Chunks of rust(lots of it). As for a flow compare... I could dump water down the heater hoses through a funnel and with 2cups of water it would back up. Now i can dump a lot more and it flows about 3times as fast. I used prestone rad cleaner. I ignored the directions and dumped it straight in the heater core and didnt dilute it. I let the first batch sit for an hour and it came out baby **** brown, and chunky. The next batch i let sit for another hour and it came out a lot better. I then blew it out with 50psi of compressed air several times back and forth, also did this with water. Made a huge improvement!

 

did you flush it with CLR.. or just water?? did you clean the OUTSIDE of the core as well? If there's crud between the outside of the fins then air can't get through... mighty have to pull the core and clean the outside of it...

 

I used rad cleaner. Didn't use CLR since it eats alum.. and the core is alum i think. I have to pull the dash to clean around the core so no i didnt lol. I will do that when i replace the core this spring/summer.

 

You can get some just by dropping the blower motor, and reaching up in there, or, using a shop vac..... might be worth a shot.

 

you could also try some liquid plumber, the kind you buy when your sink/shower drain backs up. won't be as harsh as CLR, but could help if the clog is of an organic nature. Sh*t brown doesn't always indicate rust.