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No heat, but only while driving

squeebie

New member
I'm in the north east US, and as the weather has gotten colder, I noticed my heat working inconsistently. My friend hooked it up to VCDS last night and cleared everything else, then we checked tonight, no faults, and it looks like everything is working correctly when he monitored things while it's running.

While the car is idling, the heat is as expected, very hot. Once I start driving, withing minutes the heat starts to fade, eventually just cold air blowing. Once I pull over, the heat immediately starts coming back, getting very hot within a minute.

This is a 2015 TSI SEL, dual climate control. For testing, I've tried a lot of different settings, but for simplicity sake, I was testing this on "auto" with the heat on "hi". Also, while driving and the temp drops, it doesn't help to turn on the recirculating air.

Any suggestions what this might be? Heater core seems like a common issue when the heat isn't working, although it seems odd that it would work while idling, but not while driving.
 
Last edited:

Carlosfandang0

Drag Racing Champion
Location
UK
Car(s)
2016 3Dr GTi DSG CSG
I'm in the north east US, and as the weather has gotten colder, I noticed my heat working inconsistently. My friend hooked it up to VCDS last night and cleared everything else, then we checked tonight, no faults, and it looks like everything is working correctly when he monitored things while it's running.

While the car is idling, the heat is as expected, very hot. Once I start driving, withing minutes the heat starts to fade, eventually just cold air blowing. Once I pull over, the heat immediately starts coming back, getting very hot within a minute.

This is a 2015 TDI SEL, dual climate control. For testing, I've tried a lot of different settings, but for simplicity sake, I was testing this on "auto" with the heat on "hi". Also, while driving and the temp drops, it doesn't help to turn on the recirculating air.

Any suggestions what this might be? Heater core seems like a common issue when the heat isn't working, although it seems odd that it would work while idling, but not while driving.

Possibly one of these,
  1. Stuck thermostat
  2. Air in the coolant system
  3. Low coolant
 

squeebie

New member
Possibly one of these,
  1. Stuck thermostat
  2. Air in the coolant system
  3. Low coolant

Thanks for the suggestions. I should've mentioned in the original post, the coolant levels look good.

Would a stuck thermostat cause the temp dropping while driving, but then going back up while idling? The issue is consistently reproducible in that regard.
 

squeebie

New member
Another note - Ww had VCDS hooked up while I was driving, the temp of the coolant would stay around 100 degrees, even while the heat from the vents was fluctuating.
 

Carlosfandang0

Drag Racing Champion
Location
UK
Car(s)
2016 3Dr GTi DSG CSG
Another note - Ww had VCDS hooked up while I was driving, the temp of the coolant would stay around 100 degrees, even while the heat from the vents was fluctuating.
Maybe a blocked/restricted heater core, it would need a bit of hands on diagnosis really.
 

777-300ER

Go Kart Newbie
Location
St Joesph Mi
I have heard the bag in the coolant bottle can fail and the silica balls can find their way into the heater core and restrict flow. Maybe checking to see if that bag is still sealed would be a good idea?

Personally when it is subzero my car has trouble keeping heat. Running the heat without running the car hard at high load will drop coolant temps. Ended up building a rather crappy radiator block off to keep the temps more stable and it did work pretty well.
 

squeebie

New member
I have heard the bag in the coolant bottle can fail and the silica balls can find their way into the heater core and restrict flow. Maybe checking to see if that bag is still sealed would be a good idea?

Personally when it is subzero my car has trouble keeping heat. Running the heat without running the car hard at high load will drop coolant temps. Ended up building a rather crappy radiator block off to keep the temps more stable and it did work pretty well.

I had read about those and looked, but I didn't see any kind of a silica bag in there. I even (literally) poked around in there and didn't feel anything. Do all models in all countries have the bags?
 

tstadgti

Passed Driver's Ed
Location
STL
Car(s)
2020 GTI SE DSG
Could be a bubble in your coolant, as someone mentioned above, air. So even if your coolant is at the right level, this could still impact you. I had this issue in the past (non VW product) with very similar symptoms.
 

squeebie

New member
Thanks all. My regular mechanic was like "huh, I don't know, I'd take it to the dealership".

I have a good recommendation for a VW specific mechanic, I'll bring it over there and see what he thinks. For anyone curious, I'll follow up here once he figures out whats going on.
 

Elwood

3-7-77
Location
Long Beach, CA
Would a stuck thermostat cause the temp dropping while driving, but then going back up while idling? The issue is consistently reproducible in that regard.

Yes. As soon as cold air starts going through the radiator, everything cools down. What's your temp gauge doing? You should be able to see this.
 

squeebie

New member
Yes. As soon as cold air starts going through the radiator, everything cools down. What's your temp gauge doing? You should be able to see this.

Temp gauge goes up to 200F and stays there. VCDS shows the coolant at 100C.

I agree about the cold air on the radiator while driving, although I would also expect recirculating air to make it work better, but that didn't help.
 

2slowvw

Moderator
Location
VA
Car(s)
2016 GTI
Temp gauge goes up to 200F and stays there. VCDS shows the coolant at 100C.

I agree about the cold air on the radiator while driving, although I would also expect recirculating air to make it work better, but that didn't help.
I'm guessing the thermostat is stuck partially open. When idling, there is no cold air blowing over the radiator cooling the coolant down to low temps, but when you drive and the thermostat is stuck open, the air is cooling everything down enough to make the heater air cold. Also the engine coolant temp displayed on the cluster gauge is more of an estimate.
 

squeebie

New member
I'm guessing the thermostat is stuck partially open. When idling, there is no cold air blowing over the radiator cooling the coolant down to low temps, but when you drive and the thermostat is stuck open, the air is cooling everything down enough to make the heater air cold. Also the engine coolant temp displayed on the cluster gauge is more of an estimate.

We were monitoring the coolant temperature with VCDS while I was driving. It was staying right around 100° C the whole time, so it doesn't appear to be an issue where the coolant cools down.
 
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