Flushing radiators

What is the best way to flush old antifreeze from the radiator??

The best way is to have a coolant exchange machine connected to the system so that all the coolant is replaced. Even that in the engine block.


Does anyone, other than me, sometimes use this procedure:
Simultaneously have the radiator draining through the petcock, while a garden hose is pouring water in the top of the radiator, while the engine is running?

Let it run for a while till clear water comes out the petcock, then shut everything off, let whatever water wants to drain, fill with prescribed amount of antifreeze, then top off with water.

Although the cold water from the garden hose wants to keep the thermostat closed, it seems the hot water from the block wins, allowing the dirty fluid to flow out to the radiator and drain out.

I usually just drain what’s in the radiator…Then refill with water…turn vehicle on and heat on…turn to normal operating temps…Then drain again…repeat 2-3 times.

You want to flush the complete engine, not just the radiator. Back when I did mine I would get a flush kit adapter, remove the thermostat and drain all I could, then follow the directions for the flush kit. I do not do it anymore because it took an incredibly long time to get the water to run clear, and it is not good to have dogs or kids around that stuff, not to mention illegal in some places to route it to a storm sewer. Then a new thermostat, coolant per calculated need then top off with water and check mix. The repair shops in our city send the stuff to the sanitary sewer for proper treatment. My best way is take it to a reputable mechanic.

The old way was to flush through the heater hose, clean out the heater core, engine block and let the water come out the top of the radiator. It’s important to flush out the heater. The mechanic’s machine does all that, and it’s evironmentally safe.

So that’s what I do now. Cooling systems should be flushed, not just drained.

I would buy a flush and fill kit from an auto parts store. After it is installed, you can connect the hose directly to your cooling system and flush it.

Good point. I forgot the caviot about having the heat turned on to flush the heater core also.

Just out of curiosity how do you deal with all the antifreeze water you are flushing. (note previous response)

I generally collect it in a 5 gal bucket and flush it down the toilet. I’m on county sewer, so I let the treatment plant handle it.

step 1: call your local authorities and ask them
step 2: do what they tell you to do

No! BK is right!

I can also dispense of old coolant down the toilet or any other drain inside the house. It’s carried off to the water treatment plant where the ethylene glychol is broken down from the UV rays from the sun as the waste water is held in the mixing pools. This is even stated in brochure the city sends out to residences each year on how to handle hazardous waste.


Thank you both for not poisoning the drinking water.

One can’t run anti-freeze down the sewer system here or it’s big trouble from the code enforcement police.

A now deceased friend of mine who owned a shop was working one weekend in his shop with me present and chit-chatting with him. He had a 351 Ford Cleveland sitting on a drain grate near the door of his shop and tore this engine apart as it sat on the grate. There was probably less than a gallon of residual coolant left in that engine and it ran down the grate when the heads were removed.

Two weeks later the code police showed up (door closed at the time) and told my friend that he had been dumping coolant down a drain and their tests showed it came right from that door. At first he did not know what they were talking about until he remembered that engine teardown and then it hit home.

He was given a written warning and told that if it happened again it was going to be a 500 dollar fine for starters. It’s kind of amazing that in a sewer system and town that big that a few quarts of anti-freeze could be tracked right to the exact spot where it entered the system.

That is one of the reasons I don’t do my own cooling system flushes anymore. I pay a professional to drain and refill the system every two years. If I was going to to this job myself, I would switch to a safe coolant/antifreeze that won’t pollute the drinking water. Otherwise, I would collect it and dispose of it properly.