Rotor=Motor

Hello, im taryn from longview wa. I bought a 87 rx7, had it for awhile, runs good, but found brown sludge in my coolant resevoir, ive read that it could be the coolant seals, or from using straight water, but idk for sure, would it still run if the coolant seals were out? It also idles pretty high, about 1500- 2000 rpms even when warmed up, ive tried to turn it down by the adjuster screw on top, it lowered it like 200 rpms, but fluctuates alot. Help please.