2000 Chevrolet Blazer heater blowing cold

What would cause my heater to blow cold, and what needs to be fixed? Thanks!

Low coolant level, stuck thermostat, clogged heater core. Do the heater hoses warm up at all? If the truck is getting worse gas mileage than usual, that might indicate a thermostat stuck open. What is the condition of the coolant, is there any black sludge in the radiator? A radiator cap that does not seal properly will introduce air into the cooling system which can contribute to sludging when Dexcool is used.

On my 2000 Blazer (102k miles) the intake manifold gasket was replaced at 44k miles. I switched over to regular antifreeze at the time and change it every 3 years. The radiator was replaced last summer at 96k miles. I’ve never had a problem with the heat, but I’m careful to purge all the air from the cooling system when I do any work on the cooling system.

Ed B.

Ed, thanks for the quick reply! Sounds like the worst-case scenario would be the intake manifold gasket or the sludge in the radiator – if you don’t mind me asking, about how much did it cost you to replace the gasket on yours?

Start with the simplest and work your way up to total disaster. Fluid level is check one. Step 2 is to make sure hot fluid is getting to the heater core feel the temp of the hose going in and the hose coming out, step 3 is to make sure proper vent controls are opening and closing (how I am not sure). Is there a fuse to check that operates heat controls? I have heard a computer reset helps partial ac problems but never heard it applied to heat, but nothin to loose!

Here is a link to a good article on Dexcool with a recommendation to switch the radiator cap. http://www.imcool.com/articles/antifreeze-coolant/dexcool-macs2001.php

Do a search on “Dexcool sludge”, there should be plenty of horror stories. Remove the radiator cap when the engine is cold, if there is a layer of black crud on top of the coolant you have sludge. If the heater core was clogged with sludge (extreme case) that would cause the heating problem.

I mentioned the intake gasket because it is a known problem for the Blazer not because of your heating problem. In my case, the coolant level started dropping slowly (~1 pint/month) after I changed the coolant. My mechanic found a small amount of coolant in the oil during an oil change and the dealership confirmed the leak. Any coolant in the oil will displace the oil from the main bearings and ruin the engine. The dealer paid for half of the repair (goodwill warranty), but I estimate the repair would run $400-500.

If you’ve had any work done on the cooling system make sure all the air is out of the system. I drive my truck up on ramps and run the engine with the radiator cap removed and the heater on full. As the engine warms up most of the air trapped in the system should make its way to radiator. I watch the coolant level and add to the radiator as needed. The level in the coolant overflow tank may drop over the next few days as any remaining air is expelled from the system but should stabilize. If the level in the overflow tank continues to drop there is a leak somewhere in the system.

Two simple things to check.

  1. With the engine warmed up and the heater running are both heater hoses warm/hot? A warm heater hose and a cold one would indicate a clogged heater core. If both hoses are hot then the problem may be in the heater controls.

  2. If the engine does not seem to warm up fully and mpg is down then the thermostat might be stuck open. This is a fairly simple and cheap repair.

Good luck,

Ed B.