fuel Injection that has had this symptom from 1985 to 2007. An additional symptom was that it would ping a lot. A mechanic found that someone, possibly at the factory, had drilled a small hole in a plastic fitting for the EGR valve. After replacing the fitting and a vacuum operated valve it ran very good. While the engine doesn't die or ping now, sometimes the idle drops down and the engine gets a little shakey. My experienced guess is that it is an inherent intermittent, possibly temperature related, problem with this year model design for cars with the Center Fuel Injection. The problem can be effectively fixed by fixing all vacuum leaks and assuring all vacuum operated actuators are working; this greatly minimizes the frequency and amount of the intermittent slow idle and it eliminates the ping. The ping is probably caused by the vacuum system giving inaccurate input to the electronics that then advances the spark too much. Lee Jordan