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Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 #5429346 05/14/20 06:27 PM
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jac962 Offline OP
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95 Chevy G20 Van, 98k miles. SBC 350 5.7L. Recently bought from a third party seller. There's a rusty residue all along the coolant system. My first thought is that someone mixed dex-cool with regular green. Top of the radiator, inside of the heater core valve and thermostat house all look "rust stained". I didn't notice any sludge deposits or anything, it just seemed stained. Can't wipe it off or anything but no signed of deposits at least from the parts I replaced. i can't tell if this is the "sludge" they talk about with dex-cool and regular green mix:
[Linked Image]

I'm assuming no ones ever changed the coolant, not for a long time at least, and it's just gotten rusty from sitting. If it looks like the dex-cool was mixed with green let me know. Either way,I want to flush it and get everything out and I've heard of so many different ways to do it, just hoping for some clarification on what has worked best for people. I've heard:

  • Flush with straight water with a hose
  • 50/50 water and white vinger mix
  • fprestone radiator flush and water
  • cup of cascade dishwasher detergent, + water
  • caustic soda + water


Which one is the best option for my issue? Or at least, which will likely be effective and won't damage my system? Anything I should look out for when flushing? Assuming I need to flush from the radiator as well as the block. Haven't done a coolant flush in a while and the last flush I did was on a 325i... any help appreciated.

Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429351 05/14/20 06:36 PM
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bdcardinal Offline
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I would so the dishwasher detergent flush followed by a citric acid flush. That should clean everything out. Obviously do it with no t-stat in the system and maybe consider new hoses when its all over.


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Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: bdcardinal] #5429356 05/14/20 06:43 PM
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blufeb95 Offline
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Originally Posted by bdcardinal
I would so the dishwasher detergent flush followed by a citric acid flush. That should clean everything out. Obviously do it with no t-stat in the system and maybe consider new hoses when its all over.

So use the Prestone it's a sodium citrate solution.

Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429359 05/14/20 06:45 PM
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bdcardinal Offline
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We actually have a giant bag of citric acid here for really dirty systems. Downside is it uses a ton of water.


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Opinions expressed are my own.
Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429371 05/14/20 06:56 PM
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eljefino Offline
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Water!

The dish soap is if you get oil in the system from a head gasket failure. It's got chlorine in it so it's pretty corrosive.

The citric is if you get lime in the system from mineral-heavy water.

Leave the thermostat in, drain the system, refill with decent water. Not limey tap water but if it tastes ok I'd run it.

Drive to work. Drive home. Cool, drain and repeat all week.

Use distilled for the last water cycle if it excites you, then drain, then fill 1/2 total capacity with 100% dexcool and top with water.

Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429373 05/14/20 06:58 PM
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Fattylocks Offline
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I did a reverse flush with a garden hose when my LT1 powered '94 Chevy got a little cranky. Unbelievable brown slugfest.


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Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429381 05/14/20 07:13 PM
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Gebo Offline
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I'd do several distilled water flushes. Make sure you turn the heater to max hot.

Does your car call for Dexcool? If it does, I'd mix it 50:50 before adding. I'd then run the Dexcool for
a week or so and drain and refill with fresh 50:50 Dexcool:Distilled water. Then sleep easy.

Last edited by Gebo; 05/14/20 07:13 PM.

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Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429402 05/14/20 07:40 PM
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Chris142 Offline
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That is not a dexcool disaster. Once rust starts it wont ever stop. Just like a rusty car in detroit. You can only slow it down. That mess is caused by running straight water or letting antifreeze go too long.

The bottom of the water jackets are full of rust chunks. The water pump is much higher so draining it there will not get the chunks out.

You must remove the drain plugs or a freeze plug on each side to flush the water jackets out.

Last edited by Chris142; 05/14/20 07:41 PM.

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Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429421 05/14/20 08:06 PM
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jac962 Offline OP
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Thanks for all the help. As I suspected I'm getting mixed answers here. Sounds like most people are saying flush with water, backflush with water, lots and lots of water. I'll try that first and do the last flush with distilled and fill with 50/50. If it still seems to be gunked up I'll try the prestone and then if all else fails resort to another method. Thanks again.

Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: Chris142] #5429426 05/14/20 08:12 PM
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jac962 Offline OP
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Originally Posted by Chris142
That is not a dexcool disaster. Once rust starts it wont ever stop. Just like a rusty car in detroit. You can only slow it down. That mess is caused by running straight water or letting antifreeze go too long.

The bottom of the water jackets are full of rust chunks. The water pump is much higher so draining it there will not get the chunks out.

You must remove the drain plugs or a freeze plug on each side to flush the water jackets out.


Are you talking about the drain plug bolt in the block, or the freeze caps? I believe there's an actual bolt that can drain the block do I have to remove the freeze plugs too? Can't imagine it's much fun trying to get those out.

Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429449 05/14/20 08:49 PM
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Chris142 Offline
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Originally Posted by jac962
Originally Posted by Chris142
That is not a dexcool disaster. Once rust starts it wont ever stop. Just like a rusty car in detroit. You can only slow it down. That mess is caused by running straight water or letting antifreeze go too long.

The bottom of the water jackets are full of rust chunks. The water pump is much higher so draining it there will not get the chunks out.

You must remove the drain plugs or a freeze plug on each side to flush the water jackets out.


Are you talking about the drain plug bolt in the block, or the freeze caps? I believe there's an actual bolt that can drain the block do I have to remove the freeze plugs too? Can't imagine it's much fun trying to get those out.

Either. Which ones you can get out. Often the block drains are not acessable in the car. Easier to knock a freeze plug out. If you dont you wont ever get it clean.


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Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429481 05/14/20 09:40 PM
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clinebarger Offline
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Originally Posted by jac962
Originally Posted by Chris142
That is not a dexcool disaster. Once rust starts it wont ever stop. Just like a rusty car in detroit. You can only slow it down. That mess is caused by running straight water or letting antifreeze go too long.

The bottom of the water jackets are full of rust chunks. The water pump is much higher so draining it there will not get the chunks out.

You must remove the drain plugs or a freeze plug on each side to flush the water jackets out.


Are you talking about the drain plug bolt in the block, or the freeze caps? I believe there's an actual bolt that can drain the block do I have to remove the freeze plugs too? Can't imagine it's much fun trying to get those out.


One side will have a Knock Sensor in the block drain hole (7/8"/22mm) & the other side will be a 9/16" headed steel 1/4" pipe plug. Using a 1/2" drive impact with 9/16" wobble socket is the best way to remove the pipe plug, Replace with Brass!!

The odds of ruining the Knock Sensor is pretty high if original.


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Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429483 05/14/20 09:42 PM
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Jethro_Bob Offline
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"cup of cascade dishwasher detergent, + water"

Not this, they removed the phosphates decades ago for environmental reasons. It no longer has a high enough acid content to be effective in your cooling system.

Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429492 05/14/20 09:58 PM
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IMSA_Racing_Fan Offline
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I've used 2 bottles of Lime-A-Way and a Prestone flush T kit to unplug a heater core in a Lincoln.

Re: Rust in coolant system... which is the RIGHT way to flush the system? SBC 350 [Re: jac962] #5429534 05/14/20 11:22 PM
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On old brass/copper radiator cars, I still use oxalic acid and washing soda neutralizer. On a rig with an aluminum radiator, I normally don't use anything and just back flush with a LOT of water. If that doesn't cut it, a quarter cup of citric acid powder per gallon works a lot better than the Prestone cleaner. Just don't leave it in there too long.

If a fellow would just drain and fill the radiator with a 50/50 mix once a year or so you don't gotta worry about flushing at all.

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