After a couple of years, getting some residue from evaporation on the Expansion Tank is pretty much normal. When I drain and replace the Coolant, I often remove the tank and clean it well.
Short Answer: Change out your coolant with the Mopar Stuff or Zerex G-05 (aftermarket equalevalent). Thats what I did with my '02 Caravan (Mopar HOAT Coolant) and the gunk never came back, I also changed it every 3ys/60k miles, instead of the recommended 5yr/100k miles. Use Distilled Water.
DexCool does NOT mix with other anti-freezes, HOAT does, but its NOT recommended. i.e. a little bit of other coolants mixed in DexCool can cause big problems, a lot of other coolants mixed in with HOAT may cause some problems.
Chrylser dying their later HOAT the same color of the earlier more commonly know DexCool is Dumb, and people (who don't check their owner's manual, like an idiot) can mistake it for dexcool and add dexcool to their HOAT. That would create some problems, depending on how much dexcool you added.
DexCool has an entirely different approach to anti-corrosion, it will last for 5-6 easily, if your cooling system is functioning 100% and nothing else gets mixed in it, it doesn't get low or have leaks, etc. Basically, the slightest thing goes wrong with DexCool and the problem becomes a 10 times worse with Dexcool.
HOAT has sort a hybrid approach to anti-corrosion that is closer to the old proven green anti-freeze. Like the old green anti-freeze, you can have problems with your cooling system and that does NOT effect the coolant and its still protects adequately. BUT, its anti-corrosion additives get consumed while providing that protection, and once consumed the protection is reduced. That is why I've seen multiple recommendations to change out HOAT coolant ealier than recommended, like at 4 or 3 years/60k-80k miles, don't wait for the 5 years/100k miles recommendation.
In my '02 Mini-Van with HOAT, I've gotten that orangish sludge at the very top of the coolant level at the radiator cap, it was just a little. I drained all the coolant (NOT just the radiator), flushed with Distilled water, and refilled with Zerex G-05 (aftermarket HOAT anti-freeze) 50/50 mix with distilled water. (My normal coolant change). Never had the orangish sludge come back, done coolant changes every 3 years.
The slick, oil feel that the coolant leaves behind, I've gotten that everytime at coolant changes, even the fresh HOAT seems to have a little bit of that feel on the surfaces, but yes, it does seem a little worse with older HOAT. I've concluded (right or wrong) that it was just a property of HOAT anti-freeze.
HOAT uses the silicates and a tiny bit of phosophates (NOT sure, maybe its just Silicates) of the old green anti-freeze and some new anti-corrosion additives that coat the metal of the cooling system to protect. Perhaps the new addtives give the slick, oily feel.
DexCool uses a chemical that stops the ionic reaction that causes rust, it doesn't coat the any of the parts, thus it doesn't repair any of the damage from cavitation like other anti-freezes, and the chemical will soften certain plastics and rubbers often used in cooling systems, causing leaks. SO, you never want to use DexCool unless its in a vehicle specifically designed for dexcool. If the coolant level gets low in the engine and air is in the system, DexCool, since it doesn't coat the metal parts, has to be in contact with the metal to protect. So, with Dexcool, if there is air in your cooling system, the exposed to air metals starts to corrode and it corrodes fast, and the rust sluffs off and forms the orange foam and gunk.
Last edited by Mongo; 11-05-2010 at 09:24 AM.