In the old days (yes, I'm plenty old enough to remember them) I flushed a "few" crankcases with kerosene. The engines, AND the oils were WAAAaaaayyyy.... different then. Too many people insisted on running oils like Amalie and Havoline that turned to tar. This was aggravated by road draft tubes in place of PCV valves along with 160 degree Summer thermostats.
I've pulled off valve covers and seen a big hunk of black tar that was molded in the form of the valve cover. Those oils were TERRIBLE!
I flushed a few with kerosene by getting it hot as it would get then draining the oil, then pouring in a quart of oil and the rest kerosene. I would crank it and run it to about a count of three, then pull the plug again. All this was done while everything was at operating temperature.
All that said, with todays oils I wouldn't do that on a bet. If an engine is sludged in todays terms, it would be CLEAN by mid sixties terminology. If you think it's "sludged" simply change the oil and filter by pulling the plug when the engine is at operating temperature and let id drain OVERNIGHT. Those last drops that come out will be the real sludge. Keep repeating the hot oil drains about every 500 miles until the innards are clean.
Depending on the engine, use one of the diesel oils like Rotella, Delo or Delvac for this flushing process. They have lots of detergents for soot dispersal in diesel engines and will eventually make the oil in a gas engine SQUEAKY clean.
My $.02,
Doc
I've pulled off valve covers and seen a big hunk of black tar that was molded in the form of the valve cover. Those oils were TERRIBLE!
I flushed a few with kerosene by getting it hot as it would get then draining the oil, then pouring in a quart of oil and the rest kerosene. I would crank it and run it to about a count of three, then pull the plug again. All this was done while everything was at operating temperature.
All that said, with todays oils I wouldn't do that on a bet. If an engine is sludged in todays terms, it would be CLEAN by mid sixties terminology. If you think it's "sludged" simply change the oil and filter by pulling the plug when the engine is at operating temperature and let id drain OVERNIGHT. Those last drops that come out will be the real sludge. Keep repeating the hot oil drains about every 500 miles until the innards are clean.
Depending on the engine, use one of the diesel oils like Rotella, Delo or Delvac for this flushing process. They have lots of detergents for soot dispersal in diesel engines and will eventually make the oil in a gas engine SQUEAKY clean.
My $.02,
Doc