Heater Core Flush Procedure

What's the best way to flush the heater core? Had to use the heat today and all I got was slightly warm air.....

This process takes 25 minutes. 5 minutes to flush, 10 minutes to do the initial bleeds on the system, and 10 minutes to find the bucket and bring the hose over, and then put everything back when you're done.


This is for GM b-bodies withOUT the mechanical fan. If you have the mechanical fan, just disconnect the hoses directly at the heater core and use a garden hose sprayer to gently flush back and forth in the heater core pipes until clear water freely flows out.

0. Put a bucket or pan beneath and slightly ahead of the serpentine belt tensioner.
1. Disconnect heater core hoses at the water pump. These are the two smaller hoses facing the passenger side, towards the battery. The lower hose and pipe will dump a bunch of coolant out of the engine into the bucket. Try to make sure it doesn't wee all over the optispark behind the crank damper.
2. take a garden hose, put it up against the lower hose, run until it goes clear.
3. You can switch to the pipe to backflush if you want, but all it did on my car was bubble stuff through the engine and spit out clear water anyway.
4. Repeat spraying water into the the lower hose until the heater core water runs clear or it hacks up a hairball.
5. Reassemble cooling system, fill up the expansion tank and screw the cap on, bleed it (put car on a steep hill, open the bleed valve on the thermostat housing cover until water runs out). Run the engine for a couple of minutes until the thermostat starts to open - watch the temperature gauge on the dash; once it comes up, shut it off. Open the cap and let all the air bubble out. Fill it up some more. Close it up. Start the engine again and let it run for a couple of minutes. At this point the heater should start getting hot. Repeat until you've replaced all of the coolant you lost. BTW - keep the rest of the cooling system closed up when you do this. Do not try to be slick and hook a hose to your bleeder valve, open the reservoir cap and dump the extra fluid back in. It is self-defeating. Trust me on this. Just open the bleeder valve and put a rag or paper towel next to it; leave it cracked open a turn until coolant starts coming out, then close it up. Drive around for a few heat cycles, then repeat.

done.

Don't run a lot of pressure through the heater core unless you like replacing it. Reasonable flow back and forth is usually all it takes. Lots of bubbles usually helps loosen stuff too.

BTW - if you have a cooling system that mysteriously keeps producing crud like my 9C1 does, you may have to do this every year for a few years. It gets easy.

FYI: The heater core connections are two different sizes. Just know that going in. That's why I find it easier to flush the heater core at the water pump; no worries about getting a hose the right size.

Another thought. Follow these instructions, take pictures and write a tech article for the Wagon FAQ.

More edits above.

This should be an interesting little project for me....

stockfornow wrote:

This should be an interesting little project for me....

I don't know about "project" - it should take you an hour or so.

The flush did it! Nice and hot now.

Thanks for the help guys.

Sorry, no article. Didn't take the time for pictures and such.....

Anybody have any idea what happens when you take out the restricter in the heater hose? More flow would possibly keep the heater cores from clogging up so much.

these engines (LT1) are of the reverse flow variety. That restrictor is to prevent back flow out of the core, thus preventing all that nasty gunk that has collected in the core over the summer from entering the engine. (That's my story and I'm sticking to it

Seriously though, it's a one way valve to prevent back flow. I don't have one and everything works fine.

Heater cores clog because they have tiny passages; not because they don't get enough flow. They are essentially tiny radiators mounted behind the glovebox with a blower fan next to them.

The following statement is purely secondhand conjecture until I verify it:
The restrictor may be there to limit total pressure through the heater core. This is particularly true when the thermostat is closed, as coolant is flowing only through the heater core and a small bypass portion of the water pump/thermostat.

The following statement is my own personal experience:
Shortly after a cold start and before the thermostat opened, I did a rolling 4500rpm burnout for about 5 seconds. A few minutes later I discovered that one of my heater core hoses had literally blown off of the end of the heater core and progressively dumped all of my coolant (unfortunately I was also a few minutes from home at this point). Once I got the car refilled and back on the road, I tightened the hose clamps a bit, but that lesson taught me that there's quite a bit of flow/pressure through the heater core prior to the thermostat opening, and it's made worse by high water pump RPM. So, word to the wise.

Wisdom:
You might consider that GM never does anything for no reason. That restrictor is there because at least two engineers thought it needed to be. I'm not saying the car won't run satisfactorily without it, but just keep this in mind.

The older I get, the less I am willing to just go in and tinker with something the factory did unless I have a really darn good reason.

stockfornow wrote:

Sorry, no article. Didn't take the time for pictures and such.....

So it probably didn't take very long?

sherlock9c1 wrote:

stockfornow wrote:

Sorry, no article. Didn't take the time for pictures and such.....

So it probably didn't take very long?

Longest part was trying to figure out which hoses ran to the heater core

So what did it cough out?

My 9C1 hacked out a wad of RTV the size of a spitball the last time I flushed it. Probably from when I did the water pump.

sherlock9c1 wrote:

So what did it cough out?

My 9C1 hacked out a wad of RTV the size of a spitball the last time I flushed it. Probably from when I did the water pump.

Just looked like a lot of dirty coolant. Once it ran clear for a bit I checked the heat and sure enough, HOT!

Anyone have tips on flushing the entire LT1 cooling system? I just checked my maintenance records and the Caprice not only has a clogged heater core (again) but it's due for a complete flush.

I usually just hose everything out with a garden hose until it runs clean, then take the coolant expansion tank off and fill it with a bunch of road dust (very small gravel), throw some water in to make a slurry, and shake the living daylights out of it until it's clean inside.

Then, I put in 50% of the cooling system volume of antifreeze in, then top off with the garden hose.

Anyone ever use a cooling system flush product? I have a bottle of STP that I've had sitting here for the last 7 years but never used. Any pros-cons?

i,ve posted this before,. and folks always seem to have kerniptions,. but anyway,.
i drain the antifreeze,. dump in some draino and water and drive the car for...hour... day...whatever,. then flush it all out,. then add antifreeze etc,.
yrmv
pdcc
nweoi

Nick

Just updated my procedure after doing it on the 9C1. Voila, now I have heat again! Took 25 minutes counting all the fartin' around I had to do to round up everything I needed.