What is that dark discolored deposit inside the coolant recovery reservoir/tank?

What is that dark discolored deposit inside the coolant recovery reservoir/tank?

I've wondered for years.  The deposit appears granular in nature, and more toward the upper half of the degas bottle, aka coolant recovery tank, aka reservoir bottle.  (all terms used for key word searches).

The best answer I've come up with so far is a technical service bulletin from GM that stated that the GM factory used to install stop leak tablets in the cooling system at the time of manufacture.  The residue on the upper part of the otherwise translucent coolant reservoir is a byproduct of this stop leak tablet dissolving.

While initially, the deposits were isolated to the top half of the recovery tank, I now find that the entire tank interior is becoming discolored.

Coolant is otherwise very pink.  The discoloration is on the inside of the plastic bottle, but not in the coolant itself.

2005 model year with 62K miles.

Do you know more about this discoloration?

Got Picture?

Good point.  Hang on a minute.  (Posting pics for me is as enjoyable as root canals).

Ok, here is an up close view of the icky dark colored granulated deposit inside the coolant recovery tank / reservoir / degas bottle.

The view from outside the formerly translucent tank.

With the tank half way removed, to inspect underneath.  Decided to increase pic size.

Have an '07 with Almost 6 years on the original antifreeze......no discoloration of tank or deposits or floating crap in the tank........dealer said that it called for a change at 5 years but since everything looked OK and it hasn't had anything added since new I could go for another year. Probably because I never added any more antifreeze or water.

Deposits usually start building if you add anything but distilled water....dissolved salts in well water, hard tap water or additives added by the water treatment plant to soften the water and control microbial growth can do a number on untreated aluminum and other metals.

Also, I'll spend the extra bucks and get the AC Delco OEM antifreeze......not because it is better, but to be sure that there is no reaction with additives or colorants of another brand.......my favorite was a the recall of the first year ('91?) Saturns with the new "permanent" red colored antifreeze........seems if you topped off or added a different brand to the OEM mix.....it would turn the coolant to a Jello like compound and gum up the cooling system and could overheat and seize the engine. The solution was to flush all traces of the OEM coolant and install the traditional antifreeze mix.

Thanks for you input!  It made me realize that I need to provide more details about the discoloration in the photos above.

1.  The discoloration began to appear in the first year of ownership.  I did purchase it brand new, and am the only owner.  At first, the discoloration was too slight to mention for warranty.  Eventually, it was too late to mention it.

2.  In the 7 years since I've owned the truck, I have NEVER added ANYTHING to the coolant system.  No water, no antifreeze, no additives.  Nothing.  The first time I removed the cap was to take the photo above.  63K miles, BTW.

3.  The discoloration has a granular looking texture and 3D quality to it.  It isn't just staining, its... its.... ALIVE!  I don't know if it is mold, or fungus, or metal deposit  leached out of the walls of the cooling system, or some type of stop leak additive that GM installed in the factory (a possibility derived from a GM technical service bulletin that advised dealership service deptartements that GM did this for a period of time on certain vehicles).  I have not touched the residual substance, because it is beyond the reach of my fingers, and I didn't want to jar the deposit loose until I was ready to fully drain the coolant system.

I really would like to know if anyone else in has seen something similar in their coolant recovery reservoir tanks.  I'm trying to figure out if this is unique to my vehicle only, or commonplace among similar vehicles.

2003, Z-71, 120,000 mi.  No similar crud in my reservoir.  HTH

You noted your AVY has 63K on it. Have you ever changed the coolant since it was new? The Dex-cool has a life span of 5 years OR 150,000K and it's critical to change it at either interval. I changed mine (2005 Z-71) at the five year interval with around 21K or so and did not have the sludge in the recovery tank. Do you do a lot of towing? Maybe the heat has something to do with it. Maybe it was topped off at the dealership with tap water before you purchased it (tap water will contaminate it for sure). Back in 2010 when I changed my coolant I came across a very good article on Dex-Cool. Go to getahelmet.com and click on maintenance; under articles click on "why your antifreeze is red", very informative information on Dex-Cool. If it is microbial, I would remove the recovery tank and flush it out with water, then add a 1/3 bleach mixture to clean it. Hope this helps.

Another thought......granular like sand........my first guess would simply be crap from the engine casting process that was hung up in some some spot in the water gallery or heater casing and not completely flushed prior to the build. If an escalating condition could even be deterioration of the  water hoses.

Previous post correctly points out that tap water or stop leak could have been added by the dealer. I just remembered that In both Avy's I've owned since new the coolant level has always gone down in the reservoir in the first few months and I've always topped off with the GM OEM stuff purchased from the dealer.....never thought much about it and attributed to air embolisms from the initial fill up surfacing from the nooks and crannys in the system......a common occurrence after a new fill. Is it possible that the dealer topped it off with tap water at the initial service inspection under warranty  and routinely threw in a little stop leak to prevent a future problem??

If you've wondered for years this doesn't appear to be an escalating problem......only way to know for sure would be to remove and flush the bottle, refill and watch......but it would probably be a whole lot of trouble for nothing and wouldn't help to solve the cause anyway.

Yes, GMalanche, it is true, more than 5 years have passed and I have not changed (or even touched) the DexCool.  You are a better maintainer than I, changing out your coolant at only 21K miles precisely at the 5 year mark.  Thank you for sharing your experience as well as the article link on DexCool.

That being said, the problem the photos above are illustrating occurred years prior to the 5 year mark.  It began from the beginning, but was only noticeably objectionable after the warranty expired (typical luck).

A complete coolant change is on my todo list this spring, and I think I will pre-emptively replace the hoses as well, in case they are deteriorating inside.  I'll probably even purchase a new coolant bottle, because I don't see how my current one can be cleaned thoroughly enough, even with bleach.

However, like Thomcat pointed out, I can do all of the above, and STILL not know the root cause of what created this condition in the first place.  So, prior to undertaking the work, I am looking for answers and experiences from others who have encountered the same type of discoloration in their degas bottles.

I remember when Jason of JP Customs was in business, he sold a black plastic beauty cover for this recovery tank, to make the engine compartment look cleaner.  I never bought one, because it would obviously negate the purpose for which the recovery bottle is translucsent... in order to keep an eye on the coolant level.  However, now I'm wondering if the reason why there was enough interest in such a cover for a company to manufacture it had anything to do with a commom symptom of discoloration in GM vehicles back at the time (between 2000 and 2005).

I still hope that someone else will see this post and say... YES, that happened to me too, and will be able to share what they discovered, or comiserate with me as we endeavoer to find the root cause together.

You're welcome CoveredAV. Look at my reply in the "Coolant Flush" topic, showing the way I came up with to completely flush the cooling system. I've been doing this for years on other GMT800 platforms without any negative effects at all, and it saves a lot of time too. When you do yours, remove the lower hose and rub your finger on the inside; if there is a large amount of black (rubber) on your finger, that could be a sign of deterioration. Unless the hoses are bulging, soft when squeezing (not returning to their original shape) or have cuts / abrasions they should be okay. I replaced the lower hose on mine due to one of the transmission cooling lines rubbing against it (I put a length of hose cover on, as it SHOULD have been when assembled at the factory); other than that, the hose was fine. You will know for sure if the black in the tank is microbial by washing with the bleach mixture. If it is, it will vanish within a few minutes.

Just checked again on my '07.....it appears that it is not a coolant recovery tank, but what they call a "surge" tank. There are no segments in the tank but what appear to be hoses that supply and withdraw coolant through the system and with no radiator cap on the radiator itself, the radiator cap is the pressure cap on the top of the tank. The head space above the cold fill line serves the same as the head space in the old sealed radiators and just like that the pressure cap on the tank will vent excess pressure or liquid. Since fluid is constantly moving in and out of the tank there would be no place for crap to build up even if it was in the system.

Came up with an idea for a quick change...since it is not a recovery tank, but actually an in line expansion tank....when cool, siphoned 1 gal from the tank right into an empty jug and poured in a gallon of new GM Dex.....once up to temp the antifreeze cycles through the tank into the block....of course it will dilute out with the existing mix, but it will still retain the same antifreeze properties and restore new anti-corrosion and water pump lubricants to the mix. A LESS THAN 5 minute no mess operation and gets me through the year.

Old post but did you ever find the cause?