*URGENT: 2014 Mercedes GL550 (M278) Severe Coolant Leak After Overheat Warning*

Hello, I need immediate insight into a severe coolant leak on my 2014 Mercedes GL550 (M278 engine).

The car threw a "high heat turn off engine immediately" warning. The next day, after starting it briefly, showed a "Check Coolant Level" warning and confirmed 3 leaks.

Symptoms & Observations:

After driving just 10 minutes, observed high heat turn off engine indicator. Leaks from the middle of the engine area first, then both driver & passenger sides further back (approx. 6-8 inches).

Vehicle has 70,000 miles.

Questions:

What specific common M278 failures would cause an immediate, multi-location coolant dump?

Is the water pump or thermostat housing the primary suspect given the center leak location?

Vehicle has not been driven. Just need expert advice on what parts to inspect first and any other advice you can offer.

Many thanks!

 

First thing to look at is every one of the multiple plastic coolant lines connecting the turbos to the water pump. All in all they are very common and very easy to fix.

 

STOP driving the car to an overheat. Do Not drive it or start it again until the major leak is repaired. You were lucky to not kill the engine the first time. Many times since the sensors lag, by the time it shows the overheat, the damage is done.

To find the leak, fill the reservoir with water and pressurize it.

Good luck. It's not too hard of an engine to work on, just takes time and patience and the right sized tools. We are here to help, and most of us, as referenced above, have had to tackle the plastic coolant parts. Its a slippery slope so replace everything while you are in there which can include turbo coolant hoses and junctions, thermostat, heater "valve", firewall connection, and other coolant hose nipples attached to the engine. The intercooler can also leak and this can be found by boroscoping through the rear pvc port at the top of the intercooler. The parts cost is not expensive (except thermostats have skyrocketed) but if you have to take the car in to have this done to replace all of those parts at a dealer we are taking about 10k, perhaps half that at an indy familiar with the platform, and thats not to include an intercooler, which is a bit more rare to see.

editing to add that you can also do the same pressure charge on the turbo system reservoir if you have the split cooling.