How does your coolant level look? Reason I ask is because if coolant is coming out, then air is going in. Back when I had a cracked radiator, I was losing coolant like crazy, but couldn't tell where it was going. Everyone was telling me that I had a blown HG, but there was no evidence of such. One day I rigged up an old radiator cap to a bike pump and pressurized the system to about 5 psi. I did this in my garage with the engine off so I could hear hissing and find out where the coolant was leaking from. Turns out my radiator had a hairline crack that was covered by the expansion tank. You could try something similar, or you could just let the car run until it reaches operating temperature and try to see if you can find where it's leaking from.
Might just be a good time to replace the entire cooling system
http://www.318ti.org/forum/showthread.php?t=18066 I know it sounds like a lot, but there's a couple other things you can do while you're at it that will make things easier and save you time and money later down the road. First, if you're replacing the two plastic coolant fittings described in this thread, you don't have to remove the intake manifold to access the octopus fitting on the driver's side of the engine block. A lot of people remove the intake manifold and end up with bigger problems than the one they were trying to address initially. Instead, remove the alternator and the oil filter housing. The reason I suggest this is because the oil filter housing has a paper gasket and two o-rings that are known to leak. Doing this will not only give you the opportunity to replace those items, it'll also give you access to the plastic coolant fitting without having to remove the intake manifold.
The other thing, if you're replacing the plastic "Y" fitting on the back of the head by the firewall, it's a good time to replace your valve cover gasket and spark plug seals. While you have the valve cover off, it's also a good idea to clean out the orifice towards the back that the CCV hose attaches to. Removing the valve cover will give you more room to fit your hand/tools behind the engine.
The cooling system on our cars is known to trap air. Especially when there's a leak or if you open the coolant loop to replace a part. Trapped air will work it's way into the radiator which will eventually cause vapor-lock(air-lock). Vapor-lock prevents coolant from circulating through the engine and heater core. It can cause all sorts of issues: fatigue/failure of plastic cooling system parts, fatigue/failure of the head gasket, damage to temp sensors, over pressurization/fatigue on hoses, damage to thermostat, damage to water pump, engine overheating ultimately resulting in a blown head gasket and possibly a warped or cracked head, damaged cylinder walls, pistons, valves etc.
That is why it is extremely important to PROPERLY bleed your cooling system any time you suspect there's air in the system. One symptom linked to air in the system is if the heater doesen't blow hot air. If it blows warm air, you probably have air in the system. if it only blows cold air, then you probably have complete vapor lock. When the engine is at operating temp, the heater should burn your hand if you hold it in front of the vent.
The entire cooling system including the radiator, hoses, water pump, thermostat, plastic fittings and other parts I mentioned could be purchased for under 400 bucks and swapped out over a weekend. This will give you peace of mind that your valve cover won't leak, spark plugs won't be submerged in oil, oil filter housing won't spray oil all over the front of your engine, and your car won't overheat. I'm one of those "while you're in there" "do it right the first time" kinda guys. I don't like doing things over and throwing random parts at a car hoping to fix the problem.
There's an oil filter housing writeup, a valve cover writeup and a cooling system bleed writeup in the knowledge base section.
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