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Old 10-05-2022, 01:19 PM   #1
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Unknown substance in coolant overflow

I went to start our 2003 Coachman class C with a GM 3500 chassis and 5.7L engine over the weekend and started right up. I had the hood open and just happened to glance at the coolant overflow reservoir and it appeared to be empty so I took the cap off and peered in. There was something in there but it was not green but brown. I stuck a pencil in to see if i could see what its consistency was and it felt oily. I did a few internet searches to see if I could find out what it is and why and had everything from a blown head gasket to transmission oil cooler contamination. I saw a couple of searches where the remarks stated "don't automatically jump to the conclusion of a blown head gasket" and then they proceeded to name a few other possibilities. Anyone on here feel like adding their thoughts? We've had the coach for 5.5 years and it only has 28k miles on it (we bought it with 5k miles). Thanks, everyone!
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Old 10-05-2022, 01:31 PM   #2
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OK I'll offer this Stop Leak. Maybe the previous owner added some and you never noticed.
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Old 10-05-2022, 03:46 PM   #3
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Iron sludge. Probably due for a system flush and refill.
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Old 10-05-2022, 06:36 PM   #4
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My only comment there is after we took possession in 2017, I had the antifreeze changed and the trans fluid changed as we were heading out West from Fl. I wouldn't think something could happen that soon after flushing it five years ago and only putting on 20k miles. Am I correct?
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Old 10-05-2022, 07:26 PM   #5
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Do you have the older overflow tank with a flip open cap or a pressurized expansion tank with screw cap? It has been my experience that the tray at the mid-point within the pressurized tanks tend to collect solids that can be "goopy".
In any case, bring the level back up and if it was empty of fluid recheck it again soon as the system purges itself of air. Go over all your hose clamps as the hoses can become loose and develop subtle seeps or weeps. Some Chevy engines have high point air purge lines and an o-ring joint at the front of the heads that can seep almost only when hot. Good luck!!
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Old 10-06-2022, 06:51 AM   #6
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Check your engine oil and trans oil to make sure no water is getting in there.

Oil cooler failures are common.
Your radiator should have both TOC & EOC.

Cracked head or head gasket not so much with only 28k on the clock. ...unless you ran it hot.
Also, these issues usually have radiator boil over from the increased pressure.

BTW - Pretty sure an '03 should have a 6.0 rather than a 5.7.
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Old 10-06-2022, 07:28 AM   #7
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It sounds like common rust sludge. Clean the tank, fill up to the "Cold" mark and drive it.

It's a 19 plus years old engine that probably isn't used regularly, so rust can build up in the engine and is mixed up in the coolant. Coolant expands when heated, some goes up into the surge tank, the rust won't hurt anything as long as it's not excessive.

As others said, look for any leaks. If it was oil, you would have other problems.
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Old 10-06-2022, 12:10 PM   #8
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If the engine/ oil cooler/transmission cooler was leaking, when the engine is shut off pressure in the coolant system will push coolant into one of the oil systems, then suck in oil after coolant returns to air temperature if radiator cap is good. Check each ones level daily.

I lean toward unplannedtourist's explanation. Keep topping-off the coolant overflow tank until it maintains the cold coolant level for days.
Then check overflow tank contents again.
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Old 10-06-2022, 01:10 PM   #9
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Not sure if GM is like Ford or if this is your issue, but I had a similar thing happen. A previous owner had somehow caused a mixture of the green and gold antifreeze. This is not a huge problem, but not recommended as it can impede some of the protective effects of the antifreeze. The one you are supposed to use is dependent on model year.

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Old 10-06-2022, 08:38 PM   #10
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are you useing Dex-Cool, kinda red stuff.
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Old 10-06-2022, 09:44 PM   #11
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I would remove the tank and dump the stuff out. Then reinstall and fill with coolant and not worry about it. It could be left over stuff from the block casting years ago.
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Old 10-07-2022, 10:14 AM   #12
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GM used Dexcool (orange) in 2003. Looks like someone added regular green coolant. The Dexcool is a propylene glycol based coolant.
The two do not mix very well. I suggest having the cooling system flushed and the proper coolant put in.

If your coolant was actually low, look at the oil filler cap. If there is a lot of milky brown substance on the cap, there is a chance that coolant is getting into the oil. Not good.
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Old 10-07-2022, 10:46 AM   #13
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Originally Posted by dons2346 View Post
I would remove the tank and dump the stuff out. Then reinstall and fill with coolant and not worry about it. It could be left over stuff from the block casting years ago.
Our 2001 Silverado did the same thing. Removed the tank, flushed/shook it out with boiling hot water several times and flushed out the old Dexcool sludge buildup thoroughly. Then used a cooling system cleaner, ran it for a couple hours, flushed thoroughly again. Make sure to flush the heater core extensively also. If the heater core uses those plastic quick connectors replace them, as they get brittle. New Dexcool coolant and you're good to go.
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Old 10-07-2022, 11:25 AM   #14
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IF you indeed used the Dex-Cool you should Google the problems with it. it plugged my complete cooling system. once you see that mud looking sludge its probably all through your system. radiator heater core and so on, if its bad flushing won't remove it.
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