Is this sludge inside the engine or in the fuel system? They make engine flushing products that go in your oil. You add it in run for a while then change the oil. Never tried them however. What I have done is drain the oil, change the filter, disconnect the coil wire so you don't get a spark, put about two gallons of kerosene in instead of oil, crank it over for a couple minutes. Let it stand for about an hour then crank it over again. Repeat this several times then drain the kerosene and change the filter. Leave the drain plug out for several hours and the fill cap off too to be sure you get as much out as possible. Replace drain plug. Add new oil, reconnect the coil wire, run the car for 15 minutes, then change the oil and filter. Sounds pretty crazy but it worked for me.