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How To: Heater Core Flush

32017 Views 18 Replies 17 Participants Last post by  jasonlaroche
If your heater blows cold air, your engine is getting up to temp, and all your switches and actuators are working properly this is your problem....

For reference this is being performed on a 2000 S10 Blazer 2dr 4.3vin W w/AC

Basically GM uses dexcool, when neglected or after becoming too diluted with water over a long period of time it likes to turn into a nice brown sludge of a substance that clogs up your heater core and everything else in your cooling system. Im going to show you how to flush the heater core out with some simple items you can buy at your local parts store.


-5/8in. Garden hose I used a 50 footer that they had at O'Riely's for $9.99
-Adapter's for your heater hoses (on mine one side was 5/8 already and the other was 3/4 so I bought a 5/8-3/4 adapter) $2.99
-Section of hose to mate the adapter to the heater core (in my case 1 ft. 3/4 heater hose) $1.99
-Optional screw type hose clamps (I bought them and didnt use them alot of people dont line the spring type clamps I have a special set of pliers that takes them off so I like them, up to you) $1.99

Heres a picture of everything I used.

Next you'll remove the two hoses from the firewall that go into the heater core shown above.

Then youll attach the end of the garden hose attached to the spicket onto the drivers side heater core tube. Of course the adapter, 3/4 hose, and other length of garden hose which should go into some container an old garbage can or something to catch the mess thats soon to flow out of that end of the hose. Then you turn the spicket on really low at first until you fill the heater core up and water starts to drip out of the end of the drain hose. Once you have a steady flow coming out of the hose turn it up a bit at a time until your at good flowing flush rate that your comfortable with I didnt go too strong at fear of over pressurizing the heater core and blowing it up all over my passenger side floormat. When I was done I reversed the garden hose ends and flushed it in reverse or backflushed it just for good measure.

Thats it my truck is nothing short of hot inside in washington state 28 degree weather and it did not work one bit before this, I could swear the A/C was warmer prior to flushing.

Be Advised:
There is a slightly cheaper and easier way to do this in some events you wont need anything other than a set of pliers... If you own a truck where the heater hoses are both the same size or close enough to it that you can just swap the hoses from side to side thats all you have to do alot of times to get it unclogged. However keep in mind that if you try to stretch an older 5/8 hose onto a 3/4 port there is a good chance your gonna rip it, be prepared to buy a new hose. Also doing it that way just flushes that dexcool mud right back into your system... I prefer the way I did it so I can see the mud hit the ground and leave my truck for good.

For Best Results:
Do this at the same time you are flushing your cooling system. Drain the coolant take the radiator and heater hoses off, flush the block, radiator, and heater core seperately using pretty much the same method shown above and refill the whole system with new coolant im going to tell you to replace dexcool with dexcool because thats what I do and thats what chevy would want you to do. Ive been a mechanic for many years ive seen many people swap to regular green coolant. I do know the active chemical in the two are the same dexcool is 80-95 percent ethylene glycol (the stuff green antifreeze is made out of) it does however have no silicates which are harmful to seals in the coolant passages and waterpump also has no nitrite, borate, phosphate, nitrate and amine, which elude my knowledge but they arent there lol. The myth that the water pump will go out because dexcool is not there to "lube" it is not true, water pump bearings are sealed and upon entry of coolant to the bearings the pump will fail. So use green at your own risk test have proven that dexcool in its prime state will make your pump last longer as its chemical make up is not as harsh on the seals and has been proven to maintain nearly 100 percent of its concentration after 150k miles while green is 50k or so, but other than that I see no reason that you cant use green.

Enjoy the heat, hope this helps good luck.
:flame: :cool:
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You can also run water and vinegar in the cooling system after you flush. dump all the antifreeze cold, fill with water and vinegar 50/50 or more, drive around until system is to operating temperature. Flush again and fill with prestone, forget the dexcool garbage.
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Great idea! I sometimes drive a 77 F-150 with a big old Gas Guzzling 400 in it and the heater doesn't work and occasionally blows all it's water out. Tomorrow I'll give it a douche.
Thanks for the tips. I'm getting ready to do this to my 2000 S-10 FWD. Now that I'm retired and doing my own servicing, every little tip helps.
Is that's s10 things the pipe look power full and strong i think that 's an especial for car it may be a good knowledge that is explain by you on above at post is it use full for others .
I have a 97 ford taurus. I need to flush the heater core. How do I do this and what does it look like. How do I find the heater core.
How much time do you need to complete the flush? I am planning do it it over the weekend.
Hi everyone.,
You want to check your trusted mechanic.your engine is getting up to temp, and all your switches and actuators are working properly this is your problem....
Hi all
You want to check your engine any trusted mechanic.your engine is getting up to temp, and all your switches and actuators are working properly this is your problem....
I open my rad and it is pluged full of rotten coolant should I be worried of anything else or could I just bring it in for a power flush
good write up.
I'd like to suggest one slight alteration to it though.
instead of pulling the heater hoses off at the firewall, pull them off at the
intake/water pump end instead, and connect your flushing hoses
to factory hoses.
I have two main reasons for doing it like this.
first, the hoses tend to "glue" themselves to the heater core tubes, which
typically results in lots of twisting and tugging, trying to get them free from
the tubes. this can easily crack or break the heater core, turning a flush job
into a heater core replacement job...

the second reason is that removing and reinstalling the hoses at the firewall
usually results in a fair amount of water/ coolant getting spilled on your distributor,
which usually shouldn't be an issue, but still, why chance it.
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The Pics you have provided are very useful for me
dont use aluminium if it is written in the back of the bottle not to use on aluminum which is what the heater core is made of.
Good suggestions! Thank you very much!
Great walk thru with great step by step pictures. You just saved me $1184!

I live in an apartment and needed to do a radiator flush because a- the thermostat got stuck open and the engine no longer reached temp, b-it hasn't happened since I've own the truck (3 years), and c- the interior heat stopped working on a 3+ hour drive to the airport before christmas....

Because I don't have the spatial resources now to flush it myself, I took it to a shop and had them flush it. Upon picking it up I was informed that the heater core may be bad and that it will be ~$1200 to replace it. So I took to the trusty S10 forums and after a quick search found this thread. It took me about 25 minutes total once I had the parts.

My heater hoses were stuck on the ports after I removed the spring clamps, so I ended up cutting the hose right at the end of the port and cutting the stuck hoses off the ports. I'm glad I was wearing safety glasses, the system was pressurized and spewed some coolant out of the initial cut. So watch out for that if your hoses are stuck...

I had a 3/4" heater hose elbow that I placed 5/8"->"3/4" adapter on that I used to connect the garden hose to the passenger side port.

Thanks for such a detailed right up
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Nice article. Thanks for sharing. I will do this for sure when it comes time to do mine in a few weeks.
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