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How long is too long, radiator flush

1998 chevy Blazer LS

The heater didn`t work so I flushed the heater core with just plain water.
The heater blew hot air for about 3 or 4 minutes than stopped.
I flushed it again with water and it worked for about 5 minutes then stopped again.

I flushed it a 3rd time using CLR,then plain water it worked for about 10 minutes than blew cold again.

i figured that there must be crap in the radiator that was circulating and blocking the heater core so i got some gunk brand radiator flush and poured that in the radiator.
It said to leave it in for 15 minutes and then drain and flush the radiator with water.
I poured it in when i was at the store and drove 30 minutes home, by the time i got home it was dark, so my question is will it hurt to leave it in the radiator until tomorrow?

If the gunk radiator flush doesn`t work would it hurt to put CLR in the radiator and run the engine for 5 minutes or so?

I’ve used a commercial brand radiator flush product (as I recall it was made by Prestone) on my truck, and it seemed to work ok. I’d never put CLR into my truck’s cooling system tho, no telling what it would do to the gaskets. Better to trust a name brand product and follow their directions to the letter. One problem I had after using the product I used, I soon begin hearing a rattling souind coming from the water pump. I had to remove the water pump and clean out some debris that clogged in there. Can’t say for certain the flush product was the cause tho. After that I stopped using anything that plain water for the flush & drain, then filling up with a 50/50 mix.

BTW, it’s impossible to get all the flush product out on the first drain. Refill and drain several times to make sure it is all out of the cooling system before putting the final mix in.

Don’t use CLR or Lime Away in an automotive cooling system.

Both products state not to use it in systems made of aluminum or copper.

I’m pretty sure the heater core in your vehicle is made of aluminum.



You need to get it out asap. The stuff eats away at rubber.

I don’t think your problem is something clogging the heater core, I think you are not purging all the air out.

Never tried, but I’ve heard that Cascade (not the scented one) works great also.

In terms of a chemical flush that is made for automotive use, you need to follow the instructions on the bottle that tell you how long to leave it in.

If you’re using a chemical that isn’t designed to be used as an automotive flush (like CLR), that’s a bad idea. Get it out now and hope you’ve lived a righteous life so whatever deity you worship (if you worship a deity) blesses you with luck.

If, after flushing the heater with water and an automotive chemical flush, the heater doesn’t work, you need to get it diagnosed and stop working on it based on assumptions.

Let this serve as a lesson to others that the proper way to avoid a clogged heater core is to routinely drain and refill the coolant at regular service intervals. You should never need to flush the system if you drain and refill often enough.

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Today I drained the radiator,filled it with plain water, ran the engine for about 15 minutes then drained it again, I did that 3 times to get all that radiator flush out.

Then I flushed the heater core again but this time I took the hoses off.
There was dried rusty crude inside the heater core pipes,i scrapped it off.
The hoses didn`t feel very flexible so I banged them on the cement driveway and I heard crackling crunching sounds,when I held the hoses up a lot of hard shiny brownish crude fell out. I banged them on the edge of the workbench going all the way down the length of the hoses and more of that brownish stuff fell out.I flushed them with water and put them back on and the heat works great.I drove around for about a half hour and the heat remained hot and consistent.

I`m going to flush it with water again tomorrow and put antifreeze in it and test it again but I think I found the problem and fixed it.

I just bought this truck and i noticed that the previous owner had put the green antifreeze in it, but it`s suppose to use the orange dex-cool.
since i have it flushed out pretty good should i use the dex cool or keep using the green stuff that was in there?

could the green antifreeze be causing all this junk in the cooling system?

Congrats on your success. And this is a sincere question/comment for your situation: did all the crud get removed from the inside of the hoses?
Because if any remained, bits can and might break off and start moving through the system causing more blockages.
It’s a little like breaking up clogged arteries in people. Gotta be careful that bits don’t move to smaller blood vessels (like in the brain) and clog them.

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You can use dexcool, the orange dexcool compatible Prestone or any major brand universal long life coolants. Avoid the cheap “dollar store” house brands as they may have silicates in them which would damage your cooling system.

New heater hoses might be a good idea…



You should use whatever you want. If it was my vehicle, I’d use the green stuff simply because it is cheaper and I would plan to drain and refill it more often than some people would recommend. A warranty is likely not an issue with this car, so you should only use Dexcool if you plan to leave it in there a long time. If there is any kind of warranty still in effect, I’d use Dexcool.

In order to make sure you have all the chemical flush out of there, I would purchase a flush-and-fill kit from an auto parts store, the kind that allows you to hook up a garden hose and run it to force water out the open end of the radiator, but considering how many times you’ve flushed it already, that might be overkill.

I have reverse flushed heater cores sometimes to get them to work better. Remove both hoses, turn the heat control flow valve to full on, and figure out a way to connect a garden hose to the outlet side of the core, turn on the faucet, and water and gunk will come out the inlet side.

@tardrex, do us a favor will you, if you blow out any rubber hoses (radiator/heater) over the next couple of months, let us know. At the first blown hose, let us know which brand of chemical flush you used.

I’ve got a 2000 VW Polo. I will soon flush the radiator and put in new coolant. After adding the flush solution, the instructions say I need to run the engine for at least 10 minutes before flushing the system several times with clean water. Question: does this mean I need to drive the car around or is it sufficient to let it idle? Thanks!

Idling is just fine. You do not need to drive the car.


Suggest not to use a flush agent unless you know for sure you need to. There’s more risk than benefit otherwise. Or at least ask VW if they have tested the seal material used inside the engine for compatibility with the flush agent you are going to use. Perhaps VW will recommend a particular flush agent they know is compatible.

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That was my thought too. As well as a new heater core. And a T-stat. Maybe even a new radiator and radiator hoses.

I really think the current fix is temporary. Serious crud comes from contamination and/or corrosion in the system, and both are signs of bigger problems on the way. Just regarding the heater core inlet and outlet pipes alone, scraping the crud off the pipes’ insides doesn’t clear the crud off the core’s tubes… and if there’s that much crud in the heater core, I’ll bet there’s crud in the radiator too. And on the thermostat.

In short, I think the entire system needs to be thoroughly gone through by a good radiator shop.

Thank you. In the end I didn’t flush the radiator but added one can of water pump lubricant and one can of radiator leak plugger to the new coolant. Now car works fine. No coolant leak from the head gasket.

Glad you are back on the road w/a continent vehicle. Good for you. Keep an eye on the coolant level and the coolant temp gauge as those leak plugging agents can sometimes work for a while, then start failing. You want to catch that in advance so major engine damage is avoided. Replacing a few gaskets is much easier & less $$ than replacing the main engine bearings.

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Agree with @GeorgeSanJose

From recent experience of about 2 years, I had a mysterious and extremely slow intermittent coolant leak in an older car. It could have been the water pump and could have been something more major.

Had a coolant change come up and so used some plug material. Lasted about a year or so with no leaks. Then sudden faster leak confirmed to be the water pump shaft seal. The old water pump had the plug material around the seal on the inside (and some that made it through the seal to the outside), but it eventually gave way.