W278 Cooling - Behaviour (2012, 180 000 kms)
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2012 E550 178 W212
W278 Cooling - Behaviour (2012, 180 000 kms)
Thoughts on temperature fluctuations going from 90C to 105C within like a handful of seconds? And then skyrocket into the red.
Engine also seemed to be struggling right before I turned it off. It would move, but very slowly. I pulled at barely 800 rpm, creeping out of the way of traffic safely. .. Had it towed, about to start to taking things apart.
Have done a coolant dye test, don't see anything with the black light around the reservoir, under the car, around the thermostat or radiator. All dry as a bone. Unless it's under the manifold, where I can't see it?
Have also done a combustion/CO2 leak test in the coolant reservoir - negative. May run it again.
Thoughts on what would cause the 10-15 temperature fluctuations in 3-4 seconds time? Faulty water pump? Air in the lines? Leaky coolant line? Thermostat failure? ... has fresh oil change, fresh plugs, new air filters, extracted and refilled coolant a 50/50 dilution
Any collaborative thoughts would be greatly appreciated.
Engine also seemed to be struggling right before I turned it off. It would move, but very slowly. I pulled at barely 800 rpm, creeping out of the way of traffic safely. .. Had it towed, about to start to taking things apart.
Have done a coolant dye test, don't see anything with the black light around the reservoir, under the car, around the thermostat or radiator. All dry as a bone. Unless it's under the manifold, where I can't see it?
Have also done a combustion/CO2 leak test in the coolant reservoir - negative. May run it again.
Thoughts on what would cause the 10-15 temperature fluctuations in 3-4 seconds time? Faulty water pump? Air in the lines? Leaky coolant line? Thermostat failure? ... has fresh oil change, fresh plugs, new air filters, extracted and refilled coolant a 50/50 dilution
Any collaborative thoughts would be greatly appreciated.
Out Of Control!!
Joined: Apr 2019
Posts: 11,924
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
HEAT COUNTER MEASURES
Thoughts on temperature fluctuations going from 90C to 105C within like a handful of seconds? And then skyrocket into the red.
Engine also seemed to be struggling right before I turned it off. It would move, but very slowly. I pulled at barely 800 rpm, creeping out of the way of traffic safely. .. Had it towed, about to start to taking things apart.
Have done a coolant dye test, don't see anything with the black light around the reservoir, under the car, around the thermostat or radiator. All dry as a bone. Unless it's under the manifold, where I can't see it?
Have also done a combustion/CO2 leak test in the coolant reservoir - negative. May run it again.
Thoughts on what would cause the 10-15 temperature fluctuations in 3-4 seconds time? Faulty water pump? Air in the lines? Leaky coolant line? Thermostat failure? ... has fresh oil change, fresh plugs, new air filters, extracted and refilled coolant a 50/50 dilution
Any collaborative thoughts would be greatly appreciated.
Engine also seemed to be struggling right before I turned it off. It would move, but very slowly. I pulled at barely 800 rpm, creeping out of the way of traffic safely. .. Had it towed, about to start to taking things apart.
Have done a coolant dye test, don't see anything with the black light around the reservoir, under the car, around the thermostat or radiator. All dry as a bone. Unless it's under the manifold, where I can't see it?
Have also done a combustion/CO2 leak test in the coolant reservoir - negative. May run it again.
Thoughts on what would cause the 10-15 temperature fluctuations in 3-4 seconds time? Faulty water pump? Air in the lines? Leaky coolant line? Thermostat failure? ... has fresh oil change, fresh plugs, new air filters, extracted and refilled coolant a 50/50 dilution
Any collaborative thoughts would be greatly appreciated.
Your cooling does not match your engine heating. So heat climbs up and up!
These GDI engine are setup to run **HOT** just a hair away from the drama you describe.
You are running with perfect stock conditions and are getting stock experience.
You could...
-- Improve your cooling: Tstat + radiator + Belt kit
-- Disable heatsoak accumulation: MOD-X oiling
-- Limit heat production: don't race non-stop. The blow-by gases heatup the crankcase with no way to cool engine bottom.
There no single magic fix to cool engine stock. An upgraded water pump would be great. Oil cooled pistons help keep heat circulated. Running IAT pump is good to... everything helps!
Last edited by CaliBenzDriver; Jul 12, 2025 at 06:19 PM.
Thread Starter
Newbie
Joined: Dec 2020
Posts: 9
Likes: 0
2012 E550 178 W212
Thoughts on temperature fluctuations going from 90C to 105C within like a handful of seconds? And then skyrocket into the red.
Engine also seemed to be struggling right before I turned it off. It would move, but very slowly. I pulled at barely 800 rpm, creeping out of the way of traffic safely. .. Had it towed, about to start to taking things apart.
Have done a coolant dye test, don't see anything with the black light around the reservoir, under the car, around the thermostat or radiator. All dry as a bone. Unless it's under the manifold, where I can't see it?
Have also done a combustion/CO2 leak test in the coolant reservoir - negative. May run it again.
Thoughts on what would cause the 10-15 temperature fluctuations in 3-4 seconds time? Faulty water pump? Air in the lines? Leaky coolant line? Thermostat failure? ... has fresh oil change, fresh plugs, new air filters, extracted and refilled coolant a 50/50 dilution
Any collaborative thoughts would be greatly appreciated.
Engine also seemed to be struggling right before I turned it off. It would move, but very slowly. I pulled at barely 800 rpm, creeping out of the way of traffic safely. .. Had it towed, about to start to taking things apart.
Have done a coolant dye test, don't see anything with the black light around the reservoir, under the car, around the thermostat or radiator. All dry as a bone. Unless it's under the manifold, where I can't see it?
Have also done a combustion/CO2 leak test in the coolant reservoir - negative. May run it again.
Thoughts on what would cause the 10-15 temperature fluctuations in 3-4 seconds time? Faulty water pump? Air in the lines? Leaky coolant line? Thermostat failure? ... has fresh oil change, fresh plugs, new air filters, extracted and refilled coolant a 50/50 dilution
Any collaborative thoughts would be greatly appreciated.
Thread Starter
Newbie
Joined: Dec 2020
Posts: 9
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2012 E550 178 W212
Thanks for the reply.
I did forget to mention, I got the "COOLANT LOW, STOP VEHICLE, TURN OFF ENGINE" - hence why I pulled over. And cause the car seemed to be struggling to move. But I have checked it the coolant, and it's full to the line where the reservoir changes plastic color from black/white, right to the tab in the back of the reservoir, as shown in the manual.
Thoughts?
I did forget to mention, I got the "COOLANT LOW, STOP VEHICLE, TURN OFF ENGINE" - hence why I pulled over. And cause the car seemed to be struggling to move. But I have checked it the coolant, and it's full to the line where the reservoir changes plastic color from black/white, right to the tab in the back of the reservoir, as shown in the manual.
Thoughts?
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From: Jakarta-Indonesia
2014 - W212.065 - E400 ( M276.820, 3 liter Turbo) RWD not Hybrid
You need a total cooling system refresh, less radiator....but boroscope the radiator for debris would be wise.
Why do you call ur car W278, is it the engine being M278 and a typo ?
E550 I thought is only for M273 engine....?
If indeed M278, you have lots of homework for its tubocharger coolant circuit, it will break....guaranteed a 100%.
Why do you call ur car W278, is it the engine being M278 and a typo ?
E550 I thought is only for M273 engine....?
If indeed M278, you have lots of homework for its tubocharger coolant circuit, it will break....guaranteed a 100%.
Out Of Control!!
Joined: Apr 2019
Posts: 11,924
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
You may want to test that condition before this happens again & take it easy with the high heat until solved.
Last edited by CaliBenzDriver; Jul 12, 2025 at 11:53 PM.
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MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,141
Likes: 1,293
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
OP probably has a true LOW COOLANT condition. This can cause fast temperature fluctuations as when coolant does not reach the temp sensor it cannot read the coolant temp. Then suddenly sensor sees coolant and temp shoots up. I have had this happen in a Tahoe long time ago. And when looking at the reservoir it is at correct level.
I see two possible reasons for the trouble:
1. The coolant is not correct type allowing it to boil at too low temp.
2. Cooling system pressure is too low allowing the coolant to boil at engine normal temp. This would mean the “valve” that holds the pressure does not hold high enough pressure.
I have not worked on my cooling systems enough to know what that pressure holding valve is. I don’t think it is the reservoir cap, or is it?
Just a week ago I worked on a friend’s Hyundai that lost head gasket. After gasket change overheating condition remained and coolant just boiled out from the engine.
Long story short, problem was the radiator fill neck that had worn the plastic tabs that pull tension to the cap spring when closing the cap. This meant the cap did not hold the 13-16 psi pressure in the cooling system that is needed for keeping the coolant from boiling, so the coolant boiled and engine went low in coolant and overheated.
Our cars do not have the radiator cap but something in the system must hold pressure up high enough to prevent coolant from boiling.
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Out Of Control!!
Joined: Apr 2019
Posts: 11,924
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
This condition can happen when coolant boils in the overheated engine.
OP probably has a true LOW COOLANT condition. This can cause fast temperature fluctuations as when coolant does not reach the temp sensor it cannot read the coolant temp. Then suddenly sensor sees coolant and temp shoots up. I have had this happen in a Tahoe long time ago. And when looking at the reservoir it is at correct level.
I see two possible reasons for the trouble:
1. The coolant is not correct type allowing it to boil at too low temp.
2. Cooling system pressure is too low allowing the coolant to boil at engine normal temp. This would mean the “valve” that holds the pressure does not hold high enough pressure.
I have not worked on my cooling systems enough to know what that pressure holding valve is. I don’t think it is the reservoir cap, or is it?
Just a week ago I worked on a friend’s Hyundai that lost head gasket. After gasket change overheating condition remained and coolant just boiled out from the engine.
Long story short, problem was the radiator fill neck that had worn the plastic tabs that pull tension to the cap spring when closing the cap. This meant the cap did not hold the 13-16 psi pressure in the cooling system that is needed for keeping the coolant from boiling, so the coolant boiled and engine went low in coolant and overheated.
Our cars do not have the radiator cap but something in the system must hold pressure up high enough to prevent coolant from boiling.
OP probably has a true LOW COOLANT condition. This can cause fast temperature fluctuations as when coolant does not reach the temp sensor it cannot read the coolant temp. Then suddenly sensor sees coolant and temp shoots up. I have had this happen in a Tahoe long time ago. And when looking at the reservoir it is at correct level.
I see two possible reasons for the trouble:
1. The coolant is not correct type allowing it to boil at too low temp.
2. Cooling system pressure is too low allowing the coolant to boil at engine normal temp. This would mean the “valve” that holds the pressure does not hold high enough pressure.
I have not worked on my cooling systems enough to know what that pressure holding valve is. I don’t think it is the reservoir cap, or is it?
Just a week ago I worked on a friend’s Hyundai that lost head gasket. After gasket change overheating condition remained and coolant just boiled out from the engine.
Long story short, problem was the radiator fill neck that had worn the plastic tabs that pull tension to the cap spring when closing the cap. This meant the cap did not hold the 13-16 psi pressure in the cooling system that is needed for keeping the coolant from boiling, so the coolant boiled and engine went low in coolant and overheated.
Our cars do not have the radiator cap but something in the system must hold pressure up high enough to prevent coolant from boiling.
I think DAMAGES are done AFTER engine off once pump stop circulating radiator cooled liquid.
The engine core gets heat soaked by extreme heat accumulated in dry pistons.
The coolant pressure depends on the temperature. Thats when the weakest seals let go to relief pressure. Valeo radiator is not up to the task.
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Joined: Oct 2010
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From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
The Max relief pressure is set at 20Psi by the tank cap.
I think DAMAGES are done AFTER engine off once pump stop circulating radiator cooled liquid.
The engine core gets heat soaked by extreme heat accumulated in dry pistons.
The coolant pressure depends on the temperature. Thats when the weakest seals let go to relief pressure. Valeo radiator is not up to the task.
I think DAMAGES are done AFTER engine off once pump stop circulating radiator cooled liquid.
The engine core gets heat soaked by extreme heat accumulated in dry pistons.
The coolant pressure depends on the temperature. Thats when the weakest seals let go to relief pressure. Valeo radiator is not up to the task.
Highest engine temps always are when coolant flow stops and normally this is right after engine shuts off. This is when coolant can boil especially if high enough pressure is not held in the system.
My S550 with the 278 engine has REST function that I try to remember use every time I stop. This circulates the coolant and prevents boiling it. I wish this function could be made automatic as it certainly cannot hurt anything, only prevent engine overheating.
I used to have an Audi Q7 that had this function automatically come ON every time I shut it down. There was no option to disable it either.
Out Of Control!!
Joined: Apr 2019
Posts: 11,924
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
You sure about the 20psi? System pressure leak test is done at 16psi.
Highest engine temps always are when coolant flow stops and normally this is right after engine shuts off. This is when coolant can boil especially if high enough pressure is not held in the system.
My S550 with the 278 engine has REST function that I try to remember use every time I stop. This circulates the coolant and prevents boiling it. I wish this function could be made automatic as it certainly cannot hurt anything, only prevent engine overheating.
I used to have an Audi Q7 that had this function automatically come ON every time I shut it down. There was no option to disable it either.
Highest engine temps always are when coolant flow stops and normally this is right after engine shuts off. This is when coolant can boil especially if high enough pressure is not held in the system.
My S550 with the 278 engine has REST function that I try to remember use every time I stop. This circulates the coolant and prevents boiling it. I wish this function could be made automatic as it certainly cannot hurt anything, only prevent engine overheating.
I used to have an Audi Q7 that had this function automatically come ON every time I shut it down. There was no option to disable it either.
Cap protection is... inefficient!
20 to 22Psi or really 25+Psi
@Arrie your wishes have been heard!
I circulate engine coolant automatically with AUX-PMP by programming F-SAM.
On Turbo's the AUX-PMP is used on the intercooler circuit.
> PLAN-B:
Prevent accumulating extreme heat in dry pistons.
Last edited by CaliBenzDriver; Jul 13, 2025 at 05:46 PM.
