Every time I check the coolant level in the reservoir it is bone dry so I'll fill it to the line on the tank. Then about a week later out of curiosity I'll check the level in the reservoir and it'll be bone dry again. I have never noticed a leak and the engine never overheats, so where'* all the coolant going?! I'd like to add that my mechanic said I have an intake manifold gasket oil leak. Also I've read that some GM models can't use green coolant but must use a specific kind of coolant (Dexcool or something) that rapidly breaks down the gasket. I've been putting 50/50 pre-mixed green coolant in so far, I hope I'm not damaging anything by doing this.

Good chance your LIM gaskets are leaking. How does the oil look? Changing them is not to hard to do and there is a write up on it in the tech info area. If you have DEX in your engine and your adding a green coolant you will have to flush the cooling system out. Mixing the two different types will sludge up and turn black. Seeing you have a old car I will assume you have high milage. Changing the LIM gaskets woudl be a very good idea.

Next time you start your car up, see if you can smell anything sweet smelling in the exhaust and more than average steam. Both are indicative of a coolant leak getting into the intake and then cylinders.

If the oil is milky or you have a milky film under the oil cap on the valve cover gasket...you've got coolant in the oil and you better not drive it - otherwise you will spin a bearing or two and need to rebuild the motor.

I'm no expert, but the oil looks like it should on the dipstick and on the oil cap (clear, consistent brown-ish color). I don't smell anything out the exhaust nor do I see any white smoke/steam. The only time I smell anything (and if anything) is if I'm going 70+ on the highway, and its very mild and doesn't last. I have no idea whether I'm running Dexcool or need to, but I read Dexcool was only an issue for GM 95s and beyond. Right now I have 122K and am leaning towards it not being an LIM issue. It might just be a slow leak that I don't notice when driving or parked. I've been driving with a bone dry reservoir for about a month now and have never had a hint overheating, but the engine does get to 200 deg quite quickly (within 3 miles of starting up). I'm thinking about a coolant flush anyway just for regular maintenance and the fact that summer is coming.

You could get your radiator cap pressure tested. If it'* weak you could be loosing coolant while hot. Also a good idea to have the cooling system pressure tested. That will give you an idea if there is a problem.

Well I'm taking it to the shop soon to get the supercharger coupler replaced (with an aftermarket one, of course), the serpentine belt replaced, and the intake manifold gasket oil leak fixed. While I'm there I'll have them test the coolant system and see what'* up.

I had a similar problem with my 93 Bonneville. I could not keep antifreeze in the reservoir but the radiator never went lower than one inch below the filler neck. I finally located the problem after months of searching. There is a plastic clamp on the hose that goes from the reservoir to the radiator. When the hose gets old it deteriorates and swells up and cracks. I simply cut the last inch of the hose off and reinstalled the clamp. Problem solved! Apparently the hose was seeping just enough to to leak without even dripping . The anti-freeze seeped out the hose ran on to the hot radiator and evaporated before it left any sign of the problem. It sounds too good to be true but I haven't added a drop of anti freeze in over a year.

Yes the LIM gaskets were leaking and apparently before I bought the car a year ago, someone had mixed Dexcool and green coolant, so it had turned to sludge. So I took it the shop and $670 later they replaced the LIM and did a complete coolant flush so all should be well now. I didn't see any of the signs in the oil that I had an LIM leak but apparently I did.

Ouch. Glad you got it fixed.