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2008 s550 iridium silver
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Discussion Starter · #1 ·
Hi all,
My coolant suddenly began leaking and actually ran all the way out. I kept refilling w an eye on the engine temperature and was able to get it the 6 miles I needed to get it home.

My indie checked today and after a few cursory tests speculated that it might be the heater core (he also left me hoping it was a fixable hose).

I'm told this is a dealer part and after price checking they say it's in the 4k range (Pre-Labor).

Is this what people here are doing?!!?

I also see what are supposed to be "aftermarket?" replacements for $150 or so. What's the inside scoop?
 

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2007 S550
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164 Posts
Hi all,
My coolant suddenly began leaking and actually ran all the way out. I kept refilling w an eye on the engine temperature and was able to get it the 6 miles I needed to get it home.

My indie checked today and after a few cursory tests speculated that it might be the heater core (he also left me hoping it was a fixable hose).

I'm told this is a dealer part and after price checking they say it's in the 4k range (Pre-Labor).

Is this what people here are doing?!!?

I also see what are supposed to be "aftermarket?" replacements for $150 or so. What's the inside scoop?
Most likely it's a part that cost less than $100 to buy and less time than an hour to replace. Heater control valve located near the firewall.
 

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Outstanding Contributor W221 Moderator
2010 E350 P1/P2, 2008 S550 Designo, 2002 ML320
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2,360 Posts
I would guess the same as @EasyPhil has stated.

If it were to be the heater core, that does not appear to be sold as an individual part (looking at the parts catalog), so it would seem you have to replace the whole HVAC box under the dash. However, online pricing from MBUSA shows this at ~$1,000. (Note, it is the same part number for your 2008)

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2008 s550 iridium silver
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Discussion Starter · #4 ·
Thanks for the quick replies!
I tracked down a new heater control valve mentioned by @EasyPhil and have that waiting in an online shopping cart. Saw the indie today with a deposit and (diplomatically) mentioned this information. He's now putting off tearing out the dash until he checks further.

And thanks @dlafever for the core pricing (and partno). The best I previously found was $2k.

I should've mentioned this in my original post, but this is a fla car that's never seen a winter... and the 2 back seat climate zones have perfect heat.

Is that anything I should've factored in?
 

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2007 S550
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Thanks for the quick replies!
I tracked down a new heater control valve mentioned by @EasyPhil and have that waiting in an online shopping cart. Saw the indie today with a deposit and (diplomatically) mentioned this information. He's now putting off tearing out the dash until he checks further.

And thanks @dlafever for the core pricing (and partno). The best I previously found was $2k.

I should've mentioned this in my original post, but this is a fla car that's never seen a winter... and the 2 back seat climate zones have perfect heat.

Is that anything I should've factored in?
I'm going to give your indy the benefit of the doubt and say they just have not really looked at the car. This one is easy to diagnose, it's purging all of the coolant, you simply fill the system up with water, start the car and the leak location will be self evident.
 

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My 2008 S Class has 137K miles on it, do these heater contrrol valves fail often typically with this age and miles? Should I think about replacing mine just as proactive work so that I wouldn't have to worry about it failing? I'm famous for buying parts before they're needed, and sometimes never using them and then the car gets a new owner yet the parts remain in my garage. So, maybe not. In fact, I bought a new thermostat for the M273 a year ago "just in case". Crazy, I know.
 

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2008 s550 iridium silver
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Discussion Starter · #9 · (Edited)
@Tonye
The question of rear seat heat is one that emerged after further thought. That adds to my suspicion of an incorrect diagnosis. (Add to the fact that carfax shows that the car has never seen a winter)

BtW, I'm not implying malice... I just don't want to pay for someone else's lack of knowledge. I'm thankful to everyone for their input here... and have found an additional link from 2016 that treats this same issue
 

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2007 S550
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My 2008 S Class has 137K miles on it, do these heater contrrol valves fail often typically with this age and miles? Should I think about replacing mine just as proactive work so that I wouldn't have to worry about it failing? I'm famous for buying parts before they're needed, and sometimes never using them and then the car gets a new owner yet the parts remain in my garage. So, maybe not. In fact, I bought a new thermostat for the M273 a year ago "just in case". Crazy, I know.
The plastic degrades to the point where the valve fails by breaking apart where the hoses connect to it.
 

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Outstanding Contributor W221 Moderator
2010 E350 P1/P2, 2008 S550 Designo, 2002 ML320
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BTW, this problem with the control valve and the water splitters has been around since the W140 chassis of the 1990s. It is exactly as @EasyPhil describes, eventually the plastic fails. This is the reason I do a 100k mile pre-emptive service on my cars; radiator, water pump, idler, tensioner, thermostat, belt, hoses. Might seem like overkill, but here in the desert we see 100F+ for several months during the summer. It kills batteries, and anything rubber or plastic.
 

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Discussion Starter · #12 ·
I do remember changing this on my w140 and there was plenty of discussion about the need for doing so preemptively just like you say!
 
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