When I bought the '96 Blazer a while ago, it had NO HEAT. I need heat in western NY but suffered through the first winter without (never again). When spring finally arrived, I set out to diagnose and FIX the problem. The analysis was simple, grab the heater hoses and feel what was hot and what was not. Hose to heater core = very hot. Hose out of core back to water pump = not hot at all. Diagnosis = heater core is plugged...BAD BAD BAD news. Haynes manual has you pretty much dsassembling the interior dash to get to the heater core...PITA. Try this instead...disconnect the hoses to the heater core at the firewall (keep track of which is supply and which is return, although they should be different sizes, mine were). Get a new piece of RETURN hose from your local auto parts store and a few odds and ends fittings from the local Home Center plumbing section so you end up with a hose that you can attach to a garden hose and connect to the return side of the heater core. Get a length of supply hose long enough to connect to the hetaer core and snake it to the ground or into a gallon jug. Hook up everthing and have a helper SLOWLY turn on the water pressure through the garden hose connected to your house (CAUTION: I used a backflow preventer on the water line into the heater core in case the blockage was too severe and I didn't want heater core water to mix with house water, safety first)..you are now backflushing the heater core here. Hopefully, the water pressure will blow out any obstruction(s) that might be blocking the heater core. I ended up pushing out a 1/2" thick plug of reddish/orange "mud" (sediment) from the core into the bucket. Once the plug exited, nice clean water eventually followed. Next, I reconnected the supply hose to the heater core and opend the radiator cap and backflushed the radiator core until clear water flowed. Disconnect your backflushing rig, reconnect your heater hoses, check your cooling system capacity, drain off as much water as necessary to achieve the correct Anti-freeze & water mixture, pour the correct amount (and type) of coolant into the radiator, top off with clean water and cap the radiator. Run the engine until warm and check that you should now have sweet interior heat. (Truth be told, the Blazer got too hot inside at full HEAT setting...nice.)

 

Wow! Certinly sounds feasable. I would definently try this first rather than try and repalce the heater core. Thanks for the tip and good explanation. I will keep this in mind and maybe do this next summer for next winter as preventative rather than a cure to a problem.
Good write-up.

 

I had heard from a couple people about this but your post really helped. I had no heat, (I live in the Utica area, so I hear ya about NY weather) And I hooked up a garden hose and some fittings to the engine side of the heater core today, blasted it out and I finally have heat. Thanks man!

 

That reddish/brown stuff could've been your Dex-Cool coolant...

Not 100% sure when that showed up in the S-series of vehicles, though...

 

Getting ready to just this same exact thing on my blazer. I went out and bought all of what I needed to do just what you described, before ever reading this.

I wasn't too much into the idea of taking the heater core out, and back flushing it this way seemed much simpler.

 

Ran into this same exact problem last week. Diagnosed it by checking the intake and return hose temps by touch. the return was neutral plus I have never flushed the heater core properly. I wanted to do this properly seeing i live in a townhouse complex and didnt want coolant leaking all over the place so I went to Lowes and bought some 5/8" clear plastic hose with some screw clamps. Worked perfectly, hooked up then passenger side intake port first, flushed out clumps of junk and coolant into a bucket. The clear hose allowed me to watch the junk come out. Put the hose on the other side to flush out completely making sure both sides were runningclear water. I used the hose to minimize the amount of water spraying all over the distributor and other components in the engine which I highly recommend. Hooked the hoses all back up topped off the radiator and burped it. Total cost for the fix $9 and little bit of coolant i already had. Only took about an hour of my day.