The heaters in these things are very under engineered for sure. You'd think a vehicle that suffers from chronic overheating issues would produce nuclear heat, but it doesn't. I flushed my cooling system out really well and all sorts of junk came out. I pulled the heater core and rigged up a chunk of garden hose to blow hot water through it from my wash tub faucet, and another to discharge back into the sink. After lots of thunking and banging while forcing the hot water through all sorts of chunks of solidified sediment and crap came out. When the water pressure coming out is equal to what's going in, you'll know it's more-or-less cleaned out. Made somewhat of a difference in mine, which at the time was only giving luke-warm air at best. Now it's warm. lol