Hello guys, I have a huge problem with an Q7, I have a big heater problem, it has been in my shop for over one year now, with the same problem.
no particular fault codes, We have changed almost the whole car.
The problem:
heater gets cold after 30-45 minuts of driving, have had it working for 3 weeks max, but now its back down to 30-45 minuts.
problem startet with the temp gauge doing some wierd stuff, jumping for 90-130 and back down in the span of 30sec, then ofc got the MIL on.
swapped out the temp sensor, was working fine for some weeks, then it all went to H***...
car started to not blowing hot air, ice cold like only ac was working, no faults on flap motors, have checked flap motors, and they work fine, I have accses to both VCDS and ODIS.
got word that the last owner put in "radiator sealent", started chasing this problem, first with basic things, like checking coolent for exhaust gasses, nothing, fault codes, none.
then swaped out some coolent valves, was working for some days, and back to square one.
as you can se, we have tried alot, will under list all that has been changed, all in search for heat!

parts that has been changes out:
radiator.
heater core front and back.
blower motor.
aux heater.
aux heater pump.
aux heater diversion valves.
temp senders.
multple coolant flushes.
water pump.
thermostat.
coolant valve behinde engine.

so, if someone has any idea, I would really appriciate it.
and to clearify, its not a costumer car, its own by the shop i work for, we are starting to get real tierd of this car, not wanna work.

thanks in advance for any help tp solve this issue!.
best regards.

 

How many times has the thermostat been changed. I've seen instances where someone had three faulty units. Two back to back and one failed after a few weeks. Unless there's major air in the system this should not happen. Hopefully some of the more experienced guys like Dread and TDI Prestige can chime in.

 

FYI: The poster of thread is located in Norway

You should look at live data feeds on temp sensors/ map or record the data-set if possible for reference to see if thermostat is opening at correct temp, etc., bad thermostats happen all the time, or getting one that's the wrong temp for the car, etc. You can always manually test the thermostat if it is in doubt. You can get a bad sensor too, but check the low-hanging fruit in the VCDS first.

Here's what we know about the aux heater/boost electrical heater unit: It's an electrical driven heating element that has it's own coolant circuit and dedicated pump to warm the cabin quickly after a cold start. It cuts out once car thermostat opens (this is when you are going cold, me thinks) This Aux heater can also be defeated by the car's BEM module, so be sure to check your battery charge/amp hours status. Look for door handle sensor errors; see "Got Parasitic Drains" thread if you have the door handle trouble codes...those are draining your battery constantly...losing electrical powered features such as the HVAC fans, aux heater, etc, etc., is a common symptom of a BEM circuit problem.

Aux Heater must be enabled to Automatic mode in the car's A/C controls menu to function. You should also ensure the Auto Recirculation function is enabled, as this is important to the car being able to manage the heating/cooling/humidity control functions. Disabling the Auto Recirc is detrimental to the systems ability to keep the heat where you want it, as well as humidity control and and could be a big part of the problem. You might also verify that the flap that closes for the recirculation mode is functioning correctly and not stuck in place, or been eaten away by rodents to make a nest with. (yes, that's a real thing)

I'd start by looking at the BEM (Gateway Module) channels, as it will show you the battery state-of-charge levels, as well as if anything is currently disabled by the car's BEM system due to a low battery status.

Some TDI's in Europe get the actual 'parking' heater that runs on diesel fuel (mounted in void in left front fender behind the wheel liner/in front of A pillar) and offers different functional parameters from the electric 'booster' heater...if you have this you'll be able to see the additional coolant lines running to/from the parking heater, and it has it's own dedicated fuel line running back to the tank. Since you are in Norway, I'm wondering if you have this additional 'Parking Heater' or not...only found on TDI models.

When the car reaches operating temp / thermostat opens, the Aux Heater (electric booster) circuit is defeated and that appears to be when you are getting 'cold' again.

Overseas TDI's (non-North America cars) may also have an auxiliary Parking Heater (a separate, diesel-powered heating unit mounted in left front fender in front of the A-pillar) that offers different functional parameters from the electric 'booster' heater, which is meant to work only when the car is cold-started under specific temp differential demands, etc.
The Parking Heater has it's own coolant lines plumbing going up into the fenderwell behind tire, which is visible, and a dedicated fuel line running to the rear of the car. Controls for it would also be in the MMI.

 

If your battery management /charging/voltage all shows as good and there's no door handle trouble code faults, then verify the MMI settings to be sure aux heater and Air Recirculation are both set in Automatic mode (located in the A/C sub-menu on North America cars).

Verify coolant reservoir tank level.

Verify temp sensor/ hardware and observe to ensure your radiator fans aren't running when they should not be running. You've already replaced so many items, you need to look outside the box at what impacts your car's ability to create/retain heat. Also, pay careful attention to the items you swapped out that resulted in heating function being ' normal' for a period of time, such as the temp sensor, etc., but also what they link to, or control, such as the radiator fans.

Hopefully this gets you started down the right path through elimination of common feedback pathways, and internal car-control mechanisms.

Keep us posted!

 

thanks for the replys guys, Im gonna try answering your questions sugestions TDI Prestiqe, cause yes, I'm located in norway, and yes I have the diesel powered aux heater, dumb of me not mentioning it, I live here, so I'm so used to almost every q7 having tdi having it from factory, but yes, it is a 3.0tdi, and the gauge in the car is not reacting, even tho we have change the sensor, the termostat isnt opening, and i have verifyed it with both vcds and touching the hoses, but in my mind that should not affect the heating element, or am I completle wrong? should it not go through the heater core regardless of thermostat opening?
I have a flap fault, on the right flap, but blowing hot on every other air went, so Im out of ideas.

 

the parking heater can isolate the internal heater cores from the engine, with the webasto running only when the inside loop reaches temps does it open up recirculation to the engine.. if it had to warm up the engine block too it'd take forever and the goal is to get heat out the vents ASAP.. its supposed to get the internal loop up to temps and then open up and start warming up the engine.. and then by the time the webasto shuts off the engine is supposed to be at operating temps so it can take over.. if those valves never open up, the engine heat never flows into the heater cores and once the webasto hits its max run timeout you are going to lose all heat.

Sure sounds like that valve is not working.

 

That sure sounds logical, I'm gonna investigate those valves abit more, since they where swapped out for used once, maybe I can find some new ones,I'll see what I'm able to find out, and post back!

 

Checked my notes, its the N279 Valve that I suspect is your culprit.. its under the drivers/left hand fender, best to just pull out the whole webasto, replace it and then put it back.

 

I own a 2012 Q7 TDI S-line prestige in USA, I was driving towards the end of winter (but not using heater) when I suddenly experienced the same exact symptoms. My temp suddenly jumped so high the ECU reduced power and cut the throttle and within 30 seconds it returned to normal, this happened about 3 time's in the same day and hasn't happened since. No codes triggered or anything. The next day was colder so I turned on my heater and it never got "hot" it was warm from the aux heater but after about 30 to 45 min's it was blowing cold air. Since then, the only time I can get warm air is right after I start it. I suspected the thermostat and ordered a new one along with a new water pump but have yet to install it and now in the middle of summer, no issues with overheating so I don't believe either of those are the issue. I don't know if I have the electric auxiliary heater or if I have the US version of the fuel powered heater. I saw in one response that you can see it on the left (driver side) behind the a-piller and that you can see the coolant lines running to the front but I'm honestly not sure what it would look like. I have the s-line and I have the wheel vents and several small radiators which I'm sure can be for oil, coolant and mayber even brakes?. Has the op fixed the issue? How do I know for sure which auxiliary heater I have? Does anyone have pictures or a diagram of the US version of the fuel powered heater?

 

You need to get a VCDS and scan your car fully...top to bottom.
The USA cars didn't get the 'parking heater', which is diesel powered; we have the electric booster installed and referred to as the 'auxiliary heater'. You do have to be sure it's turned on in the MMI A/C Menu, as otherwise it doesn't work. Since you weren't getting heat at all, that sounds more like a stuck valve or vacuum powered flap, etc., which is keeping the heat from getting into the cabin. Don't forget to check the coolant level, as TDIs lose coolant when the EGR system eventually leaks either internally or externally.

Also, be firmly aware this car shuts down features you would consider essential, such as the blower fan and the electrical auxiliary heater, and lots of other things too, when it senses a low voltage condition. So don't forget to have your charging/battery system checked, as that is very often the root-cause of many, many symptoms encountered on this car. While on this topic, the door handle proximity sensors all go bad and turn into parasitic battery drains, which contributes to the low voltage problems. Easily resolved in minutes (the grounded door handle sensors, that is), if you care to search for "Got Parasitic Battery Drains?" within this forum.