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Discussion Starter #1
Hi,

I'm looking for some assistance in trying to diagnose my 2010 B7 3C 125TDI CFGB engine Passat which is running cold.

Initially I suspected the thermostat because even after an hour running along the freeway, the temperature would float around 50-60c (outside temperature between 5-15c). When in heavy traffic, the temperature might get close to 90c but would generally fall way once on the move again. I also confirmed these temperatures using vagcom and compared the 2 temperature sensors which rose in temperature together (also indicating a faulty thermostat).

I then went out and bought a new genuine VW thermostat (with housing) and spend the best part of a day installing it.

Unfortunately I'm still getting the low temperatures and the car is not reaching operating temperature with both sensors still rising together.

In further testing this morning I ran the car for about 20 minutes parked in the driveway at 2000 RPM, the car did reach 90c with the radiator feeling hot at about 50c (which makes me suspect the new thermostat was also opening early). However, I then went for a light (flat road) drive and expected the temperature to fall away which would have confirmed a faulty thermostat, though it did not (and would not) and stayed at the correct temperature of 90c.

I've established the following:

- Temperature sensors appear to be indicting correct values (also confirmed against similar oil temperatures).
- Thermostat has been replaced with Genuine VW one.
- Radiator does warm up at the same rate indicating thermostat is faulty.
- Car's temperature does not drop away once 90 has been reached during light driving, indicating nothing wrong with thermostat.
- If car has not reached 90c before driving, it will never reach it during usual driving conditions and will rise and fall based on driving conditions.

Does anyone have any ideas as I'm really confused about what to do, or test next and I don't really want to purchase another thermostat at $120 on suspicion the new one might again be faulty?

Would it be possible the heat is being incorrectly circulated with pump 2 (diagram 8 below) ?

To me, it's a complicated network of pipes which I don't properly understand.

Thanks for assistance in advance. :)

Regards,

Ben




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Discussion Starter #3
No, no bubbling or anything out of the ordinary, coolant is also retaining its pink colour.
 

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Ben,
I am interested to know if you got to the bottom of this problem. I have exactly the same symptoms with our 2012 125 TDI Passat. I am convinced it is a stuck-open thermostat, but the local VW dealer says there is nothing wrong and they've never heard of another Passat with a similar issue.

So now I am going to replace the thermostat myself. Do you have any advice on the best way to go about it? I can't find anything about the model and "how to" get the thermostat off with minimum effort on the web.
 

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Discussion Starter #5
Hi,

No, unfortunately I have not got to the bottom of the problem. As stated up the top I initially relaced the thermostat and on the first drive the temperature went straight to 90 and I assumed I'd fixed the issue. The next day the same issue returned.

On investigation of the removed thermostat I found it was opening about 75c instead of 88c though I think this was only part of the problem.

After a month I proceeded to relace the new thermostat assuming this was faulty with another new one. Unfortunately the issue didn't resolve and upon testing the removed thermostat I found it was working correctly.

My attention then turned to the (secondary) electric EGR coolant recirculation pump (as I believe this is used to quickly warm the engine using the exhaust gasses) and I also replaced this is as it was worn and not running smoothly. This also has not corrected the issue.

Interestingly, each time I refill the coolant system the car preforms perfectly (but only until it is turned off and re-started) which suggests the computer is doing something strange? (maybe something to do with anti pollution system valve not letting exhaust gasses through resulting in less heating of the coolant.)

As I'm not a professional with the anti pollution system or computer system and can only find little information on this issue I plan to take it to VW in the coming weeks to see if they can locate the issue.

FYI: replacing the thermostat is a pretty long and tedious job and can take up to 8 hours by the time you remove the battery etc, intercooler pipes, pull back the water pipes and remove the thermostat. (you'll need patience, lots of lights, small hands and some universals to get around corners & an allen key with a round end for the thermostat)

I'd be interested to her how you go.
 

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Hi,

No, unfortunately I have not got to the bottom of the problem. As stated up the top I initially relaced the thermostat and on the first drive the temperature went straight to 90 and I assumed I'd fixed the issue. The next day the same issue returned.

On investigation of the removed thermostat I found it was opening about 75c instead of 88c though I think this was only part of the problem.

After a month I proceeded to relace the new thermostat assuming this was faulty with another new one. Unfortunately the issue didn't resolve and upon testing the removed thermostat I found it was working correctly.

My attention then turned to the (secondary) electric EGR coolant recirculation pump (as I believe this is used to quickly warm the engine using the exhaust gasses) and I also replaced this is as it was worn and not running smoothly. This also has not corrected the issue.

Interestingly, each time I refill the coolant system the car preforms perfectly (but only until it is turned off and re-started) which suggests the computer is doing something strange? (maybe something to do with anti pollution system valve not letting exhaust gasses through resulting in less heating of the coolant.)

As I'm not a professional with the anti pollution system or computer system and can only find little information on this issue I plan to take it to VW in the coming weeks to see if they can locate the issue.

FYI: replacing the thermostat is a pretty long and tedious job and can take up to 8 hours by the time you remove the battery etc, intercooler pipes, pull back the water pipes and remove the thermostat. (you'll need patience, lots of lights, small hands and some universals to get around corners & an allen key with a round end for the thermostat)

I'd be interested to her how you go.
Changed thermostat, my problem is the same as yours! fine one day doesn't get warm the next,interested to hear if you ever fixed it?
 

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Discussion Starter #7
Hi,

Yes, still having the same issue. I took the car to a German car expert a few weeks ago and they charged me a couple of hundred dollars to tell me they thought it was the thermostat (despite me changing twice and testing the second one which I removed)

When the car is started from cold the radiator will warm before the temperature gauge gets to 50c, however the heat doesn't appear to be coming through the thermostat but rather from the electric circulation pump which pulls heat from the EGR cooler. I'm pretty sure this is the root of the problem (either the pump running when it shouldn't or not enough heat from the EGR cooler maybe because of a valve)

I'd be interested to hear any progress you make on your car.

Regards,

Ben
 

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Ben,

I am having a similar issue.
If the temperature drops sub-zero (C), the problems start.
In city mode driving agressively the temperature rises. As soon as I go above 50kmh or leave the city and hit the highway, the temperature drops, feet are freezing and all in all it is a terrible experience.
Changed 2 thermostats, since VW dealer confirmed that the first one was faulty (happens 1/100).

Long story short.
1. Brand new thermostat.
2. In city, slow speed, traffic - temperature goes till 90°C.
3. As soon as I go faster and consume less fuel, the temperature needle hits the ground.

Really looking forward on what could be the issue.
 

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Hi Ben/Cuspurs
I know your posts are from a while back but did you ever get it sorted??
I have exactly the same issue. Thermostats replaced yesterday as I was experiencing the same symptoms as you describe. All good that day for journey home...circa 60 miles.
Then next day back to the same problem :(. Exactly the same as Ben and Cuspurs dscribe.

Did you ever try replacing the coolant temperature sensor?

Does anyone think a leaking water pump might cause the issue? Feedback to the ECU that pressure is down so keep the stats open?

If anyone fixed this issue I would love to hear how. Swiss mechanics are expensive....
 

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Hi, I have the same problem.
This engine have many thermostats.
Maybe 4/2 way thermostat housing, dsg thermostat or the intermediate thermostat.
One of those can make fluctuation in the temperature!

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I had the same problem, I found another forum (http://www.ukpassats.co.uk) mentioned that vehicles fitted with DSG gearboxes have an extra inline thermostat for the DSG heat exchanger that can fail.

I checked the DSG thermostat on my 2011 TDI 125kW VW Passat, it was was jammed open and prevented the engine from warming up during cold weather. The problem was resolved by replacing the DSG inline thermostat. This thermostat is marked as item 7 on the coolant diagram included in a previous post. The DSG thermostat is located under the battery holder, and it can be replaced without draining the entire cooling system, when engine is cold. When replacing the DSG thermostat catch the coolant in a small container. Take note of the orientation of the thermostat, the flow is indicated on the housing. It took around 30 minutes to replace the DSG thermostat part number is 1K0121113A.
 

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I had the same problem, I found another forum (http://www.ukpassats.co.uk) mentioned that vehicles fitted with DSG gearboxes have an extra inline thermostat for the DSG heat exchanger that can fail.

I checked the DSG thermostat on my 2011 TDI 125kW VW Passat, it was was jammed open and prevented the engine from warming up during cold weather. The problem was resolved by replacing the DSG inline thermostat. This thermostat is marked as item 7 on the coolant diagram included in a previous post. The DSG thermostat is located under the battery holder, and it can be replaced without draining the entire cooling system, when engine is cold. When replacing the DSG thermostat catch the coolant in a small container. Take note of the orientation of the thermostat, the flow is indicated on the housing. It took around 30 minutes to replace the DSG thermostat part number is 1K0121113A.
Good to hear that you fixed the problem.
It s better before buying the thermostat to check first with VAGCOM the change of fluids temperatures (coolant, dsg and oil) during warm up and driving. An increase of temp of both dsg and coolant together means bad dsg thermo. Also with vcds you can see what cools the coolant, the air or oil.

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Good to hear that you fixed the problem.
It s better before buying the thermostat to check first with VAGCOM the change of fluids temperatures (coolant, dsg and oil) during warm up and driving. An increase of temp of both dsg and coolant together means bad dsg thermo. Also with vcds you can see what cools the coolant, the air or oil.

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HaKoU,
Thank you for your suggested diagnostic procedure; I agree that using your method you should detect an anomaly in the DSG oil temperature. It is always best to confirm with diagnostics prior to purchasing expensive parts! If you don't have access to VAGCOM you can detect a faulty inline thermostat by removing the thermostat and checking flow with cold water, there may be a small flow even if working correctly, however a larger/open flow means it has failed, then run hot water through it and the water flow should be full open.
 

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There are a number of problems that I have experienced with a 2010-2011 VW Passat, as follows:

1. Stop/Start failing to start vehicle when hot intermittently and occasional long crank times; cause: crack in fuel lift pump housing, replace fuel lift pump in fuel tank. This is a safety flaw as the vehicle can unexpectedly fail to start when trying to take off at an intersection.

2. Array of error lights on the dash, ABS, engine, steering, airbag etc. steering radio controls and horn not working; clock spring contaminated by human hair and broken wires, replace clock spring (recall in USA, but not in Australia) copy option codes from old clock spring before replacing. This is a critical safety flaw because the airbag and horn stops working, also there is a possibility that the airbag could deploy unexpectedly.

3. As previously mentioned, vehicle failing to reach running temperature when cold; cause faulty DSG thermostat, replace thermostat.

4. Intermittent driving light error; cause, corrosion on driving light bulb, replace driving light bulb.

5. No high-beam headlight; cause faulty indicator and headlight switch cluster, replace indicator and headlight switch cluster.

6. Fix the problems that previous mechanics introduced trying to fix the above faults... replace aftermarket ABS sensors with genuine, re-code the ABS module with correct installed options.

The previous owner was advised by a "mechanic" that the car could not be economically fixed and was advised that the main computer was faulty. I purchased the vehicle with all the above faults apparent, I'm pretty sure that VW are aware of most of these faults as the later models appear to have these faulty parts re-designed to eliminate the identified problems!

I hope that this post will assist some other poor unfortunates that have purchased a 2010-2011 VW!
 

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Discussion Starter #18
Hi,

Been a while but with the suggestions above I ordered the DSG thermostat and changed it last night. Sure enough the old thermostat was stuck open and after fitting the new inline thermostat everything is back to normal.

Looking at the diagram above you wouldn't think this thermostat would have such a impact on the cooling system, but obviously if water is allowed to pass, it's then finding its way through the radiator back through the electic pump and back up to the motor.

I would never have guessed a secondary thermostat would have caused this and i'm also surprised how rare this is. Even the dealer I took my car to couldn't diagnose it and asked me to charge the main thermostat once again. (and happily took $300 for the short discussion)

If nothing else, I hope this thread has helped other people having the same problem and saves allot of time, parts and headaches !
 

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Hi Ben, It's good to hear that you have this issue sorted-out. I have also noticed the fuel economy improved significantly once the thermostat was fixed...
 
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