Hi guys,
I have enjoyed this forum for couple of my projects, so I would like to share mine since I could not find it here. I got stranded in NYC due to a coolant leak /overheating and had to ship the car to Michigan where I live and take a one way rental. I got the car back today and dived into it right away.
At first glance this job looks like you will have to tear down the whole dash, but don't freak out like me. This is not the case.

Time: Less than one hour if you can extract the broken part in one piece
Tools: Basic hand tools/sockets plus extra long pilers

The part in question is 1669970659 / 1669971459. You can find some on ebay, otherwise from a dealer. The cost is roughly $20-$30. The hose that attaches to it is 1668304096, which is the first part number you will see to figure what part you need.

Process:
1. Remove large engine cover (just pull it up, snaps in place)
2. Remove plastic cover on driver side by the windshield that covers brake booster. Push it towards the windshield and pull up on the other size. It seats there pretty loose.
3. Remove the water drain pipe
4. Remove the engine compartment rubber seal (driver side part only). Just pull it in the middle and then keep pulling towards the end of the driver side
5. Remove plastic firewall cover (upper middle piece) by unscrewing two screws (8mm) and pulling it out. You will have to wiggle a bit to get it out. Now you should have pretty clear access to the clamps.
6. Remove/release pressure from the clamp firewall side using one hand and extra long pliers and pull the broken tube out first then the clamp. I recommend replacing the clamps at the same time.
7. Remove/release pressure from the clamp engine side and extract the broken part. IMPORTANT: Be really careful, if you break that piece in more pieces, they will go deeper inside that hose and let me tell you, getting these pieces out is no fun, which is what I had to do. I spent about 45 minutes shaking that hose and pushing it down to get all the pieces out and I am not still 100% sure all of it is out. I wish I knew which way the coolant flows there. If out of engine, you possibly can run some fluid out from there with the pieces while running the engine (watch the temp gauge and turn off the engine if it gets too hot!)

The installation is reversal to removal. I am waiting on my part and I will update this post once I get this done.
If any one can chip in and tell which way the coolant flows from the engine side it would be great. Also if out of the engine, at what temperature it should start flowing? I had the engine running for couple of minutes up to 80F on the temp gauge but no fluid came out.

Here is some pictures I took from the process:

Part in question is No. 20 (NOTE: If you have GL550 like I do, you will not have that other connection - I believe that is only on the diesel European engines, which this diagram is of)


Here is how it looks broken:



Removed brake booster cover and the drain connector pipe



Removed engine compartment rubber seal


Removed plastic firewall cover (upper middle piece) by unscrewing two screws (8mm) and pulling it out




Exposed part to be removed


Pliers used



Broken part out

 

I had that same damn hose broke off on my 2012 wagon. I had a local machine shop made a pipe oh of aluminum to replace it. I’ve planned to do the same thing to the gl550, but I’m scare to touch it. Because I know I’ll break more stuffs getting to that hose. I hate those freaking biodegradable plastic

 

Wanted to thank you for this! You just saved me $1500 or a lot of time! Appreciate it.

 

That's awesome Thanks for letting us know!

 

My 2015 GL550 with 80k miles currently seems to have all the plastic cooling system fittings breaking one by one. First the radiator was leaking last year, replaced it myself. Not too hard and installed Nissens brand radiator for $200 and some coolant.

A few months ago I replaced the 3-way hose off the reservoir because the plastic T broke, gushed coolant, left stranded. Fixed in a parking lot with $75 hose that was luckily in stock at the dealer.

Next, coolant line to turbos broke, gushed coolant, luckily I noticed it in the driveway. Towed to indy shop who replaced all the turbo coolant lines, thermostat, oil filter housing gasket, and re-seal of the cam covers that leak. All these are common to M278 / M157 V8s. I simply did not have the time for such a large job, so I paid the shop $3500 for this job. Thought I was good for a while. Picked up the car today, wife drives it about 6 miles and it gushes coolant and overheats again. I went to pick her up and found that the plastic piece through the secondary metal bulkhead had a chip broken off the end and the hose blew off. I called the shop prior to getting there and they sent their tow truck to fetch it back. Now this piece is nowhere near the front where they replaced all the turbo coolant lines, etc. but I'm wondering how in the world did that hose blow off so soon after this repair? (And why did the shop not give it a good long test drive after this big job?). I'm wondering if there is some pressure issue as the result of replacing the thermostat and all? If they try to charge me more than a small amount to replace the $10 plastic bulkhead tube and re-test, then I'm going to order one and do it myself in their parking lot.

After the radiator, I also had one of the pipes crossing under the radiator develop a leak from one of the "permanent" factory installed metal clamps. Replaced that line as well.

The cooling system seems to be the Achillies heel of this vehicle or all M278 / M157 due to all the plastic used. I would not expect all these random plastic failures at what I consider low mileage. I also have a 2015 ML63 with 99k miles where I have replaced the radiator, and the same coolant line below the radiator for the same leaking clamp issue. Now I want to buy another toy car and do the same big job of the turbo coolant lines and cam cover re-seal on my own time. The OEM parts online cost is only about $800 for what was listed on their invoice totaling over $1400. Even full retail OEM prices comes to about $350 less than they charged me for. I want to do this preventatively before it leaves me stranded. I'm also going to look for any other plastic lines in the cooling system that can be replaced on the GL, and maybe the primary radiator hoses which have plastic push fittings. Any one of these little fittings snaps and you are stranded.

 

My 2015 ML63 (64k miles) has recently started leaking coolant from this same heater hose connector from behind the firewall.

Just wondering, is there any need to drain the coolant before removing the hose clamps? Should I expect much coolant to escape when removing the hoses from this connector?

Here is a photo of the leakage, only a small leak currently, not gushing out yet.

 

This elbow is at the very top of the engine bay, so not much fluid should leak out. Just top up the tank.

 

The hose connection arrived today, went to install as per OP's instructions but quickly discovered this is the wrong part for my car. There are LHD and RHD versions of this part, and obviously I bought the wrong one (LHD).

1669971459 (LHD)
1669971659 (RHD)

Fortunately it's an inexpensive part, $30USD delivered. Still annoying.

Also, with the firewall pieces removed there is fantastic access to the rear of the cylinder heads, which is good timing as I have new plugs and coils arriving tomorrow, should make the job getting into the area of cylinders 4 and 8 a lot easier. I'm now wondering if this would make replacing the PCV valve a realistic possibility on the X166/W166, without dropping the engine. The E63 crowd seem to think so.

 

New hose connection elbow installed, easy job, except for reattaching the hose to the new elbow behind the firewall due to how the elbow bends off to the left in RHD variants, there isn't much room to get decent leverage to reattach. Looks easier on LHD variants as the elbow continues straight.




Have driven the ML today then re-checked for leaks (on both sides), so far so good.

 

I just want to add, if you change this part, you probably should also change #10 in the diagram A166-830-41-96. It connects to the connector at the firewall. It will fail. You will lose most of your coolant in a matter of seconds if you are driving and your engine will overheat. I had to get my vehicle towed as there was no way to do any kind of roadside repair for this.