Your coolant must be purple/pink OAT and nothing else. Mixing coolants will create the sludge you found. We always recommend factory sensors and parts when available. Someone else here had the same fan and overheating at low speed issue. I don't remember what his resolution was but you can search for it. What's happening, as you probably know, the high speed is pushing plenty of air through the radiator to cool the engine but the fan isn't doing that at low speed. It's entirely possible that your fans aren't running fast enough in general which causes the overheat at lower speeds when the hot engine should still be calling for max fan speed. You should have a fan module mounted to the fan bracket. If you don't then they put the wrong fans in it. You also want to ensure they put in the heavy duty cooling radiator and not a V6 radiator or a non-heavy duty from a prior year. It could also be a faulty temp sensor isn't triggering the fan high speed.

 

The issue has nothing to do with speed, as I can make it do it sitting still , going 10 mph or going 60mph. Here is a link to a video showing the issue .

 

I have seen head gasket failures cause. The fire ring on the gasket fails in a place which connects it to a water jacket. Under normal normal driving the cooling system can maintain as there is not a ton of heat entering through the failure. When you are on the throttle hard the exhaust gases overpower the ability of the cooling system. It basically creates high pressure steam. This can be easily tested with a “sniffer” here is an example. Good luck.

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Thanks for this tip. Took it to the mechanic and got them to do the "sniffer" test and it's the head gasket.

 

Jason:
Hopefully the current T stat is oem. When the rpms go up, so does the heat generated by the engine. For some reason, the cooling system can handle the heat when it's being driven gently, but not when you are working it harder.

Making sure that the radiator fan(s) are operating correctly is a box that should be checked asap.

Don

 

Sorry to hear, but at least it is now figured out and you can get her fixed!

 

Jason:
As shelbymopar89 says, sorry to hear the news, but at least you know now.

A couple of thoughts. Unless your budget is really tight, I'd have them do both sides. How many miles on it currently?
If you are anywhere near or over 100k, I'd have them eyeball the lifters since the heads will be off anyway. If they look good, leave them be. If not, throw in a new oem set while they are accessible.

Don

 

Thanks Don. They are doing both sides. And yeah. We are well over the 100K mark. She sitting at about 187k.

 

Ok.
At that juncture, assuming the current lifters are original, if you plan to hang onto it, I'd probably do the lifters anyway just because you are right there. Going back in 6 months later paying the same labor again would suck.

Don

 

agreed it would. I’ll talk to them tomorrow and see just how much more that’ll cost me.
Thanks for the input. I plan on keeping her. Love my Durango!

 

If you had the same old normal thermostat put in. . . Buy a FAIL SAFE OPEN style. They're more expensive, so it's a new engine. I booked my old one and a new one, normal style, and they both got hung up on the spring. The FAIL SAFE opens have a sleeve that eliminates this malfunction. Trust me, I know only from a same issue.