Flushing heater core steps

[mpierce9]

Hello, 1975 K5 Blazer 350 with A/C. I'd like to flush the heater core.

Here's why (skip down to The question if you don't care and just want to read the question)
It's cold outside. Heater never blows warm air. I checked the cold/hot door attached to the cable on the dash panel, this is working as expected. Put on a new thermostat earlier this week, the temp gauge reads med to med-high after driving for about 10 minutes. There are two hoses attached to the heater core housing. One hose is attached directly to the radiator, the second hose is attached to the water pump. The hose attached to the water pump going into the heater core gets very hot. The hose attached to the heater core going to the radiator does not get hot at all. Seems like there is something blocking the water pump hose from the radiator hose somewhere inside the heater core.

The question:
Is this as simple as unhooking the inbound and outbound line and plugging a water hose up to and just holding down the trigger until it runs clean and clear?

[Tx Firefighter]

I take the hoses off on the other end and elevate them up higher than the heater core. Then pour CLR type of product into the hoses to fill the core up. Let it sit a few minutes and then lower the hose ends so it drains out. Then flush thoroughly with water from a garden hose.

The CLR stuff really cuts to gunk out of the heater core. You might be surprised the nasty stuff that comes out of an old core.

[Gumby]

On top of that ^ I like to use the U-DO-IT car wash pressure wand to rev flush the sucker out.

just loop a hose to bypass it for the ride over.

But I have come across one that would not work no matter what in my wagon, several cold winters and never success on a flush, a new one [only $15] was the only fix, though we all know price isn't even the big factor in that decision, its how much work you wanna do to change one

[mpierce9]

So, still having the issue of my heater not delivering warm air.

I took two distinct steps.

1. I flushed out the heater core. in the manner below using CLR. Removed the top and bottom hose plugged a garden hose in one end turned it on full blast for about 30 seconds (till the water was clear), then repeated on the other side for about 30 seconds also until the water was clear. At this point I hook everything back up.


2. after hookinh back the heater core I did a full flush of the cooling system. Full radiator drain, Prestone cleaner flush, rinsed it out two times then a full replacement of coolant.

I drove the truck about 1 miles, temp gauge moves up to mid range but the heater never gets warm. What am I missing here?

[Tx Firefighter]

Possible air lock in the heater core. Try jacking front of truck up very high and letting it run for awhile to try and make sure the core is burped.

Check heater hoses with engine warm and running. Is one of the hot and one cold ?

You might have a blend issue in the box under the dash.

[mpierce9]

So you bring up a good point. . . This is my first attempt at all of this stuff so I'm not sure what is working as it should and what isn't.

When I flushed and refilled the cooling system I ran the car a bit without the radiator cap on. After the car got a little warm the radiator kind of vomited the fluid out. I mean projectile style, it went on the side and wheel and all that out into the driveway.

During the "burping" process how long do I leave the cap off while the car running? Do I put the cap on while the car is still running and hot? Do I stop the car and let it cool before re-installing the cap? What should I expect. Sorry for the first timer questions. I've checked so many things that i believe you could be accurate with a good "burping" being necessary and want to make sure it is does correctly.