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CHANGE EVERY TWO YEARS!! ESPECIALLY IF IT'S DEX-COOL
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Post CHANGE EVERY TWO YEARS!! ESPECIALLY IF IT'S DEX-COOL 
Good info on death-cool

http://www.sancarlosradiator.com/dex-cool.htm

"The "recommended" service intervals of Dex-cool coolant is 5 years 100K miles and in some cases 150K miles. These extended service intervals have not proven to be any thing but trouble for the consumer. In some cars, the coolant tends to turn to a thick, gooey, sticky, muddy, gritty, orange corrosive gel that fouls the radiator, heater, plugs coolant passages in the intake manifold and heads, and many times starves the water pump, resulting in overheating, and leaking. Mounting evidence suggests that Dex-cool reacts with plastic sealing surfaces, allowing leakage at the intake manifold gaskets.  The best thing to do without voiding any warranty, is to change your coolant every two years with the recommended coolant. CHANGE THE COOLANT EVERY TWO YEARS!! ESPECIALLY IF IT DEX-COOL!!  DEX-COOL IS THE WORST OFFENDER.  DEX-COOL DOES NOT LAST 5 YEARS OR 1000,000 MILES*. 

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Post Still using Prestone DexCool - Changing it every 3 Years 
I wanted to write my long term experiences for quite a while on this as I'm still using DexCool from Prestone.

My Truck was built January 1999. In March 2005 (with original coolant still installed and never changed yet) I had problems with the intake manifold gaskets. Loss of sealing due to deformation or the carrier material as well as deformation of the o-rings produced leaks.

I replaced the gaskets with updated ones from GM (GM#89017465) and since then running DexCool from Prestone, that I change religiously every 3 years. With every coolant change I replace the thermostat and the radiator cap with brand new OEM. I completely flush my system (remove the thermostat, flush it until clear tapwater comes out, drain the tapwater, fill in half-of-system-capacity undiluted DexCool, install thermostat and top off the rest with clear tapwater, check for concetration and adjust if necessary).

Since then my cooling system works flawlessly, the coolant that I'm changing always comes out as DexCool colored, but otherwise crystal clear and clean liquid. However the removed coolant usually does no longer test ph-neutral if tested with ph-strips (hence I can see of course changing every 2 years might be even better).

Based on what I could find so far on the internet, one possible issue with DexCool the additive 2-EHA, that purportedly affects the silicone content of gaskets and is therefore incompatible with such gaskets. Further it appears that DexCool von Prestone has the 2-EHA additive, while DexCool from Valvoline (Zerex DexCool) does not. This is evident if you check the safety data sheets of both coolants.

Personally, I will continue using Prestone DexCool. It worked for me, also the hoses in the truck look as good and as soft as new, so I think that 2-EHA could have to do with that.


PS: check out FelPro MS98000T gaskets. Those will be my next replacement if the currently installed GM#89017465 ones fail again.

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