I used this stuff today and I'm pretty sure it cleared out a bunch of rust. When I got the wagon, I cleaned up the water outlet housing (got the plastic bullet moving freely) and put in a new t-stat (the old one was pretty much rusted away). I could see a bunch of rust clinging to the insides of the motor. I did not want to use a harsh flush and I have no hose available where I park it. I bypassed the heater core, as I knew it was pretty much clogged up and installed a new radiator after a few drains and fills of coolant. I installed my new heater core this week (not hooked up yet) and now it was time to tackle the crud in the block before installing new hoses and hooking up the heater core. I decided on the Evapo-Rust stuff because they say it will not harm anything, is non toxic and from past experience with regular Evapo-Rust on rusted tools and stuff. I drained the sytem and loaded it up with the quart of cleaner and distilled water. I let it run for an hour (per instructions) then after a sufficient cool down, drained the mix. P U did it STINK!! Smelled strongly like sewer or rotten eggs (take your pick) and was just about black. It went in crystal clear. The black is the converted rust. I did two subsequent fills and drains with just distilled water and the mixture came out less black each time. I finally filled it back up with 50/50 Green Zerex and distilled water. I'll run it a day or so like this and do one more drain and fill and get the heater core going again. Anyway, until I change out the water pump (next project somewhere in the near future), I guess I will not know just how clean it got, but I KNOW it HAS to be atleast 50% cleaner than it was.