I run a cheap Chinese stainless steel ultrasonic cleaner I picked up on that jungle-themed website for a song. I use it to clean various parts, injectors, delivery valves, lift pumps, small engine carburetors, etc.

They will NOT remove gummy or mushy debris. Scrape that crap off before you start cleaning. They are not magic, if you have excessive rust or debris, the ultrasonic cleaner will not clean it off.

They WILL do a good job of cleaning parts you can't readily access, removing varnish and carbon, and getting parts like injectors extremely clean.

If you have a stainless steel unit with a stainless steel lid and no internal heater, you can use stronger solvents. I routinely use Xylene and lacquer thinner in mine for nasty jobs. It works extremely well. Any time I'm using something flammable, the fire extinguisher is nearby, the steel lid is installed, and I'm standing nearby. Be aware that even a unit without a heater like mine will heat the fluid up in ~30 minutes.

If you're ever in doubt whether or not your ultrasonic cleaner really is ultrasonic, put a square of aluminum foil on top of the fluid and run the unit. The foil should very quickly develop holes and begin dissolving. If you don't get holes, or the foil doesn't look any different, you don't have a real ultrasonic cleaner.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]