Black waxy sludge in coolant

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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 ?
 
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Cincinnati, USA
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.
 
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34,654
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NY
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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Toronto Canada
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.
 
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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.
 
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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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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.
 
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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.
 
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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.
 
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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34,654
Location
NY
Originally Posted by roachslayer
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.
I did the LIM gasket on a 2000 Buick century and had the mess you describe. There was zero water in the oil, but oil in the water. The car was losing coolant, leaking it externally. Pressure testing showed where the coolant was leaking from.
 
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UPDATE: There is NO oil in the coolant. Screw opinions, I took to SCIENCE. Lab tested the coolant, found no oil at all. The issue is: -High iron corrosion in coolant -Heavy magnetic precipitate Apparently, this caused BLACK dexcool rather than rusty orange. Weird. But the issue is not BECAUSE of dexcool, rather, i believe it is the result of "normal" engine wear and simply not changing the dang coolant for the entire life of 200k miles and 13 years on this vehicle! Iron corrosion is said to be due to water pump and/or cylinder lining and block. This could be bad if the coolant showed this epic amount of corrosion inside a year or so, but should I be surprised at 200k miles? Again, NO issues with compression, so although aged for sure, I don't have any significant leak yet. Now the question still remains... how on earth am I going to get MAGNETIC sludge out of my freakin engine cavities? I mean this stuff is literally layering every square inch of everything from the radiator (of course) to the heater core, hoses, reservoir, thermostat housing... all ducting everywhere. It's caked on, and nothing seems to flush it.
 
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You are not making sense. Coolant gets hot. That's its job. If hot water and detergent cuts the sludge you already have a solution. Obviously the lab did not do a thorough test if they can't identify the waxy substance. Yes you probably have some iron after years running the same coolant but that is not at all waxy. The metal is due to not changing the coolant often enough. The waxiness is due to Dexcool. You are making a mountain out of a mole hill. Dump some hot detergent solution in, run the engine a minute to circulate it. Flush it out. If your hoses are degrading, replace them. Same for the radiator. They won't last forever, especially if the coolant is never changed and extra especially if it's Dexcool that was never changed. As far as the engine itself, there is no reason to expect doom. You don't report any problems with it including no overheating so why would it get worse from flushing out what you can? It won't. If you would prefer to get some citric acid and put that in with your hot dishwater, go for it but don't leave it sitting in there long and then flush again with baking soda then again with clean water. Realize that finding heavy magnetic precipitate in the coolant, means it's coming out with the coolant. That does not mean you have this big magnetic barricade blocking up your engine. Whatever corrosion has already happened has happened and there's no need to put more thought into that - just to do the timely maintenance the prior owner apparently did not do. Just flush the thing and be done already. You are not the first person to deal with a vehicle whose prior owner let the maintenance go for a long time. As far as some others who state Dexcool is a long lasting solution, even GM themselves state the lifespan is 5 years or 150K mi. Yes GM used some questionable gasket materials but even if they did not, Dexcool is not supposed to stay in a vehicle more than 5 years before it's replaced.
 
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Sunny Florida
Blaming Dex is typical but not likely. We run all our fleet trucks to component failure here before any service to the cooling system based on decades of experience. This can easily equal the OP's mileage estimate of 200k, sometimes more. All cooling system components look like new on teardown. And before someone gives out the old line "there's air in the unsealed system", our Silverados are sealed overflow bottles and our vans are not. I seriously doubt that Dex can be the cause, but it is easy to use as a scapegoat. And collapsed hoses indicate a bad radiator cap or a bad thermostat to me as well...
 

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So if you know it's iron … didn't folks flush with TSP back when big iron blocks were common … ? The dishwasher detergent trick took a hit when phos got the boot …
 
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