Cleaning inside radiator and heater core... soap?

[jmgtp]

My radiator and heater core are 20 years old. No failures to date but I did run a bottle or two of some kind of stop leak through them a few years ago when I had a freeze plug on the old motor leaking (I ended up pulling the head and doing it right).

I'm sure that left some sludge in there, the underside of the radiator cap had some brown goo on it. Other than boiling the radiator out what can I do to clean it? Has anyone heard of putting a bar of soap under the radiator cap and driving for a few days then flushing the coolant out? I've heard of it, never tried it.

Any other products/chemicals that are engine/waterpump/gasket safe and capable of dislodging any gunk?

[60dgrzbelow0]

Don't get too enthusiastic with using the Garden Hose at perpetual WOT...or you could damage or rupture the core... just modest bursts to dislodge "The Mung". This article describes the procedure(s) very well.

http://www.ehow.com/how_5206315_clean-heater-core.html

[pocket-rocket]

I've restored heat in my Ranger and the wifes Daewoo. Although I removed the heater cores and proceeded to run some hot water through them while beating them like rented mules on the sink in such a way it didn't bend fins. I got a good amount of "Mung", as Bob calls it, dislodged so coolant could flow again. It sure beat $150 for OEM cores. I won't ever buy a $50 special again, and I suggest that no one here does it either. Way fewer fins and hardly any heat, if any at all.