Hi people, this will be a lengthy and juicy one.
I have a Mercedes 2016 C63 S

It only has catless down pipes.
I’m trying to work out whether I’m dealing with a cracked head/head gasket issue or something else in the cooling system. About 2 months ago I had the thermostat and water pump replaced. It was driving well when one night it kept saying low coolant. I would top it up drove around for 15-20 min and it would say it again. If I turned off the car the coolant would drop rapidly again but I can’t see any coolant leaks??? If I leave the coolant cap off and let it run to operating temperature the coolant gurgles and bubbles over the reservoir.

I thought maybe the system needed to be bled properly. I took it to my mechanic he said he bled it 4-6 times and nothing changed. He reckons it’s a cracked head or a blown head-gasket which is weird because I don’t think these are renown for this issue at all! Can’t find anything on the internet regarding this issue.

There’s no milky oil
My coolant is still red
And there’s no smoke billowing out or misfires.

He’s taking it as the coolant is leaking back into the motor?? Or exhaust gases are getting into the coolant.

I let it idle for around 20 minutes and it stayed around normal operating temperature fine. The fans kicked on as normal, but when I pressed the accelerator and revved it a bit, the temperature started climbing and the fans became much louder. I’m not getting constant massive clouds of white smoke from the exhaust. I have noticed bubbling/pressure in the coolant reservoir at times, and I’ve been trying to bleed the cooling system for days in case there’s trapped air in it.

At this stage I’m trying to figure out whether this sounds more like a head gasket/cracked head problem or something like a thermostat, water pump, blocked radiator, airlock, or general coolant circulation issue.

 

Hope this helps:

 

1. Why ?
1.1 DIY?

2. Engine Cooling Circulatory System
or the other was repaired ?

3. From min to max reservoir is 750 ml till lamp glows (low coolant).

4. If you make to max. how much km/miles is it if it lamp glows again ?

5. Have you checked all Radiators ?

 

In all my years I've never heard of anyone having an M177 with a head gasket issue. It's not that!

The fact it started after thermostat/water pump work makes me suspect your mechanic didn't do a proper job or there's a bleeding/circulation issue now. But he thinks he's bled it 6 times? Alright. Now, you need a proper pressure test and combustion (block) test before anyone says cracked head. Surprised he hasn't recommended these next steps honestly, I'd bin this mechanic and start off new with someone who knows these cars.

 

@63samgbloke The proper way to fill the coolant is with a vacuum fill process. I'm skeptical of a mechanic who says they bled it multiple times. You drain it once, put the vacuum fill tool on the coolant tank and let it suck out all the air, then let the coolant flow back in under vacuum.

If you had a cracked head gasket that was leaking coolant into a cylinder you would have white smoke out the exhaust. It's also easy to inspect with a snake-camera in the spark plug hole because the top of the piston will be steam cleaned versus having a coating of carbon on it.

I think the first step you need is to take it to another European car specialist or a Mercedes Dealership to have the coolant system properly filled using vacuum, so there's no air in it.

 

You can do CO test in coolingwater.