99 C280 heater blowing air, but no hot air
 

Hello, I am new to this forum. I am thankful for a site like this because it helps me make decisions. Anyways, I have a problem that I could not find a solution to here. My benz is a 99 C280 (Freaking love it). Here's the story:

First off, the part of the expansion tank that connects to the radiator hose is cracked so it leaks. I have to constantly put water as the water is leaking. Ok so keep this in mind.. And yes, I am actually going to replace the expansion tank tommorow as I just a new one today.

While waiting for the expansion tank, it was still neccessary to drive around because i have no other car. you can see why i always have to refill the expansion tank with water since it was leaking. No i did not over heat the car. anyways, I was told that a way to keep the engine cooler was to keep the heater on. As you can imagine, I was driving with the heater on because I wanted to keep from over heating my engine because the expansion tank was leaking. So, while I was driving yesterday, I heard various knocks coming seemingly from the dash. It sounded like either bolts snapping, or something snapping under the hood. It sounded as if a bolt snapped, or tore apart, because what it was supporting was too heavy. And it wasn't just one snap, it was like 3 or 4. Well after that, I noticed that the heater stopped blowing hot air.. After reading some post, I want to clarify that IT BLOWS AIR but not HOT AIR on heater. The A/C does work FYI. That is my problem.

what was those snaps or knocks or whatever, and why is my heater not blowing out hot air. Can anyone please suggest to me what to do. Thanks in advance :)

P.S. Anyone have a guide for replacing the expansion tank. It would be very helpful for me tommorow.

Your problem is coolant loss...
 

As in many modern cars (Jaguar S-Class and BMW 7 series come to mind) the Mercedes C of this era has a high-mounted (relative to old systems many of which were below the engine centerline) heater core.

The heater core thus can be a good indicator of low coolant level. Without your expansion tank, the system needs a good "burping" after you refill it. That means removing the coolant cap on the engine.

Without opening my hood this is memory - but my C has a large bulbous plastic fill cap without markings that is up at the top front of the easily-removeable engine trim cover.

Pop off your trim cover and look - you'll find a spot where you add coolant to the system AFTER you properly repair the expansion tank. Only when the system is without air pockets will your heater and defroster work properly.

Do not drive this car much - and do not trust your temperature gauge 100%.... replace that tank NOW<