Proper way to flush coolant from radiator

So I’ve got two different sets of instructions here, one from the bottle of Prestone Super Radiator Flush and one from the guy who sold it to me, who I really tend to trust as he seems to know what he’s talking about.

In a nutshell, here they each are


Car off and cold, remove cap, open cock valve, put garden hose in mouth of radiator. Start engine, adjust garden hose flow so it’s refilling the radiator at exactly the rate it’s draining. Run engine like this until the liquid draining out is clear water. Close cock valve, remove garden hose, pour in bottle of Flush, put cap on. Run engine until it gets to normal operating temperature. Turn off car and wait until engine cool. Remove radiator cap, open cock valve, put garden hose back in, and just like before, run engine and flush the system until it’s clear water coming out. Turn off engine, leave radiator cap off and cock valve open so it can drain completely. Close cock valve, and fill radiator with coolant-water mix.


Engine off, radiator cap off and cock valve open, drain the radiator. Close cock valve, pour in bottle of Flush and fill radiator with water. Put on cap, and run car with the heater on maximum hot temperature for 10 minutes after the engine reaches normal operating temperature. Shut off car, let engine cool, remove radiator cap and open cock valve and drain the radiator. Close cock valve, fill with water, put cap on, run for 10 minutes with heater on full blast after reaching normal operating temperature, like before. Shut down car, let it cool, remove cap and cock valve, drain radiator. Close cock, and fill radiator with coolant-water mix.

They both involve draining the original contents, running the engine hot with the Flush and water in a pressurized system, draining that out, and filling with coolant. The key difference is whether to use a garden hose and do the flushing with the engine running.

Which method is the way to go? Is it important to use a garden hose to flush it out, is it important to run the heater, both, neither? Do you use some other Method Three?

Oh and I should mention, based on my dad’s recommendation, I already got a bottle of water pump lubricant to put in after the flushing is all done.


Id say #2, you want to run your heater because of the coolent lines going to it. If not your leaveing the old stuff in there.

after your done with the flush and your draining everything, inspect all of your hose’s - while your at it syndrome kicks in here because its probably the best time to replace them and the thermostat.

if your radiator leaks let me know I gotta guy that does presure testing, flushing, recore and if needed welding on them in Hoffman Est.

i just did a complete system flush last night with the same prestone product. I flushed with a garden hose attached to the heater hose using an adapter, then i dumped the prestone in. it says on the bottle to drive for 3-6 hours then flush again and can be done over a few days so its still in there. i will flush it out again then fill with 50/50.

So two votes against flushing with the engine running, and zero in favor of it?

i should have been more clear, i did it with the engine running during the water flush except i had an adapter attached to the heater hose.

Ah! Thanks for clarifying.

My Prestone bottle makes no mention of driving. Interesting.

With engine off and cool, remove radiator cap.* Drain radiator then close draincock.
Pour entire contents of Prestone® Super Radiator Cleaner into radiator and fill with water. For systems larger that 12 quarts use two bottles. Replace radiator cap to fully closed position.
Drive a total of 3 to 6 hours (normal driving) - may be intermittent and over a period of several days.
With engine off and cool, remove radiator cap, drain system, and close draincock. Refill with water and replace radiator cap to fully closed position.
Run engine with heater on highest temperature setting, for 10 minutes after reaching normal operating temperature.
Stop engine and allow to cool. Remove radiator cap, drain system and close draincock. Add enough recommended coolant to achieve a 50-70% concentration. Top off radiator with water. Replace radiator cap to fully closed position. Run engine for 20 minutes to mix coolant/water.
If equiped with a non-pressurized coolant reservoir, rinse, drain, and refill with a 50-70% solution of coolant and water. NEVER SYPHON BY MOUTH.

  • The cap on newer vehicles may be located on a remote, pressurized overflow reservoir. Follow instructions given for radiators, but add product and coolant to reservoir. Much longer running time may be needed to introduce product into system.

So I ended up following the directions on my Prestone bottle.

Drain coolant =

Fill with water and Flush, drive around a bunch with the heater on full blast (it was in the 30’s so no problem there), drain =

Fill with water, drive around with heater on, drain =

Based on how crappy that water came out, I really wish I had access to a hose to do the thing where you keep water coming in and draining out until it runs clear… But it’s already getting down to the 20’s here and I just had to be sure to have good coolant in there.