My add coolant light came on. I noticed the overflow reservoir was completely empty. When I opened the cap and looked inside, I could see coolant in the pipe/tube all the way at the bottom of the reservoir, however, covering the entire bottom portion of the reservoir was this brown/light brown clay gunky stuff.

I have since bled air from the coolant system and topped it off to a normal level and the overflow tank level is where it should be but the brown gunk is still at the bottom. I don't know what it is or what to do about it. This is on a '86 NA with 110k miles, my oil pressure gauge stopped working a month or two ago but the car has been the same as far as power/driveability. Any thoughts?

 

The gunk is just buildup from years of not being changed. It's a mixture of sediment from the water, corrosion from inside the engine, bits of rubber from the hoses, etc. Just pull the resevoir out and wash it down. Or if it's inside the bumper, use your hose to wash it out.

Now, find your coolant leak so the buzzer doesn't sound again.

 

Rust and mineral deposits from within the cooling system. Clean it out and use distilled water to mix your 50/50.

 

awesome, so it's normal for a poorly maintained 20 year old car. Thanks for the replies.

Edit: I've had the car for less than a year. I was told all the fluids were routinely changed. I don't think any of the fluids have ever been changed judging from all the other fixes I've had to do to this thing that could have been easily avoided with proper care. So sad.