Sudden Overflow from the coolant reservoir

[ChevMali99]

Driving at a comfortable 70mph, traffic is moving then suddenly I'm caught in a bumper to bumper - stop and go traffic for almost 30minutes. Temp gauge oscillates between dead center to probably mid way towards the red. I get to my destination, Put the car on park and suddenly steaming antifreeze violently drains out the reservoir through the top hose on it leaving it entirely empty (actually hard to tell since it's hard to see the coolant levels through the material the reservoir is made off), or at least that immediate chamber.

Is there some sort of thermostat that may need changing?

[slls]

I would say your radiator coolant fans failed to come on.

[ChevMali99]

Uhm... I've read through the board and it seems that this is not uncommon for Malibus. I refilled the the reservoir up to the full line, turned the car on, and monitored the temperature and the fan. As the temperature rose to the previously mentioned level the fan turned on. At least that seems to be working.

Now here's something I didn't mention, while I'm sitting in B2B traffic I tend to put the car on neutral and shift back to drive once the traffic inches up. I do this often, especially when I feel the car's idle get a mildly rough. Could this action overheat the car? Is the mild rough idle a sign the temp is out of wack?

At any rate, about a month ago I replaced the reservoir with one off a junk yard. The original one developed a hairline crack and was constanly leaking. The replacement had lots and lots of that dex-cool black gummy resedue. I cleaned most of it by running hot water and picking out the chunks out of the thing with pliers and hooks. The question is - what is that gunk?!