My 92 civic si hatchback has been my daily driver for 11 years. Apart from a new timing belt and water pump and replacing plugs, wires, cap and rotor every once in a while all I've ever really done to keep up with it is get the oil changed. I recently got the bug to start cleaning and tuning it up in hopes to have some fun with it and get it running strong. One of the first things I noticed is that the coolant reservoir is completely empty and there is sludge in the bottom of it, I checked when the engine was completely cold. I can only assume it's been like that for a while since my last oil change was about a month ago. I know I should be checking this often, but I have not. I popped the radiator cap off and there is fluid in the radiator. I have to assume the system is really low on coolant, but the temp's been running perfectly in the middle of the range, even through the hottest part of the summer. Is it possible for the system to sufficiently cool the engine even when it is starved for coolant? Should I be concerned about damage done to the engine by running it so long with an emply reservoir even though the temperature guage has indicated normal the whole time? Does the sludge (coating about half the bottom of the reservoir and about 1/8" thick) indicate internal problems? I immediately filled the reservoir and I'm waiting to see how the level holds up to see if it's leaking somewhere. Also, I've never seen/heard the cooling fan actually come on either while driving or while letting it idle while working on it. I'm wondering if the engine is in fact getting hot and the car ( the temp guage and mechanism that turns on the fan) doesn't know/sense it.

I'm just getting started learning about this stuff so any help/input is greatly appreciated.

LB