Heater Taking Too Long Too Activate?
Thread Starter
Member
Joined: Aug 2025
Posts: 104
Likes: 20
From: Texas
2015 ML350, 2020 BMW X7, 2005 LS430
Heater Taking Too Long Too Activate?
15 ML, woke up this morning to a icy car, ~31 degrees here in Texas, and as soon as I got in I started the heated seats and heater. Took a solid 10 minutes for the heat to blow and defrost the windshield, and even longer for the heated seats to actually generate heat. Not sure why but my thermostat was stuck at 0 with the needle at the bottom for 10 minutes, could that be why the heat wasn't engaging? Also I just replaced the engine thermostat a couple months ago so thats brand new. 10 years ago when I got in the car in cold weather the heat would start right up and it would blow really hot. Do you have any suggestions how to fix this issue?
MBWorld Fanatic!
Joined: Jan 2016
Posts: 2,748
Likes: 880
From: In the Shadow of the Tetons
2013 ML350 Bluetec
A new thermostat doesn't guarantee it is a good one. What brand? Just idling trying to warm up? If you weren't driving, the engine will take forever to warm, especialky if you're already calling for heat.
The heated seats take forever...
The heated seats take forever...
Thread Starter
Member
Joined: Aug 2025
Posts: 104
Likes: 20
From: Texas
2015 ML350, 2020 BMW X7, 2005 LS430
No idea about the brand, it was dealer installed. Heated seats usually take a minute or two max for mine. Yes I get the engine takes time to idle but the needle shouldn't be stuck at zero.
MBWorld Fanatic!
Joined: Jan 2009
Posts: 3,829
Likes: 592
From: NY & FL
MB’s of all kinds.
Sounds like a stuck open thermostat to me. I would have the dealer replace it as it seems the one they installed was defective. What made you replace the thermostat months ago?
Thread Starter
Member
Joined: Aug 2025
Posts: 104
Likes: 20
From: Texas
2015 ML350, 2020 BMW X7, 2005 LS430
Dealer was replacing the entire cooling system, so radiator and hoses and all that, and thermostat is a common failure so I thought I'd get it out of the way, plus I only had to pay for parts since everything else was already disassembled.
Out Of Control!!
Joined: Apr 2019
Posts: 11,923
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
To be noted: a cold engine can hardly warm-up and circulate coolant throughout the engine at IDLE.
Drive the cold engine to warm it up or rev-up to 2kRpm then the HEATER CORE will get supplied with 200°F coolant that can warm up cabin occupants.
Point being the efficient water pump at idle is useless at moving coolant or heat...
Drive the cold engine to warm it up or rev-up to 2kRpm then the HEATER CORE will get supplied with 200°F coolant that can warm up cabin occupants.
Point being the efficient water pump at idle is useless at moving coolant or heat...
Junior Member
Joined: Nov 2017
Posts: 33
Likes: 9
2012 ML 350 (W166)
15 ML, woke up this morning to a icy car, ~31 degrees here in Texas, and as soon as I got in I started the heated seats and heater. Took a solid 10 minutes for the heat to blow and defrost the windshield, and even longer for the heated seats to actually generate heat. Not sure why but my thermostat was stuck at 0 with the needle at the bottom for 10 minutes, could that be why the heat wasn't engaging? Also I just replaced the engine thermostat a couple months ago so thats brand new. 10 years ago when I got in the car in cold weather the heat would start right up and it would blow really hot. Do you have any suggestions how to fix this issue?
Last edited by vrmb; Dec 5, 2025 at 04:21 PM.
Trending Topics
Out Of Control!!
Joined: Apr 2019
Posts: 11,923
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
poor heat circulation
This engine has a hard time moving heat with its undersized pump, it's really slow.
The heater-core circuit is always on Max heat summer or winter -
It has no control valve that can fail.
If I were to warm up this car in Canadian Winter... I'd rev' it to 1500/ 2000Rpm to generate heat to warm up my windshield before driving.
MB World Stories
The Best of Mercedes & AMG
Manual Mercedes? 6 Times Sindelfingen Let Drivers Have All The Fun
Verdad Gallardo
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Verdad Gallardo
8 Classic Boxy Mercedes Designs That Have Aged Like Fine Wine
Verdad Gallardo
Flawlessly Restored Mercedes 190E Evo II Heads to Auction
Verdad Gallardo
Electric Mercedes C-Class Unveiled: 11 Things You Need to Know
Verdad Gallardo
Mercedes EQS Gets A Major Update: Everything You Need to Know
Verdad Gallardo
5 Underrated Mercedes-Benz Models That Don't Get the Love They Deserve
Verdad Gallardo
Mercedes 300D Has Pushed Well Past 1 Million Miles and It Ain't Stopping
Verdad Gallardo
10 Most Reliable Mercedes-Benz Models You Can Buy Used
Verdad Gallardo
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
If there was a negative change in behavior after work was done, I agree with @C280 Sport to have the dealer fix the problem.
Thread Starter
Member
Joined: Aug 2025
Posts: 104
Likes: 20
From: Texas
2015 ML350, 2020 BMW X7, 2005 LS430
To be noted: a cold engine can hardly warm-up and circulate coolant throughout the engine at IDLE.
Drive the cold engine to warm it up or rev-up to 2kRpm then the HEATER CORE will get supplied with 200°F coolant that can warm up cabin occupants.
Point being the efficient water pump at idle is useless at moving coolant or heat...
Drive the cold engine to warm it up or rev-up to 2kRpm then the HEATER CORE will get supplied with 200°F coolant that can warm up cabin occupants.
Point being the efficient water pump at idle is useless at moving coolant or heat...
Thread Starter
Member
Joined: Aug 2025
Posts: 104
Likes: 20
From: Texas
2015 ML350, 2020 BMW X7, 2005 LS430
Was this done by a Mercedes dealer or an independent shop? Just want to make sure we are on the same page.
If there was a negative change in behavior after work was done, I agree with @C280 Sport to have the dealer fix the problem.
If there was a negative change in behavior after work was done, I agree with @C280 Sport to have the dealer fix the problem.
Out Of Control!!
Joined: Apr 2019
Posts: 11,923
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
filter plugged up
Yes done by a mercedes dealership. The weird thing I noticed yesterday was that the heat would blow very strong on the passenger side but barely on the driver side. I had this same issue last summer with the AC with the driver side getting barely any cold air in the brutal texas heat and the passenger side and rear vents were getting plentiful amounts. Before taking the car to the dealer to get it checked out I tried a $10 bottle of freon re-charge from walmart and it INSTANTLY blew ice cold air from all the vents including the driver. The driver side did still have a little bit of less pressure compared to the passenger after re-charge but that could have just been placebo. Not sure what the issue is now? Do you guys have any idea?
It can seriously restricts cabin air circulation, both hot/cold - In a pinch you can simply remove it out for circulation.
Last edited by CaliBenzDriver; Dec 7, 2025 at 04:41 PM.
Thread Starter
Member
Joined: Aug 2025
Posts: 104
Likes: 20
From: Texas
2015 ML350, 2020 BMW X7, 2005 LS430
MBWorld Fanatic!
Joined: Jan 2016
Posts: 2,748
Likes: 880
From: In the Shadow of the Tetons
2013 ML350 Bluetec
K&N cleanable re-oilable rock filter? Or do they make a paper filter that actually works at catching fine particles that kill engines?
Take a ride down a dusty road following someone. Then look on the "clean" side of your intake, downstream of the K&N filter. Then put a paper filter back in.
Take a ride down a dusty road following someone. Then look on the "clean" side of your intake, downstream of the K&N filter. Then put a paper filter back in.
Thread Starter
Member
Joined: Aug 2025
Posts: 104
Likes: 20
From: Texas
2015 ML350, 2020 BMW X7, 2005 LS430
So I just discovered this on my ML. On full heat the air blows really hard on the passenger side and rear ac vents, but for the driver side it's near nothing, both temperature wise and air flow. Then I turned the dial from HI to LOW and the driver side vents blowed at full pressure, and this includes both the dash vents and footwell vents, which previously both were blowing barely any air. You guys have any idea what the issue is?
Out Of Control!!
Joined: Apr 2019
Posts: 11,923
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
So I just discovered this on my ML. On full heat the air blows really hard on the passenger side and rear ac vents, but for the driver side it's near nothing, both temperature wise and air flow. Then I turned the dial from HI to LOW and the driver side vents blowed at full pressure, and this includes both the dash vents and footwell vents, which previously both were blowing barely any air. You guys have any idea what the issue is?
Perhaps a temp sensor got disconnected during service ...
