best way to flush crud out of cooling system???

so I posted a thread a while back about having some overheating issues when towing my new to me boat and still have not resolved the issue. I still have not pin pointed it but am now leaning towards the radiator.

so far i have installed a tru cool 40k (thinking my stall is heating up the coolant), "flushed" the system 3 times, pressure washed the junk out of the fins of the cooler, the ac condenser and the rad. i did a ton of research on the issue and noticed that my upper rad hose is higher than the pressure cap, so i bought a surge tank that I mounted higher to get rid of any possible air in the system (which aparently can cause major heating issues). this has all helped some but im still having heating problems on hills and in hot weather.

i have the larger 40inch rad for the hd trucks installed. single pass, single core rad. put all new coolant in (dex) after motor swap, and noticed it turned to brown sludge. put flush though it and went back to green coolant. keeps turning brown with junk. at the point now where its just straight water and still getting crud....

I am now leaning towards the issue is the radiator itself. clogged with crud. allowing enough flow to make the truck happy unloaded, but with the boat, not enough flow. so i bought an all aluminum crossflow dual 1 inch tube core rad. this baby is twice as thick as what i have. and from the searching ive done, this is better than 3 or 4 row smaller tuber cores.

my question is, I still have brown sludge in the system. even after all the flushes. new pump. new stat. i dont want to put he new rad in with all this crap in there. how do i flush it??? i cant pull the stat because its built into the water neck. i want to run the motor with fresh water running through it but idk how to do that since i cant eliminate the stat. what are your opinions on this? anyone done it? im almost sure the new rad will fix my issue but i dont want to clog it with residual crap floating around. how do i go about this? thanks!

a new radiator will fix it but you can try some CLR and water just run it through your system and flush it that way

im finally going to have a day to do this tomorrow. i still dont really know how to go about this. I literally want to put the hose into the intake hose and let it run out of the outlet hose for a while to clear it out. but what do i do about the stat? is my only option to basically just buy a new stat/waterneck and rip out the stat so i can connect the hose? i really dont see any other option. anybody done this before? will it hurt (crack) the block/heads by running cold water through it like that? its iron block alum heads.

sounds like you are getting oil in the cooling system. maybe warped heads. Just a thought there aluminium and the can warp and crack just like any other car or truck after they been over heated. try a compression check and look at the spark plugs for oil or coolant damage.