what do you use to clean a radiator that been on dex .and do you do it whit the engine running or not

What I did was drain the radiator completely,
remove it,
hose it off real good(mine was really bad)
Straighten out bent fins with needle nosed pliers
I then put a hose into the top and ran water thru it til clear
I then reversed the procedure(ran it through bottom to top)
Let it dry
checked for leaks(buddy at a local shop for $5, figured why not??)
Reinstalled radiator
refilled with good quality 50/50 GREEN coolant(I have a 95)
Burned rubber!!!!!!!

I added a little water pump lube too, just to be safe.

YMMV

Bob

I simply drain and flush. Flush through the heater hoses...(both) throught the filler neck, throught the top radiator hose.. then fill and run the car for a moment to cycle the water. Flush again. Repeat until you see nothing but clean water. Drain and refill with the new coolant.

If you believe there is buildup or sludge... consider a flush chemical available at your autparts store.