Heater core issue?

[2012jsw]

Not getting heat like I used to, and AC last summer wasn’t working for diddly. Hot/cold blend door seems to be functioning. I crawled under the dash, watched it function and then disconnected the arm and moved it manually so see if there was any additional movement the servo motor wasn’t getting, all looked good. Ran up to temp and tested the temp of the heater core tubes in the cabin
100 degrees on upper and 85 on lower tube. Checked the heater hoses feeding the heater core in the engine bay and saw closer to 125 if I recall. I’m thinking heater core is plugged and not allowing full flow, hence the lower temp numbers on the inside tubes.
I know the valve on the AC compressor is a known issue/fail point. I’m thinking I have 2 different issues here that just seem to correspond in a similar way (blend door not functioning)

thoughts? Opinions? Additional testing? I have vcds, not sure what to use on it to verify anything hvac related that I couldn’t see/test/feel in person. Open to help and suggestions on the vcds side though since I’m about as literate in it as a 5yr old doing calculus

 

[Mozambiquer]

Not getting heat like I used to, and AC last summer wasn’t working for diddly. Hot/cold blend door seems to be functioning. I crawled under the dash, watched it function and then disconnected the arm and moved it manually so see if there was any additional movement the servo motor wasn’t getting, all looked good. Ran up to temp and tested the temp of the heater core tubes in the cabin
100 degrees on upper and 85 on lower tube. Checked the heater hoses feeding the heater core in the engine bay and saw closer to 125 if I recall. I’m thinking heater core is plugged and not allowing full flow, hence the lower temp numbers on the inside tubes.
I know the valve on the AC compressor is a known issue/fail point. I’m thinking I have 2 different issues here that just seem to correspond in a similar way (blend door not functioning)

thoughts? Opinions? Additional testing? I have vcds, not sure what to use on it to verify anything hvac related that I couldn’t see/test/feel in person. Open to help and suggestions on the vcds side though since I’m about as literate in it as a 5yr old doing calculus

Pretty common issue..I just did one last week. Flush the system, replace the heater core and refill

 

[2012jsw]

System was drained and refilled @ 140k with water pump/thermostat/radiator. Then 1/2 drained @ 170k with an oem Thermostat swap. Currently at 175k

Thinking I can get away with squeezing the heater hoses and just swapping the heater core.

 

[Mozambiquer]

System was drained and refilled @ 140k with water pump/thermostat/radiator. Then 1/2 drained @ 170k with an oem Thermostat swap. Currently at 175k

Thinking I can get away with squeezing the heater hoses and just swapping the heater core.

I'd flush it. I like using the blue devil flush. Works pretty good for these.

 

[prsa01]

S

Pretty common issue..I just did one last week. Flush the system, replace the heater core and refill

So is this a known issue, like on the passat? Must be at least somewhat less common since I've read way less about it in the jsw.

 

[2012jsw]

S
So is this a known issue, like on the passat? Must be at least somewhat less common since I've read way less about it in the jsw.

Def a known issue with the coolant that they were using.
removal of the heater core looks very simple…so that’s a positive!

 

[oilhammer]

I can't say I've ever seen an A5/A6 have a clogged core. Plenty of NMS and a few NCS cars, but the NMS is by far the worst for it.

My 2010 sedan has nearly 320k miles, all stock, and its heat still works great.

 

[2012jsw]

I can't say I've ever seen an A5/A6 have a clogged core. Plenty of NMS and a few NCS cars, but the NMS is by far the worst for it.

My 2010 sedan has nearly 320k miles, all stock, and its heat still works great.

you know better than I and I truly don’t mean that sarcastically. With that said, I’ve def seen/read a good amount of info regarding clogged cores in these.i know the Passat was worse, but ive seen a good chunk of jetta data to go with it.

Checking temp going in at the heater hose being 125*ish and then the aluminum tube at the heater core being 100* and the exit tube from the heater core being 20* cooler (fan on low) makes me think the core isn’t moving liquid efficiently. Heat coming out of the vents is barely “warm” and def nowhere near hot enough for typical operation.

checked the servo/blend door motor and linkage and it all functioned as it should. I don’t believe these suffer the blend door fate of the earlier VW that uses the foam as seals. The blend door has a rubber/silicone feel to it. So I don’t believe air is bypassing around it

changed the thermostat to the vw oem one when I did my turbo work a few thousand miles ago. The heater issue was a problem with the aftermarket thermostat and also with the new oem(zero heater difference) so I feel like that can be ruled out of the equation. Correct?

 

[turbodieseldyke]

125 going into the heater seems awfully low. Should be much closer to operating temp. What temps are you seeing on the other coolant hoses?

 

[2012jsw]

125 going into the heater seems awfully low. Should be much closer to operating temp. What temps are you seeing on the other coolant hoses?

I checked upper and lower radiator hoses, don’t quote me on the number, but I think my high number was 175ish. This was sitting in the driveway holding rpm at 2k until the gauge had just hit operating temp and then I started checking temps with a laser temp gun. So in reality the whole system most likely wasn’t at full heat sink and actually operating at the thermostats 190*

 

[2012jsw]

125 going into the heater seems awfully low. Should be much closer to operating temp. What temps are you seeing on the other coolant hoses?

Also part of why I think the heater core is clogged, it’s struggling to push hot water through it at a rate fast enough to keep the numbers anywhere that they should be. I don’t have an overheat issue under typical operating conditions, so I think water pump is good. It was replaced with the timing belt 35k miles ago with a Tbelt kit from one of the major online retailers (Id,euro,ecs) don’t recall exactly which retailer. Kit was of decent quality with a gates or conti belt and it included an oem or equivalent water pump(metal impeller if I recall)

 

[scream7595]

You can flush the heater core out backwards with air and water at the same time. The dealer replaced mine with a new one under warranty and the replaced heater core stopped up also. When I flushed mine out I got a a lot of sand out of it. were it came from I have no Idea. The dealer is the only one who had added any coolant to my car. I was told it could have been what was left in the water system when the engine was cast. Make sure to flush from the cold hose side of the heater core through to the hot house side. This solved my heater problem, hopefully it will help you with your problem.

 

[turbodieseldyke]

I checked upper and lower radiator hoses, don’t quote me on the number, but I think my high number was 175ish. This was sitting in the driveway holding rpm at 2k until the gauge had just hit operating temp and then I started checking temps with a laser temp gun.

Still, all hoses should read about the same. The heater hose goes straight from the engine to the heater core. Whether the core is blocked or open, that hose should have read 175. The heater-out hose (going to egr cooler) would read lower if your fan is on and blowing heat into the cabin. The lower rad hose would read lower if you were pushing air thru the radiator, which you weren't.

 

[oilhammer]

It flows the other way.... engine ---> EGR cooler ---> heater core ---> back to engine.

When they get really bad, and again, only seen this on the NMS and a couple NCS, they'll flag an EGR cooler inefficiency DTC. Because that circuit is not moving enough and the temp sensor on the end (outlet) of the EGR cooler will be picking up the low pressure EGR temp too high as compared to the EGTs from the other sensor.

I've successfully flushed a lot of the cores with a combo of soaking them isolated with Permatex flush, then the garden hose going backwards and pushing all kinds of granular crud out.... brown and whitish. It just depends on how long and how bad they've been messed up.

The CVCA cars have it happen, too.

 

[2012jsw]

What egr cooler are you guys talking about?
Car went on a diet with the turbo! Issue existed way before that though!!

 

[2012jsw]

It flows the other way.... engine ---> EGR cooler ---> heater core ---> back to engine.

When they get really bad, and again, only seen this on the NMS and a couple NCS, they'll flag an EGR cooler inefficiency DTC. Because that circuit is not moving enough and the temp sensor on the end (outlet) of the EGR cooler will be picking up the low pressure EGR temp too high as compared to the EGTs from the other sensor.

I've successfully flushed a lot of the cores with a combo of soaking them isolated with Permatex flush, then the garden hose going backwards and pushing all kinds of granular crud out.... brown and whitish. It just depends on how long and how bad they've been messed up.

The CVCA cars have it happen, too.

this is what I was referring to on the heater core not moving the fluid fast enough to keep the temps up through the core itself or even allowing the high temp water to make it to the core at a rate that keeps the temp up.
My concern with flushes/cleaners is that they would corrode the aluminum and possible cause a future leak in the heater core. Is that not a concern? I ordered a heater core from Napa, it’s a valeo which is the current brand with the VW stamp next to it in the car already. So OEM replacement. It was like $180 which I feel isn’t a terrible price for what looks like 2hours of work

 

[2012jsw]

IMG_9995.jpeg

• 2012jsw
• Feb 13, 2025

 

[oilhammer]

Never done one, looks to be a bit easier than the others, though. As the connections are right at the core tank, instead of at the end of a tube.

 

[2012jsw]

Never done one, looks to be a bit easier than the others, though. As the connections are right at the core tank, instead of at the end of a tube.

Looks incredibly simple!

 

[turbodieseldyke]

It flows the other way.... engine ---> EGR cooler ---> heater core ---> back to engine.

Ahhhhhhhh so.

edit to add:
1- radiator
2- thermostat
3- water pump
4- tranny cooler
5- oil cooler
6- egr cooler
7- heater core
8- reservoir
9- block heater (optional)

 

[oilhammer]

Yes, that pic is a bit confusing without the key. But, for our discussion, #6 is the EGR cooler, and #7 is the heater core.

(#1 is the radiator).

Obviously the hoses are not to scale. But, as always, the hottest coolant exits the engine from the flywheel end of the engine, and the small hose goes off to the EGR cooler (where equipped.... early 1Zs didn't have them, nor did any of the IDI diesels or gassers), then to the heater core. That's why when you delete the EGR, the cabin heat suffers. Because the EGR is working almost right away after a cold start, so you get some heat into the cabin from that... the coolant exiting the EGR cooler is higher than it is exiting the head.

The issue with the CR TDIs, vs. the older TDIs, especially post-Dieselgate "fix", is that the EGR duty cycle is so high, and the EGTs so high due to the rest of the DPF/etc., that it cooks the coolant that it can literally boil the additives right out of suspension, so now instead of a liquid you have a liquid with random solids in it. And the next stop in that circuit is the heater core, which has very small passages and ends up acting as a filter where all the solids get trapped.

 

[2012jsw]

Alright typed this as I was checking temps

•Metal Pipe that feeds coolant to the back of the motor(tested near oil filter housing) - 165*
•port on block near the oil filter housing-180
•Best reading off upper radiator hose -150
•Upper heater core feed on engine side of fire wall - 150
•Lower heater core core feed on engine side of firewall - 110
•top of heater core itself -110
•Bottom of heater core -70
•temp at center vent on fan speed 1 - 77*(higher fan speeds fro temps on vent)

 

[turbodieseldyke]

Ambient air temp?

 

[2012jsw]

Roughly 60 at the time according to my digital temp probe that I used for the vent temp measurement

 

[2012jsw]

@oilhammer @turbodieseldyke
The large red and brown wires feed that finned metal unit that sits right in line with the heater core
Isn’t this the auxiliary electric heater to help get cabin temps up before the heater actually works from coolant? I recall there being a specific setting that i read about on how to activate this heater. Can you guys confirm or deny? Recall the setting for it?

IMG_0018.jpeg

• 2012jsw
• Feb 14, 2025

 

[turbodieseldyke]

Before the coolant gets warm, you turn the heat dial to max, and turn the fans high. Not sure which setting, but higher than "1".

 

[2012jsw]

Before the coolant gets warm, you turn the heat dial to max, and turn the fans high. Not sure which setting, but higher than "1".

I thought it was something I had read when I got the car years ago!

happy to report that the car will officially cook your a** out of it now!! Before the temp gauge even thought about moving (3 min of driving at neighborhood speeds) it was already blowing warmer air than the old core did at full operating temp.
Amb. 39 Fahrenheit
Middle vent on fan speed 1 - 150+

IMG_0019.jpeg

• 2012jsw
• Feb 14, 2025

IMG_0022.jpeg

• 2012jsw
• Feb 14, 2025