SO, not Monte related, but 3800 related. My mom's 2005 LeSabre had NO HEAT. Earlier this year I replaced the heater hoses and radiator hoses (all original). I did not know until recently it had no heat. When I learned of this, I found one hose would be hot and the other not hot at all! Seemed pretty clear this is a clogged heater core. I suspect the old hoses were coming apart on the inside and building up crud in the veins of the core.

I tried something and it seems to have worked. I pulled the heater hoses off. The bigger of the two (which was the one that got hot), put the end into an empty bottle to catch stuff. The other one, I wanted to send compressed air into (water was my second choice). I did not have a way to go straight into the hose with the air nozzles I have so it would build pressure. I took a spare hose, sleeved it inside the heater hose. First time I sent air in, it blew my sleeve out. So, next step, I reconnected, held the connection and sent air in. It blew a bunch of junk out! It pushed the coolant out and black flakes. Now no issues moving air. Put the hoses back on, topped the coolant and heat works PERFECTLY.

Thought this might be worth sharing in case anyone else bumps into this issue.

 

Remove both hoses from the heater core and get about 6 ft of new hose for flushing purposes. Dump a bottle of "CLR" into the system and let it sit for 30 minutes. Then run water both ways through the system. Usually when you run it opposite of the normal way the gunk will start coming out. I'm not a big fan of using any compressed air.

 

I ruined the walls to my last garage from doing this. Well, the walls and all the clothes my wife and I were wearing.

I used almost the same method except my catch bottle, the hose barely fit inside the opening creating an airtight fit.

I put about 100psi into the heater core for a second but nothing happened. Gave it another shot for a second, nothing. Gave it that third shot- About 15 years of built up crap came exploding out.

Ill never forget that sound. Slight gradual hissing that immediately turned into ringing ears.

CLR works great. I make a small loop with fishtank hose and a drill pump and cycle it for a few minutes.

 

I went back and forth on the idea of air vs water. And if I used water, consider something to flush the heater core with (to get other gunk). I had the pressure dialed down on the compressor and figured give it a shot. And I was tempted to do a few other things, but after the air busted a blockage free, air moved freely, figure what the heck, see what happens by putting it back together. So far, so good. And hopefully it stays good.