Air in the cooling system?

[1998_K1500_Sub]

Yep, turned out to be missing the hole in the thermostat. I got a new one that isn't a custom brown color. Thanks for all the input.

So why the custom brown? Rust in the system at some point in its history? You ran (only?) water in it for a time, didn't you?

I've never seen a thermostat discolored like that in a good system, but maybe I've lived a sheltered life.

FYI, that's where my temp gauge runs too, coincidentally.

 

[SAATR]

So why the custom brown? Rust in the system at some point in its history? You ran (only?) water in it for a time, didn't you?

I've never seen a thermostat discolored like that in a good system, but maybe I've lived a sheltered life.

FYI, that's where my temp gauge runs too, coincidentally.

I have a hard time believing that you've never seen a GM vehicle's cooling system painted Dexcool Brown.

 

[1998_K1500_Sub]

I have a hard time believing that you've never seen a GM vehicle's cooling system painted Dexcool Brown.

Looking closely at the picture it looks like a rust accumulation to me.

I ran straight water in an engine once for a short time and I was surprised at the crap that appeared in the water (coolant) in short order... that's when I came to appreciate that anti-corrosion additives are in antifreeze to do a job.

(edit)

That darkness on the thermostat looks like fine rust particles, e.g., from the block, that mobilized in the coolant and settled-out / accumulated on the thermostat... any by my reckoning, other parts of the cooling system such as the radiator tubes.

If you say it's Dexcool then I'll believe you

 

[SAATR]

Looking closely at the picture it looks like a rust accumulation to me.

I ran straight water in an engine once for a short time and I was surprised at the crap that appeared in the water (coolant) in short order... that's when I came to appreciate that anti-corrosion additives are in antifreeze to do a job.

Most of the engines I dyno run on straight water for a few hours, so that rusty tinge is familiar to me.

Looks like the red/brown crust of an old Dexcool motor to me, but you may be right. Confirmation bias being what it is, I see brown crud in a Vortec, I jump to that conclusion.