Hi folks,

My w212 e200 2.1 is losing coolant - rapidly. It will go through half of the expansion tank after a short 30km spin. The car has 500,000+ kilometres. I understand leaks are generally either external or internal. Hears what I know so far:

External:
The car has been pressure tested for over 2 hours and there was no loss of pressure whatsoever. No visible coolant leaks anywhere. My mechanic was bewildered and said he could do no more.

Internal:
No milky oil, no white smoke from the turbo, and no fruity smell from the oil or the exhaust. My understanding is that this rules out a head gasket leak.

After doing some delving, it seems the heater control valve and the heater core can cause coolant loss. I'm just finishing removing the rain covers under the wiper linkages and will be able to inspect the valve and its hosing tomorrow.

Has anyone experience with the heater control valve / heater core sucking coolant?
Are there any other possible culprits?
The car has been without coolant in the coolant tank for quite some time; is it possible that the engine is playing 'catch up' to replenish lost coolant?

All help is greatly appreciated.

 

The cap relief valve of the expansion tank is known to leak. It never shows up on pressure tests because then it's removed during tests...

If you can't see the leak or smell it, mix in some UV die to make the leak shine under UV-Light.

Good luck with it 🤞

 

E550 here, it was my cap.

 

Stealth or intermittent issues are hard to diagnose but I'm glad you don't see milky oil which is another great testimony of Mercedes-Benz engine . I take it from this perspective as well . Well, I had coolant loss in other cars and replacing the radiator solved the problem for good

 

Does this car have the transmission fluid cooler integrated with the radiator? Leak in that would let coolant leak in the transmission.

Leak being this severe would show in transmission fluid volume so removing the drain plug would show if volume is up.

But first that reservoir cap should be replaced as it surely can let coolant out if the spring in it has got weak.

 

UPDATE:
It's not the radiator cap. The coolant in the reservoir tank is emptying merely minutes after its filled - even without the engine running.

 

There's a gurgling sound when I turn the ignition which sounds like coolant running through the system.

 

Car is a manual 6-speed, so no auto transmission. The clutch, flywheel and rocker cover were also recently replaced, if that matters.

 

engine oil heat exchanger?

 

Possibly, but I'd imagine there'd be some sort of visible leak underneath.

 

I re-read your OP, you've done your homework, that's an interesting case!

Pressure tested, manual gearbox, no drips, no crankcase mayo.... 500KKm

 

Long shot, but is it leaking into the washer fluid reservoir? If it's going down while not running and not leaking underneath, not going into the oil, it must be contained somewhere, if it was going into the transmission it would be coming out the breather on top of the transmission and dripping out on the ground.

 

I read in WIS, some car with heater option does run its coolant pipe to washer fluid bottle to warm up wiper washer fluid.... Nice one Pierre, it is a possibility.

ILJ wrote :
UPDATE:
It's not the radiator cap. The coolant in the reservoir tank is emptying merely minutes after its filled - even without the engine running.

How many liters have you lost/top-up so far ?
Now keep filling up the coolant, just in case what your are experiencing is air void , lots of it. However if exceeding 1 liter top up, something is probably wrong.
How long by mileage have you owned this car when you started "loosing" coolant ? Just in case you bought the car pre-owned.
Was there any work related to removal of coolant before this "loosing" coolant event ? Previous owner or yourself.

I believe your aftercooler is air to air, so unlike engine with water to air aftercooler/charge cooler, your charge cooler uses no coolant.
Not a suspect there.

The HVAC heater core leaking coolant and dumped along with condensation water is still a possibility.
Are you already using the blue coolant or still amber color coolant ? Amber coolant if mixed with HVAC condensate water may seems like just water.

If you can use vacuum pump to evacuate air bubbles in your coolant system circuit, it may indicate leak by vacuum decay or/and it may help remove more air bubbles and if indeed no leak, your coolant "loss" issue may stop there then.

 

Over 5 litres gone so far. I am trying to access the HVAC heater core, but the rubber on top of the linkage bolt is stuck.

 

I would never say never, but if it was leaking in the interior from the heater core, it would smell sickly sweet in the cabin and if it was coming out of the condensate tube from A/C I think it would leave a puddle underneath, I wonder if its accumulating on the underside panels, then dripping off while moving? Wow, this is interesting, five liters is a lot.

 

It may not necessarily be leaking from the heater core, but all the coolant may be getting sent TO the heater core via a faulty heater control valve.
Or, the heater core is faulty and is demanding all the coolant in the system. But it's most likely the former.

 

Try checking the heater core. It might be leaking inside the cabin thats why its not leaking out the engine. Hopefully you get to resolve this.

 

You can diagnose a leaky heater core under 5 ses... even a pinhole would turns the whole car into a humid steem bath with wet condensation over the windshield. 🤪

As of now: no leak can be found...
The system may just have been half-empty by a dry O-ring

 

This is actually wrong. The heater core's valve can fail without any smell or symptoms in the cabin. The valve isn't actually in the cabin, its still in the bay, under the windscreen and behind the wiper linkage.

See below, the valve is on the left of the second picture. A leak in that vicinity will simply spill onto the fabric and dry in. Can't be seen from underneath, above, OR in the dash.
This still remains the prime suspect.

 

yes, that makes sense. The valve is kept external and leaks inconspicuously... but then that's not a core leak, right?

 

People (myself included) seem to be using heater core / heater control valve collectively as its one system, but truth be told the W212 apparently has a redesigned HVAC system from the W211 setup in the pictures above, so I won't actually know how its setup until I've the last cover off. I'd imagine there's still some sort of valve/hosing connected to the cabin though.

 

Rear of engine block to HVAC heater core if for M276 is as follows :



Item plastic pipe 1 and 2 of left image. And then right side image hose no 180 ( same as your photo ) which is engine block plastic pipe to intermediate firewall/bulkhead/heat shield and then to hose 150 . Hose 150 ends at the aluminum pipe of HVAC heater core which is at the true firewall of the car.


Visual of hose no 180, I called hose 1. Your photo shows this hose unclipped.





If hose 2 ( item 150)

Hose 2 is the hose no 150.


Visual of hose 2 ( no 150)



End of hose 2 ( #150) then goes to heater core block, which is also where the HVAC expansion valve is.

Left image with no 2 is the heater core block, the shiny aluminum is the refrigerant TXV / expansion valve.
Right image with no 2, is the hose 150 starting point at the intermediate bulkhead/heat shield. The hose has its own rubber bulkhead fitting, as per your photo.

So those are locations are where leak can occur. Total 2 plastic pipes and 2 rubber hoses.



There is one more last one though, hose no 250 ( most right side diagram ).
This is hose and pipe combo from HVAC heater core to coolant circulation pump no 10.
I call this as hose no 3




In your car the P/N maybe different.....

 

I do not know anything about your OM651 cooling system, but MB likes to share similar design layout for W212 different engines, not 100% the same but
its a good guide.

Since your car has achieved the insane 500,000KM ( awesome !!! ) and if you intend to keep it, I am sharing my 10 year preventive program for your
reading pleasure. Find attached my excel file.

To me any plastic and rubber hose which carry coolant, oil, vacuum ( brakes and turbo boost ) are critical and if I want headache free ownership
10th -20th years , the file represent what I will replace regardless of their condition by year 10th. I have not decided yet if I would proceed ownership past 10 years. I got 2 years to decide by 2024. Labor for components difficult access is the nightmare, the components themselves price wise would probably be
okey.

I have not even consider in the list my radiator and aftercooler, which its sides are plastic

Anyway we put it, your car have done 500,000KM, you are already a winner today ....

 

Left taller pipes (2), that is to heater core. This huge HVAC box / system is a Left Hand Drive car one



Heater core stand alone, yours would be very similar this one from W204

 

At 5 liters loss of coolant and it pass pressure test, I think the test method is flawed or lacking of pressure... what was the pressure used ?
Do a 1.35 BAR pressure test, because our radiator/recovery cap is a 1.4 BAR one.
The actual leak may only leak at higher pressure...we don't know.
If you need to, do pressure test from engine off to while engine is running...rev it up if you need to.

It does not make sense a 5 liter loss can't be detected by pressure decay.

If your leak is only at higher pressure, that means engine running condition and car is moving.
If the leak droplets falls to very hot parts it will evaporate and you wont be able to track it.

My stand alone turbocharger coolant system has a super minor leak and it will leak only when I pushed the car super hard.
Like fun at track or extended redline RPM.
The culprit is the o-ring at the quick coupler of the small coolant recovery bottle for the turbo.
The only trace I can find, due to super small the leak is and only when car is driven hard, are the bluish stain of the blue coolant I am using.
Since the leak is like at best under 50 drops, only the stain is the give away.
The cooling fan also blows away the minor leaky coolant, so no wet liquid to trace, purely the blue stain of a blue coolant.

I tried changing the o-ring but could not find exact dimension and still is leaking super minor.
So I have to buy complete hose assy ...dang. Just because the o-ring is not sold separately.

ADD: Due to the loss of coolant is so very very little on the turbo cooling system, 2+ hours pressure test can't show anything too. 20 drops is 1 milliliter approx. 1 ml is 1 cc air volume equivalent. My gauge can't read 1 cc ( 1/1000 of a liter ) air loss on a system worth easy 2.5 liters.