A4 B6 Cooling System and Heater Issues.

[old guy]

Here's what I have learned after 13 years, 230k miles and having opened and bled the cooling system more times than I can count.

I have bled the system hot. I have bled the system cold. I have bled the system with the engine running. I have bled the system with the engine off. I have flushed the heater core numerous times. I have replaced the heater core.

I have replaced the rear coolant flange. I have replaced both the lower and rear CTS. The rear at least three times. I have replaced the thermostat and the “J” plug. I am on my 4th or 5th coolant reservoir. I have tested and replaced the reservoir cap at least three times. I have performed four complete TB/water pump services. I have replace the head.

I fully understand the flow through the “small loop” and the “large loop” and how it is controlled by the mapped thermostat. I have logged the entire system numerous times with VCDS to where I know how it works and what to expect under the different conditions.

WHAT ADVICE CAN I GIVE?
If you have a 1.8T your cooling system holds 6.9 qt. (6.5 L) of a 50/50 mix of coolant/distilled water. Premix it so you know you have added enough. If you want to make sure you have drained as much as possible you are going to have to do more than just remove the lower CTS. In addition to this you will also have to remove the return line from the oil cooler as well as loosen the turbo coolant return line banjo fitting where it attaches to the block and let it drain down.

The best way to refill your cooling system (without any aids such as a vacuum fill) is to do it with a totally cold system. Open the hard pipe bleeder screw and expose the bleeder hole in the upper heater core return line. Prop the coolant reservoir up 90° from the normal position. This will increase the static pressure.

SLOWLY add the coolant into the system. (By the way, this is the procedure that Diagnosticator continually preached). And by slowly I mean take at least 20 to 30 minutes adding a very small amount at a time. When you see a steady stream of coolant coming out of the hard pipe bleeder close it. When you see a steady stream coming out of the heater core return hose bleeder push the hose back on and clamp it. Return the coolant reservoir to its proper position (don’t forget to plug the coolant level sensor back in). Put the reservoir cap back on. Done.

SO WHY DON’T I HAVE HEAT?
Well for starter the heating system in the B6 (in my opinion) is poorly designed. I posted my reasons for this statement in this thread: Clicky click®

In order to have good heat you need to maintain totally restriction free circulation through the heater core. Any air in the heater core is going to cause problems as I explained in the link. Also any sediment collecting in the crossover tubes is going to cause a problem. Especially during low flow conditions such as at idle or low engine RPM’s.

From what I have seen there are two conditions that cause restrictions in the horizontal crossover tubes. There is a scale build-up that occurs from mineral deposits and then there is a sediment build-up from rust and metal particles.

A strong chemical flush can be effective with removing the scale build-up.



But it really doesn’t have much of an effect on the sediment particle build-up.



If a chemical flush doesn’t get your heat back a heater core replacement may be your only option. I went through this cycle a few times. After about six years my heat dropped off. I flushed the system and it came back. The drop off occurred two more times. Each time a flush cured the problem but the problem came back at shorter intervals. I finally replaced the heater core. What a pain in the ass. Clicky click® But I’m glad I did because I now have excellent heat that starts coming through the vents by the time the temperature gauge reaches the first notch.

When I dissected the heater core I found that the horizontal crossover tubes were almost totally clogged up with the rust colored gunk you see above. When I removed the gunk and dissolved it in water I found that it consisted of small densely packed rust particles. It looks exactly like the particles that you see collecting in the bottom of the coolant reservoir. Apparently there just isn't enough force behind the heater core circulation to keep then from settling out on the horizontal surfaces. Once this occurs your heat will continue to drop off until it becomes altogether nonfunctional.

SOMETHING TO TRY
As I am writing this there is something that just occurred to me. One thing that may be effective would be a high pressure spray through the heater core. I never tried any more pressure than I could generate with a sump pump. The chemical flush can remove the scale but it may take mechanical rather than chemical means to remove the sediment. A high pressure blast may be just what the Doctor ordered to remove the sediment deposits. If anyone tries this please add your comments to this thread.

Happy Heating!

[refueler]

Thanks Old Guy great write up every thing I know about the B 6 cooling/heating system you have taught me. Right now my 2002 B 6 I think the thermostat is leaking by, because on cold mornings (-10) the temp gauge struggles to reach the first line on the temperature gauge the heat is warm but not really warm. Once the outside temperature reaches ~32 and the car has been driven about 20 miles it will eventually reach full operating temperature at that point I have normal heat. I keep expecting to see a check engine light or the engine to start running like crap because the engine is not running at the correct temperature. I guess I might have a thermostat replacement in my future. Thanks again for sharing your knowledge.

[old guy]

From your description it sure sounds like you need to replace your thermostat. If you can monitor your cooling system measuring blocks you should be able to see that pretty quickly.
Good luck!

[Garf]

From the looks of the stuff in the heater core, why does every one blame the heater core design which does the job perfectly when it is cleaned. What I see is the coolant carrying junk into the heater core then it solidifies cause the clogged system. What about a pre-filter before the heater core or does the coolant that needs some form of additive that prevents it from creating the scale altogether in the system?

[old guy]

The problem isn't necessarily with the heater core design. It's from the lack of positive flow through the heater core that allows the crud to build up.

[b1337]

thanks for this post, i have been chasing a air bubble or t-sat or jplug or head gasket/ no heat/over-heat issue for a couple weeks now, but also discovered some breather hose disintegrations and several oil leaks (rear main) and something under the turbo, so dealing with those too plus holidays. i will get a better log tmro morning from straight cold start up

[Protection]

Thanks Old Guy for sharing your wealth of knowledge. I'm curious as to often you have found a failed thermostat to be the cause of cooling/heating system problems? I frequently see responses on here, in other forums, and on Facebook Groups, 'sounds like a stuck thermostat'. I am usually skeptical, as a thermostat is a pretty simple mechanical device, and they are usually changed every 60k miles or so when the timing belt and waterpump are done. In over 30 years of car ownership, and current ownership of 2 B6 A4s (one I have owned since new in 02) and a B5 S4, and two other vehicles, I have only experienced one failed thermostat (on A Mitsubishi pickup with 150k miles on it).

I rarely see a follow up post, where the OP states that it was the thermostat.


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[eljay]

Thanks old guy! Your guidance was immensely helpful to me. I have followed the bleeding procedure and never had issues when refilling coolant.
Also, looking at the specific measuring blocks is very helpful, if nit required, to properly diagnose anything related to the cooling/heating system operation and knowing which component is the culprit.

Cheers.

[old guy]

Thanks Old Guy for sharing your wealth of knowledge. I'm curious as to often you have found a failed thermostat to be the cause of cooling/heating system problems? I frequently see responses on here, in other forums, and on Facebook Groups, 'sounds like a stuck thermostat'. I am usually skeptical, as a thermostat is a pretty simple mechanical device, and they are usually changed every 60k miles or so when the timing belt and waterpump are done. In over 30 years of car ownership, and current ownership of 2 B6 A4s (one I have owned since new in 02) and a B5 S4, and two other vehicles, I have only experienced one failed thermostat (on A Mitsubishi pickup with 150k miles on it).

I rarely see a follow up post, where the OP states that it was the thermostat.

More often than not it isn't a failed thermostat. However It does occur. The good thing is that if you have VCDS or similar software you can confirm it by monitoring the two Coolant Temperature Sensors. And if you have an IR sensor you can do pretty much the same thing without the software. I suspect because of the cost of the mapped thermostat it typically doesn't get changed along with the timing belt service.

Edit: I should note that with an IR sensor you can only check for a stuck thermostat at idle. You will need VCDS or similar to catch a t-stat sticking during a dynamic situation.

[Garf]

The problem isn't necessarily with the heater core design. It's from the lack of positive flow through the heater core that allows the crud to build up.

Is there a way to increase the flow? The hose has a ID of 3/4" which is massive flow so is there a restriction coming from the engine area? When I flushed the heater core with a pump, the flow was terrific.

[Ian1006]

Inline electric water pump wired to a relay and a switched power source to run whenever the engine is on...old guy brought it up in another thread and some people are waiting for the diy ;)
i plan on selling mine soon otherwise I would install a pump along with my new heater core in January and do the diy.

[refueler]

Thanks old guy for your input. Looks like I'm going to learn how to monitor cooling system measuring blocks with my vcds.

[Garf]

I am sure a getto pump will do it, but is there another way to increase the flow without a pump?

[old guy]

I am sure a getto pump will do it, but is there another way to increase the flow without a pump?

Not that I can think of without restricting the flow to the upper coolant return hard pipe.

[Garf]

Not that I can think of without restricting the flow to the upper coolant return hard pipe.

Well, it is a bummer. This car is made in Germany were they have cold winters like Canada, and still screwed up on a heater design.

All my past American and import cars never had heater core issues.

[maurizio]

This should be a sticky

[deyrag]

Is this poor heat performance limited to the 1.8 or are any of you 3.0 guys having similar issues?

[imnuts]

I don't believe the 3.0 has the same heat issues as the 1.8T as the cooling system piping is much different. I'd imagine though that one could experience similar buildup in the heater core regardless of engine. I've been fortunate to not have any heater core issues in my ownership, but there have been a few issues here from 3.0 owners, usually when refilling the system.

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[Protection]

Is this poor heat performance limited to the 1.8 or are any of you 3.0 guys having similar issues?

I believe the 1.8 is more susceptible to to trapped air than the 3.0. I've only drained and refilled it twice, but the 3.0 has never given me any problems with the heat/coolant. And my 3.0 was poorly maintained by the prior owner, whereas my 1.8t has always been pampered, and has still given my problems bleeding the cooling system.

Oh, and thanks for the response OG, I appreciate your input.


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[deyrag]

thanks guys, i was thinking it was more of a 1.8 thing as i have never had any issue with getting nice hot air out of the vents on a cold morning.