Mine chews through some coolant here and there. The spark plugs show no sign of coolant and neither does the oil. Engine temps are fine, and I notice a leak coming from mine every spring... still don't know where it comes from, but I'd imagine it is the culprit. A compression test can tell you a lot. If your head gaskets haven't been replaced, it might be wise to do so. Pay close attention to details and you will find the cause eventually.

 

Its helpful to take a Sharpie or similar ink marker and start marking the coolant levels. Start now and continue to check the level every few days, and re-mark. Then you can really figure out how much you are losing.

Usually if it is near the throttle body you might have a little bit of residue on the valve cover.

Likely head gasket, but may as well check everything else first.