Golf Mk6 2010 1.6 TDI CAYC - Overheating - Bad interior heating

(Problem solved: Blocked heater matrix / heat exchanger - replaced with a new one)

Hello everyone!

I am trying to help a friend and his missus with their overheating Golf mk6 2010 1.6 TDI Diesel CAYC Hatchback.
The symptoms are very weak interior heating (worst at passenger side), as well as overheating engine under load (uphill) at high gears (low rpms). While the engine coolant temp rises to 110 degrees celsius, the interior heater is blowing cold air. As long as the RPM's are around 2k it seems to be able to keep the temperature around 90 degrees celsius, maybe abit higher.

The car has been like this for maybe a year. So it's seen alot of a 100+ celsius temps during the last year.

It felt like the return hose on the heater matrix was not as warm as the inlet hose on the heater matrix.

Tested coolant flow at the coolant expansion tank return hose, seemed fine and increased with RPMs. Flow compared to my 2008 Skoda Octavia BMM engine. Color of the coolant was closer to clear/yellow than red. Coolant level around 1-2 cm below minimum before removing the cap.

Car has dome 134 000 km / 83k miles. No sign of a timing belt / water pump replacement in the service history book. The change interval stated in the service manual is 210 000 kms.

No obvious related fault codes in VCDS.

Test drive with VCDS hooked up, monitoring engine coolant temp, oil temperature, coolant temp at radiator outlet, radiator fan

Ambient temp while troubleshooting: 4 degrees celsius
Engine coolant temp as high as 115 degrees celsius (needle in dash went above 90 when VCDS showed 110 and up)
Oil temp follows coolant
Coolant temp at radiator outlet: between 13-25 degrees celsius. Radiator hoses cool to the touch.
Radiator fan as high as 90% engaged when the coolant temp was highest

My first thought is obviously a faulty thermostat because the radiator isnt getting any action, and a blocked heater matrix.

I have seen some forum posts on the CAYC engine where replacing the thermostat ain't helping, but bypassing/changing the heater matrix has helped. Could the coolant flow be disturbed resulting in thermostat not opening because of a blocked heater matrix?

Attached is the VCDS auto scan and production data sticker, as well as a picture of the coolant return flow to expansion tank at 2000 engine RPM.

Grateful for any comments or advice. Thanks.


Attachment 38927Attachment 38926
Attachment 38925

I would fit a new thermostat unit first, they are well known for failure, revised part number 03L 121 111 AN then if that fails to help, the revised water pump part number 03L 121 011 P and cam belt kit 03L 198 119 F. If the additional electric pump fails, these usually come up as a code but it would be worth doing an ignition on output tst to listen for it running, it is on the front of the engine. Golf 6 doesn't tend to suffer from heater matrix problems.

https://digital-assets.tecalliance.s...5.0.384f7bb938

I see, thanks for the reply. Would you recommend getting that expact part number from the official dealer, or would a replacement part from a premium aftermark brand like FEBI BILSTEIN 39224 or MEYLE 128 228 0014 likely be sufficient?

Another question; will a faulty thermostat affect the interior heating? I can't really understand why the heating doesnt work properly when the inlet hose for heater matrix is hot, and the coolant is above 100 degrees celsius, if the heater matrix isnt blocked in some way.

I didn't check under the old number 03L 121 111 S, Febi and Meyle are excellent

https://digital-assets.tecalliance.s...5.0.384f7bb938

The parts store checked the VIN number ( WVWZZZ1KZAP165385 ) and changed the thermostat to MAHLE ORIGINAL TI 225 92D
Attachment 38928
Original part number 03L 121 114 B.
Seems like it changed from a simple thermostat housing to the 4/2-way valve with thermostat around the middle of year 2010.

Hopefully abit easier to change this one.
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Removing and installing thermostat
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I did another test run yesterday, before replacing anything.

Noticed intermittent function of the thermostat and interior heating. I also confirmed the VCDS temperature readings with an IR thermometer. Interior heat was bad at the driver side and worse on the passenger side. The return from the heat exchanger was about 20 degrees celsius lower than the inlet. (ca. 70 / 50 degrees celsius).

Decided to bypass the heat exchanger in the engine bay with a 19mm ID 90 degree radiator hose and couple of 19mm hose couplings, because I believed that the insufficient flow through the heat exchanger cooled down the coolant too much to open the thermostat.

After bypassing the heat exchanger the thermostat opened as it should, the coolant stuck to 90 degrees celsius and the radiator outlet temperature reading increased instead. As it should.

Partly clogged heat exchanger confirmed to be the cause of overheating at least.

Thanks to this thread for pointing me in the right direction!

A PQ35 platform blocked heater matrix is rare and one hell of a job to replace.

4 hours into it. Got the old heat exchanger out. Can confirm one hell of a job, felt like I had to rip the whole interior out. :D

Anyone know the "book time" for it?

Alot of gunk:
Attachment 38955
(Cleaned up the snow afterwards ofc)

A laughably optimistic four hours start to finish not including de/re gas.