[supereal]

Trying to use a solvent, gas or other, with the engine in the car is not a good idea. If any of the sludge is dissolved, it will be circulated in the system where it can cause bearing damage. When you remove the intake manifold on an old Ford that has not been apart for a long time, often years, you will find the valve chamber full of "black Jello" sludge to give you an idea of what the rest of the engine looks like. Best to leave the sludge in place until overhaul time.