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Black waxy sludge in coolant #5099035 05/08/19 12:42 AM
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roachslayer Offline OP
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What would cause a wax like residue to coat the entire coolant system, reservoir, radiator, engine, etc? I’ve got a black waxy residue that will not flush out.

Another interesting characteristic is that the solidified residue liquifies when heated and floats to the top like oil. But goes solid again when cooled.

A significant factor: 200k miles, coolant probably never been flushed, aged Dexcool here in a 2006 Chevy 5.3l suburban.

I’ve identified these possibilities:
  • Burnt oil (head gasket leak)
  • Dexcool mixed with air and/or other contaminant
  • Dexcool eating at gaskets and seals


The problem with the head gasket theory is that compression, leak down and block tests all pass! No signs of combustion gasses getting into coolant. That said, I can’t rule this out completely... perhaps it’s a small leak that only happen under load?

I’ve never thought of oil becoming wax, but then again, I also don’t understand Dexcool chemistry. But then I learned that overheated oil actually does solidify, also known as engine sludge that can build up in the case due to being burnt. But never heard of this building up in the coolant! Unless perhaps it’s literally burnt and accumulating slowly over a long period of time through an undetectable head leak, now completely overtaking the coolant system.

Anyway, I’m frustrated. Even if my head gasket is fine, the engine is doomed as there is no way to get this crap out. Not even a radiator specialty shop offers a heated flush, which is is the only way this junk will ever have a possibility of leaving the system!

Anyone else ever seen anything like this? Ideas ?

Re: Black waxy sludge in coolant [Re: roachslayer] #5099057 05/08/19 02:50 AM
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Dave9 Offline
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Dexcool does that, it needs changed every 2 to 5 years (depending on who you ask).

IIRC GM had some chemical flush recommendation, then a neutralizing agent was put in then a second flush, though some have just ran the engine up to temp before pulling the hose to drain it.

Dexcool may have been eating at your gaskets and seals but if they aren't leaking yet, just keep an eye on them. At 200K mi they're not doing as bad as some intake manifold gaskets that tended to fail under 100K mi. (arguably) from Dexcool.

Re: Black waxy sludge in coolant [Re: roachslayer] #5099058 05/08/19 03:09 AM
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demarpaint Offline
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I've seen it in Dexcool cooling systems. Pressure test the cooling system to rule out a bad head gasket. If it passes muster Google Citric Acid Flush, and do that.


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Re: Black waxy sludge in coolant [Re: roachslayer] #5099096 05/08/19 05:08 AM
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Astro14 Online Content
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Sounds like left over stop-leak...


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Re: Black waxy sludge in coolant [Re: roachslayer] #5099097 05/08/19 05:09 AM
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I sense a shot at Dexcool here but the problem is 13 year old coolant. Also, plus one on the stop-leak thought.

Last edited by Snagglefoot; 05/08/19 05:25 AM.

If you want the job done right......do it yourself.
Re: Black waxy sludge in coolant [Re: roachslayer] #5099117 05/08/19 05:47 AM
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In my experience Dexcool is an excellent long life coolant when not messed with. The poor reputation it developed was from using the wrong material for the gaskets during the late 90's and early 2000's. This was taken care of many years ago.

If this sludge liquefies when warm, could it not be removed by draining the existing coolant, fill will water, run up to temp and drain? Repeat this water fill/run and drain a few times.

If the sludge is some left over stop then maybe there is some hidden problem addressed by a previous owner.

Re: Black waxy sludge in coolant [Re: roachslayer] #5099118 05/08/19 05:48 AM
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atikovi Offline
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So what problem is this "black waxy residue" causing? You don't mention overheating. Flush and fill and don't worry about it.

Re: Black waxy sludge in coolant [Re: roachslayer] #5099141 05/08/19 06:13 AM
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Does the truck have an oil cooler in the radiator?


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Re: Black waxy sludge in coolant [Re: roachslayer] #5099142 05/08/19 06:14 AM
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Chris142 Offline
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Have you put purple power in the cooling system,ran it then flushed that out?


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Re: Black waxy sludge in coolant [Re: roachslayer] #5099222 05/08/19 07:44 AM
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roachslayer Offline OP
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@atikovi, I forgot to mention, yes, overheating is the problem, even when little or no load, without a trailer.

@Chris142, yes, transmission cooler in radiator. Already tested and not leaking according to radiator shop. Also I dunno what purple power is.

@demarpaint, citric acid has no effect on this residue. Already tested that in hopes it would react.

@Dave9, I would love to know what chemical GM was recommending. So far nothing I try has any dissolving effect except dishwasher detergent, but again, it really isn’t very effective without heated water.

All, if I simply drain when hot, it will leave the black residue behind, I have been experimenting with this. Though liquified when hot, it floats in top and it tends to stick to the walls of whatever surface when the water is drained out from underneath it!

Seriously guys, this is a nasty, nasty predicament. I think without finding a chemical that will dissolve it AND prevent it from sticking to walls on its way out, I am screwed.

Last edited by roachslayer; 05/08/19 07:51 AM.
Re: Black waxy sludge in coolant [Re: Astro14] #5099228 05/08/19 07:47 AM
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demarpaint Offline
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Originally Posted by Astro14
Sounds like left over stop-leak...

I've seen leftover stop leak, usually it is brown, although in this case I bet there is oil mixing in too causing the black color the OP described. I'd pressure test the cooling system.


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Re: Black waxy sludge in coolant [Re: roachslayer] #5099259 05/08/19 08:16 AM
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roachslayer Offline OP
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Thanks, I will do a pressure test.

In the meantime, here are other coolant related factors:

  • The coolant didn’t seem low as if it’s been leaking out
  • No physical signs of leak
  • The radiator hose collapses flat once the system cools down.


Regarding that last point, to me it seems this means it has pressure, no leak, granted it’s actually negative pressure! I couldn’t hold a vacuum if it had an external leak. But it raises the question... why does it do that? It shouldn’t.

Also btw, worth mentioning, no water is in the oil at all.

Last edited by roachslayer; 05/08/19 08:18 AM.
Re: Black waxy sludge in coolant [Re: roachslayer] #5099471 05/08/19 11:28 AM
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Collapsed hoses are usually caused by a faulty thermostat. So there is one issue already along with the overheating. Also coolant hoses degrade from the inside out, in other words the hose may look ok but the inside may be completely worn out. You may have to change the thermostat and hoses, that would be a perfect time to completely flush the system. I would stay away from purple power (its a degreaser btw) since it WILL attack aluminum if you feel the need to do a flush Wynn's makes a degreaser/ coolant flush or simply search in google "best coolant flush to remove oily residue" and you'll get a shopping list of products you can use.. I have had DexCool in my 05 GTO since new and have never had any issues with it.

Re: Black waxy sludge in coolant [Re: Gimpy1] #5099533 05/08/19 12:34 PM
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Yep, I've got new hoses and thermostat ready to put in once the flush issue is figured out.

My overheating isn't likely thermostat related though, as I physically tested it in hot water, perfectly operational.

@Gimpy1, the issue doesn't seem to be oil, but rather, wax. Degreasers just aren't working so far. Hot water AND dishwasher detergent seems to be the only thing that will loosen this stuff. I've tested a couple off the shelf flush solutions so far - nadda.

Re: Black waxy sludge in coolant [Re: roachslayer] #5099598 05/08/19 01:21 PM
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Originally Posted by roachslayer
Yep, I've got new hoses and thermostat ready to put in once the flush issue is figured out.

My overheating isn't likely thermostat related though, as I physically tested it in hot water, perfectly operational.

@Gimpy1, the issue doesn't seem to be oil, but rather, wax. Degreasers just aren't working so far. Hot water AND dishwasher detergent seems to be the only thing that will loosen this stuff. I've tested a couple off the shelf flush solutions so far - nadda.


I HAD a Dexcool problem (past tense). Also had REAL dirty cooling system. Probably never changed on almost 15 yr. old car. I paid for a shop to flush my system (wallet flush). Then I used Prestone flush, BG coolant flush, then Irontite flush, then...Finally, I had a co-worker tell m to use citric acid. You can buy it in the canning section at Wal Mart or Farm Supply stores (it is used to clean water lines for livestock and to can tomatoes). It's NOT expensive. I used 3-4 tablespoons/gallon of water.
I spent an entire Saturday flushing my system; I think I flushed at least 8 times and it was STILL dirty. Used Citric acid and distilled water, ran till hot, drained the system, rinsed a couple times with ONLY distilled water...My cooling system now looks like new coolant. I would post you pictures (after each flush, I saved a plastic cupful) but I ended up recycling everything and I deleted the pics off my phone. Try Citric Acid.


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