Thermostat Failure?
 

Has anyone experienced this before?

Typically, my car runs between 180-190F when driving normally and will jump into the high 190 to low 200's on a hot day while in stop-and-go traffic. Well, about a week ago, when the ambient temps rose into the high 80's and I was stuck in traffic, the CLT went up to about 213 - which was a bit alarming. After I got home and let the car cool down and checked the radiator and found that the level was low (which kinda made sense since the temps were higher than normal). Anyway, after that incident and re-filling the coolant, while going to work I noticed that the CLTs were lower than normal - like 160-175 at most.

My question here is; a) Has something like this happened to anyone else, and b) does this mean that my thermostat is screwed? My guess is that the element in the thermostat that opens it got damaged somehow and it's stuck open and not managing the temps correctly.

One important fact; I have a coolant re-route that's been in place for years and I've never had this happen before. Because it's a re-route, the thermostat is on the back of the head and is relatively inaccessible with the engine in the car. I damned sure don't want to have to deal with trying to take that apart and risk it leaking (because I couldn't properly get to it). I'm seriously considering putting in an inline thermostat in the upper hose just to be able to regulate the CLTs again.

Thoughts?

Buy a supermiata reroute, thermostats are easy to access. The ones on the back of the head are indeed a pain in the ass. The inline ones take FOREVER to regulate temperature. They oscillate like crazy, from ~215 to 160.

I will say that the SM one has a decent sized bypass hole that keeps it from warming up quickly, so something to consider if you drive it on the street and/or winter often. It otherwise maintains ~196 (the default thermostat) degrees very well.

Thanks curly. I'll take a look at the SM one.

We recently reduced the bypass hole on the Qmax reroute to speed warmup to more closely match OEM rate. Helps the NB's from throwing a P0128 code for warming up too slowly. Doesn't affect peak flow.

Oh genius, not that I would swap just to get a smaller bypass...that would be ridiculous...

As to your original question OP, it does sound like a failed thermostat. It initially fails when hot, then when it cools, it sticks open instead of re-closing. Theres a chance it's radiator cap related, but unless you were pissing coolant out of the over flow, you could probably rule that out. A failed cap also wouldn't cause the steady ~20* drop in operating temperatures.