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His - '09 ML-320 Bluetec, Hers - '05 E-320.
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My local indie shop is stumped as am I. Engine warms up and maintains temp as expected, and cabin heater begins to warm the interior. After a short while however, the cabin air discharge temp starts to cool and I'm sensing simply a re-circulation of cabin air (based on temp). I can manually increase interior thermostats to call for more heat and the fans speed up but the discharge air temp actually feels cooler. This tells me the water flow through the heater core is not sufficient to warm the amount of air being moved across it.

My shop thinks there is something floating around in the engine that blocks/restricts water flow to the heater, yet the engine NEVER overheats and maintains a constant 185 to 190 farenheit regardless of outdoor temperature. I'm told the climate control system isn't reporting any fault codes. I'm really open to suggestions...
 

W163 Crash Test Driver
2008 ML350 (Me!), 2010 GLK350 (Wife), 2003 ML-350 (Deceased)
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This is where a MB Star computer comes in handy. It could be a faulty, semi-broken or stuck mix flap or temp sensor (air temp). Have you/they verified that the heater core itself or it's hoses are cold? Remember MB has the heat and A/C working most of the time and the flaps mix the air to the desired temp. Just because the air is cold, doesn't meant the heater core isn't hot.

Any codes?
 

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2009 ML350 164.186 / 272.967 / 722.9
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49 Posts
I don't want to divert this thread, but hopefully Ph or someone can answer a related question: If outside temp is 30 degrees F, and I set both sides of the system to 68 degrees and AUTO, the A/C comes on. I do understand that is the default. And that A/C is useful for dehumidifying. If I turn off the A/C at that point, do I risk any damage? The system does not try to turn it on again. Thanks.
 

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Dallas, Texas
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7,376 Posts
My shop thinks there is something floating around in the engine that blocks/restricts water flow to the heater, yet the engine NEVER overheats and maintains a constant 185 to 190 farenheit regardless of outdoor temperature. I'm told the climate control system isn't reporting any fault codes. I'm really open to suggestions...
Reading codes is just small portion of the job. Reading actual values is next. This is done in two steps. With vehicle parked overnight read actual values of all temp sensors. They all need to read same +/- 2 degrees. then with engine warmed up, temp set to MAX, blower set to MIN.

Let's say in-car temp sensor drifted positive. Interior temp is reading 80F, air mixing flaps will move towards colder setting.
 

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Dallas, Texas
Joined
7,376 Posts
I don't want to divert this thread, but hopefully Ph or someone can answer a related question: If outside temp is 30 degrees F, and I set both sides of the system to 68 degrees and AUTO, the A/C comes on. I do understand that is the default. And that A/C is useful for dehumidifying. If I turn off the A/C at that point, do I risk any damage? The system does not try to turn it on again. Thanks.
If humidity is low you can turn A/C compressor off without any adverse effects. That type of compressor turns all the time anyway. Turning it off puts it in 0% delivery mode.
 
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