No heat at idle
 

Hey Guys,
I have a C350 with 40k and I have noticed that when I come to a stop light the heat is becoming less and less hot. Once I start driving it is fine again. Does this car have a coolant circulation pump?

Sounds like you may have air in the coolant system. The system needs to be topped off with fluid and bled properly.

The engine temp is stable. This happens every time I come to a stop or the engine is at idle. Coolant level is full.

maybe someone would be nice enough to go and try this in their w204? Its 18 degrees in Chicago right now and my issue happens when I am in "drive" and stopped (600 rpm) with the temp dials on the red dot and the fan all the way up. The temp of the air blowing tapers down until it feels like just the fan is blowing, no heat.

Just a thought .. with the ambient temp below freezing & the engine not producing much heat at idle, it may be struggling to maintain its efficient operating temperature.

The system may be clever enough to stop you pulling heat into the cab? This may be BS.

I think the only coolant recirc pump is driven by the serpentine auxilliary belt. At idle this pump is not circulating as much coolant through the system hence not picking up much heat from the head & block, therefore another reason for the poor performance of the heater.

It will be interesting to hear from another member who lives in a cold climate. Can't help you here mate as we are having 28- 30 ' c days here !!

Is your engine running at its normal operating temp ? ie around 90'C . It not the thermostat may have failed open.

I understand that there is also an electrical pre heater which operates to warm the cab almost instantly from cold. I do not know how this affects the system.

After some more testing, it has to just be so cold that the coolant going though the heater core gets cold. When I put it on recirculate, it starts getting hot again (since its not taking 18 degree air and trying to warm it up to 85) only down side is my windows fog up and I have to crack open a window.

Based on last 2 posts, I'd add turning down the heater fan speed a bit (if possible on a C250), to bring up the temp.

Check:

1) rad fan should not come on at those temps

2) during a cold idle, water exiting the t-stat and hosed to the radiator, should be cold at the T-stat exit point (T-stat ~closed, all hot water from rad goes through heater coil). Hand check. ---> verify
If hot, T-stat stuck open.

3) if sustained cold weather, see if MB had partial radiator blocks.

.

I got this checked out at Loeber Mercedes in Lincolnwood this week and they are telling me it is a known design flaw. New models have a aux coolant pump to alleviate this issue. Most people don't notice it so I guess I am special. They are changing the diverter valve and doing a flush for good measure. :V crap mercedes

Mine does this only when the heater is on. Once the temp goes above about 1/4 it will stop doing it. I haven't thought much of it though.

My wife has complained about this. She starts the car to warm it up and lets it run for about 10 minutes....gets in and the car is blowing FULL cold through the defroster.

The underlying assumption of the process is incorrect. Given the large amount of oil in the pan, these engines warm up most quickly under load. She should start the car with the HVAC on AUTO, heated seat on three stripes. and drive at moderate speeds until the engine is warm. Prolonged idling is wrong for several reasons...waste of gas, pollution, and very ineffective warming of the car.

Yup....got all that. It doesn't change the fact that the heating system is not operating correctly. Anyone gained insights on why this is occurring?

Hotelcrazy maybe right. I have people complain about this same problem. The first thing to check is the coolant, if it is low or has a air pocket in the heating system it will act like that.

I will check coolant level. Seemed fine when I leave it on "auto" today.

I'm new to the W204, but I'm wondering if (in the endless search for efficiency) if the water pump is "clutched" - ie the water pump only spins/works when needed.

The coolant level may or may not appear to look right, it's to your best interest to have the system bled either way since you can't see air trapped in the system.

I am not sure what settings my wife had the heat set in when she said it was blowing cold. All appears to be working normally for me.

Is your auxiliary coolant pump operating? The primary function of the Aux coolant pump is to provide heat to the cabin for a while after shut down but it would sure help in this situation at idle if the system has no air in it. Dealer might have to switch it in at idle using Star to set the ECU. Under normal conditions the engine water pump will suffice.

The other issue could be with the blender flap actuator on the HVAC that blends cold & hot air.

This is the W203 diagram but the W204 essentially operates in the same fashion.

https://mbworld.org/forums/attachmen...-heaterbox.jpg

Aux coolant pump.
 

I have been doing some searching. I have never seen a W203, W204 or W209 without an Auxiliary Coolant pump. However on checking some pictures of US cars I note that some have them omitted. Cost saving measure maybe? So much for all the Benz speak that it is not only for cabin heat but to prevent hotspots in the engine on heat soak & cool down.

If your car has no Aux coolant pump maybe you should fit one.

W204 with pump attached to surge tank below.

https://mbworld.org/forums/attachmen...1&d=1392045089

W204 without pump attached to surge tank below. US car.

https://mbworld.org/forums/attachmen...1&d=1392045907

W203/W209 with aux coolant pump etc.

https://mbworld.org/forums/attachmen...1&d=1392045320

Very strange!

Thanks Glyn. I check and my 2010 has the aux pump. I went out this morning, fired the car up and put the defroster on. I let the car idle for few minutes while I cleared some ice and snow in my driveway. I jumped in and everything worked fine for me....ice was melting on the windows. So I have never been able to replicate what my wife had occur last week.

I duplicated this with my 2010 W204. Pretty much as 95 described and never noticed it up until he mentioned it.

My settings are Auto, 70 DS, 72 PS. AC is not activated. Coolant level is to spec, car warms up fine. While driving, as the car warms up - there is hot air mostly via floor and side dash vents. At stoplights, the air quickly cools down. Immediately upon driving from the light the air warms back up. I'm not in recirc mode and my windows don't fog up at all.

This is only while the car warms up, though. Once the car is at full operating temperature - the cooling of the air stops happening.

I'm in Chicago area also, and this was the case this morning with -3 indicated.

Hmmmm....wondering if it is designed to limit heat to the passenger compartment to get the engine up to operating temp sooner at idle.

Does your car have an Auxiliary Coolant Pump fitted?

Glyn,

I have read that the c300 has a three-phase cooling system helps the engine warm up very quickly. When the engine is first started, no coolant circulates. Then, as the engine warms up, coolant begins to circulate within the engine, but not through the radiator. Only when the coolant temperature reaches 221 degrees Farenheit (or 189 degrees F. under high load), coolant also circulates through the radiator. Coolant circulation through the heating system for the car's interior is controlled separately.

Is this a possibility??

The auxilliary coolant pump takes heat from the engine block to supply the heater heat exchanger. Heating is almost instantaneous after a cold start in our SA mild climate. Guess it would take a little longer in a freezing Illinois winter.

Here is the operation of the multiphase electronic thermostat. Designed to achieve maximum efficiency out of the engine.

https://mbworld.org/forums/attachmen...1&d=1392211016

https://mbworld.org/forums/attachmen...1&d=1392211016