Dirty Coolant

Got a 2001 Monte Carlo SS (3800 V6).
What would cause the engine coolant to become dirty? I noticed some "sludge" (for lack of a better term) around the top of the radiator
cap the other day and immediately had the coolant flushed and filled when I had Jiffy Lube change my oil.
Is dirty coolant a normal occurance, and is a flush/fill a proper way to address the issue, or is this a sign of serious problems to come?
Thanks for any info.
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Read this link,
http://www.imcool.com/articles/antifreeze-coolant/dexcool-johnbrunner.php
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Yours is a 3800 series II engine, I believe. People here will correct me if I am wrong, but you know the inherent issues with the decomposing FI plenums on these engines, dont you? (When ours went bad, water went into the cylinders, but bear in mind that if you have a plenum failure, or head gasket, or a crack, strange and unpleasant things can happen.)
Assuming your engine is sound, antifreeze does, eventually, deteriorate, even that Dexcool crap. It should be flushed out and replaced with a good antifreeze at recommended intervals (or before). IF your system is not functioning properly and you get an air leak into the coolant, corrosion will start sooner and more severe than you might think. Rusting iron plays a number with antifreeze, particularly with the Dexcool crap, which is based on organic acids...organic acids form salts with iron which are nasty, goopy, ugly.
You might want to do a little diagnosis or detective work and not just assume that a flush will bring you back to health. That could be all it is, but it might be signs of other things going on.
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What type of coolant do you have in the system? I'd use only distilled water, especially with today's new coolants.
Besides the Dexcool related issue, also check head gasket.

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Up to milage in the mid 50K's I know for sure there was Dex-Cool in the system - because it was the original stuff. Then I had it flushed and filled (I know that's way earlier than needed, but it's a long story having to do with NTB stripping the threading on my radiator when doing a transmission fluid flush/fill, and then them paying for a new radiator for me). I *believe* when the new radiator was installed the mechanic put in traditional antifreeze due to concerns that Dex- Cool apparantly eats away at the head gasket (I could be wrong, and need to double check my paperwork). So from milage mid 55K to recently (120K), I didn't worry too mauch about it, until a low coolant indicator turn on. That's when I noticed the dirt buildup around the radiator cap. After my most recent flush/fill (few weeks ago) there is dex-cool in the system.
So if I understand that link posted earlier, one culprit for the contamination may be a faulty radiator cap?
"The culprit: low coolant! All vehicles that showed contaminated cooling systems shared one common condition: low coolant. Low coolant level in the radiator allows a “beachhead” type of deposit accumulation in much the same way that ocean waves deposit sand on the beach. This material can collect on the drop-center valve at the bottom of the radiator cap and prevent it from sealing. Without that seal, the coolant boiling temperature is reduced to 226°F. Sealed to 15 psi, it will not boil before 265°F. The lack of pressure and premature boiling allow an already under filled coolant reservoir to completely empty itself and leave the radiator tank with a substantial air pocket or beachhead. Always test the pressure cap and replace it if it fails to hold proper pressure."
I have a feeling that it would be prudent for me to take the thing to a professional to have it looked at.
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After my most recent flush/fill (few weeks ago) there is dex-cool in the system.
*************** Dexcool is not the panacea many would believe. The early versions were pretty poor, actually.
Essentially ALL the good and premium quality antifreezes on the shelves nowadays are totally compatible with Dexcool.
I dont believe your problem came from having Dexcool in the system, and certainly not from having the radiator flushed and replaced with another good or premium brand of antifreeze.
It is still somewhat in question whether Dexcool damaged gaskets, or whether the poor quality of gaskets that GM used were the bigger part of the problem. There were a lot of rumors about Dexcool, not all of them founded in fact.
It is not in doubt that the GM series II 3800 engines had a prone to fail plastic fuel injection plenum, and that the gaskets that GM used were also prone to fail.
In those engines, I believe it is not a matter of whether the plenum will fail, but when it will fail. The gaskets are, IMO, somewhat less of a problem, although I have opened some of those engines with relatively low age and mileage and have seen some ugly gaskets.
You problem might have nothing to do with gaskets or plenum. That is why I suggested a bit of detective work first. A really good mechanic -not necessarily a dealership mechanic - should have the experience and equipment to avoid a lot of trial and error.
I hope you will tell us how this finally plays out.
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Besides the old news about Dexcool damaging cooling systems and engines (see: http://www.consumeraffairs.com/news04/2006/08/gm_dexcool.html ), air in the system, tap water, and mixing coolants (prior to Dexcool compatible formulas all over the market these days) were not good news.
If the radiator cap's vacuum valve was plugged, then you would probably see collapsed radiator hoses. As soon as you remove the cap, air would rush back in and they swell back up.
If you see bubbles in the reservoir when hot, then the head gasket may be leaking.
Sure, get a competent cooling system shop to look at it.

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I finally took the car to the shop, to a guy I trust, and it turns out the lower intake manifold gasket had failed. So, I had that replaced, along with the upper gasket, valve cover gaskets, spark plug wires (127K miles) along with some other minor stuff. I think it might be time to look for a replacement car (hoping that I can have this one last 6-8 more months to get milage out of this repair!).
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Unless there are other problems, there is no reason to believe it won't be worth running longer. The intake gasket problem is very well documented, and your replacement should gain you many thousands of miles of life out of the car. If you like the car and other aspects of the car seem to be in good shape, then press on and enjoy the car.
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-Mike-
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Listen to Mike. I agree, cars are just nuts and bolts. Keep the repairs up, rust down and drive 'em. A car only is unusable when you decide not to maintain it anymore. Change the fluids and filters on time, regular.
Mike Marlow wrote:

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