2014 E350 4matic
WDDHF8JB9EA780202
106k

Heater doesn't stay warm. When I turn up the temperature to 72, it will start to blow warm air for about 30 seconds to a minute then go back to blowing air as of I have the temperature on low (AC OFF). If I move it up 2 to 3 degrees higher to 74 or 75, it will do the same thing again and blow warm air for about 30 seconds to a minute. The only way to have warm air blow out consistently is to turn it up to 85 or higher right away without moving it up 2 or 3 degrees at a time. But the air isn't hot like the temperature that it is set on. Just warm.

I've been trying to search and figure this out, but I only found different years, models and engines have a similar problem which they said it was the blend door actuator or the heater valve / switch over valve.

I can't seem to find anyone with this problem for w212. And I can't find new blend door actuator replacement or heater control valve for this model.

Anyone have any ideas? Or did I over look the solution during my research on this issue?

Thanks

 

Any coolant level problems?
Engine overheating?
Coolant leaks in engine bay, or inside cabin?

 

No coolant level problems, no overheating, and no coolant leaking in the engine bay or cabin

 

I would suspect vent flaps that direct air over the heater core. Either the intake, or exhaust flap isn’t opening fully. There is probably a module that controls it. Are you getting any codes, or warning lights?

 

No codes or warning lights.

I didn't even realize I had this problem until one night I noticed I wasn't getting hot air and I started playing with it every night on my drive home. And it would do the same thing every time.

For now the solution is just crank it up all the way and leave it there for warm air. But would be nice to have it working like it should.

 

This could be a "simple" temperature sensor issue. If you had a cheap scanner you could read all temp sensors at the air vents. I have a iCarsoft MB II but there are much better scanners not costing more. I have later purchased the xentry system for total control of the car but the iCarsoft was able to read all temp sensors.

One thing you could try is to go to cooling with AC on (Set it to AUTO) and set it to say 70. See if it blows much lower temp air in the car as if the same sensor making it not blow hot enough it might tell the system to blow much cooler than you are asking for too.

 

Right now I have the Launch Scanner Elite V2.0(made in China version). It has a lot of features but I'll have to double check if it can read the temperature sensors for the vents.

If it doesn't read the temperature sensors I'll try AC on Auto to 70° to see if it's blowing cooler than what's displayed.

 

Your engine is M276, it has no stop valve for coolant flow to heater core of HVAC, it can only do blend door.
Also I do not believe your car has the extra electric heater grid in your HVAC system, I do not know what is the option code number for it, but I do not see anything mentioned of heater
in your data card, except for :
- 873 SEAT HEATER FOR LEFT AND RIGHT FRONT SEATS
- 875 HEATED SCREEN WASH SYSTEM

Your HVAC is the basic one like mine Code 580
- 580 AIR CONDITIONER
- 919 AIRCON COOLING POWER PACKAGE

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By logic of operation, you heater core coolant temperature would be at all times 90-95C or 194 to 204F, it probably can blow hot air as hot as 60C/140F read by middle vent sensor when you do max heat. Assuming you are at 10C / 50F ambient temp and thus needing mild heater core assist......... controlling heat outout accurately when the system is this primitive ( coolant based heater core without stop valve ) won't be easy purely by blend door blending.

If you turn on your AC, it may do better at controlling the heated temperature range you want because AC cooling side can do easy 30C / 54F delta.
If the heater is electric, it is easy, the heat value can be controlled...... but you can't control coolant based heater core without a stop valve .

Imagine a fire place in a cabin, fix fire strength 3-4 logs burning, you adjust the heat by sitting closer or further away from the fireplace...

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NOTE : I am assuming your coolant flow to HVAC heater core is still good.

This coolant flow is engine RPM dependent due to coolant pump is belt based to crankshaft pulley, here again very primitive, no way to control LPM/GPH flow of coolant to your HVAC heater core.
Yes there is a small 12V DC pump which aids the circulation for heater core when needed, but that is all you got and whenever thermostat opens up, coolant flow is heavily diverted to radiator.

Here is a simple example of coolant pressure at the bank 2 rear azz output feeding the heater core of HVAC core vs RPM vs Coolant Temperature.
The only data I do not have but I know the temperature range is ...... is the thermostat full opening to flow coolant to radiator.
I am in the tropics, so its always hot.




This is the location of the coolant pressure sensor.... on my M276.820
So it is truly reading heater core input in terms of pressure and heater core output goes back to mechanical coolant pump INPUT .... with a Tee to 12VDC pump for heater assist.
This means the 12V DC pumps works on suction mode.

This the best location to tap coolant pressure , I been testing a few location and this is by far the best.





Azz view of ur M276 coolant output pipe to heater core



2 pipes




I suggest use your AC cooling section ( compressor ) when you want better final temperature control when using heater core.
So your AC compressor is the 2nd temperature "blender".

 

It's very special that heater core not get hot enough when this engine stores heat in wholesale Qty.

Let your Launch do a full scan report you can post here for help.
The control module is named AAC.

 

I did a full scan and I was able to to check the function of every flap, temperature sensor, and blower in real time. No fault codes and everything seems to be working like it should. I was also able to get data on the cabin temperature sensor and showed me the cab temperature in real time.

Other cars/brands I have owned just keeps the heat coming on whatever you have it set on until you turn it down yourself, maybe this car is cutting off the heat because it's already at the temperature I have it set on in the cabin. But I do find that strange because I don't have it set on "AUTO"

It's been a bit on the warmer side here so it's hard to test out my theory, but I did remember a few nights ago that warm air was coming out for longer than expected. It did eventually stop and turn to normal fan air blowing.

I'll leave it be as it is for now. If I find anything new or any updates I'll post on this thread again.

Thanks for every ones input.

 

On a side note,

The picture of the 2 coolant pipes that goes to the heater core, located on the back of the motor that S-Prihadi posted, I see it's all plastic, so I'm wondering how often do they fail?

Looks like a pain to get to.

Might be a good idea to be proactive and replace that before it actually cracks and leaves a mess behind or get stranded somewhere because all the coolant leaked out.

 

The Air-Control module actively dries the A/C Evap coil using hot air. That may explain what you experienced.
You can quickly configure your Air con with 50+ settings using Launch scanner VARIANT coding.

Heater core is known to be always fully circulated with engine 200°F coolant - No water valve available, only a flap door. Nearby heat bleeds into 2°C A/C.

 

I did see some variant coding related to the A/C. I'll look into it some more when time permits and play with the configuration. I did read somewhere that someone brought their w212 to the dealer and they just did some software stuff and now they have consistent hot air. So definitely that can be the solution.

 

You need to remove vacuum pump to do these two pipes.
You will also need to do 2 hoses downstream of these pipes if you are in there
1 hose from end of 2nd pipe to fake firewall engine (outer) side
1 more hose from inner fake firewall ( inner side ) to the heater core itself.

When you are there, get the o-ring seal for coolant temperature sensor, replace urs with new one.
The first pipe is holder or locker for the coolant temp sensor.