SQ5 BiTDI Coolant Temperature Fluctuates

[wilco184]

Hi,

I have an issue with my 2013 SQ5 where sometimes the coolant temperature only gets to around 75 Deg C on the dash and fluctuates a little either side of this. Sometimes it'll be rock steady at 90 which obviously is fine.

I've investigated a little with VCDS and it seems that the ECU is asking for the coolant temperature to be 75 Deg C sometimes. The 'coolant temperature at radiator outlet: specified' value is 75 or sometimes 90. When it's set at 75 and the coolant temperature creeps up to 76 or 77, you can see the map controlled thermostat F265 duty being increased to lower the coolant temperature, causing the small fluctuation. I can't find what affects this specified value.

Has anyone noticed this with their own BiTDI or could anyone suggest why the car would want to lower the coolant temperature like this?

Many thanks,

Matt.

 

[Lartza]

Hi Matt,

Did you manage to find the reason for the weird behaviour? My 2015 SQ5 TDI (CGQB as well as yours) has started doing the same excact thing. The specified value drops to 75.

It has started dropping the value since summer and warmish weathers arrived.

Thanks,
Lauri

 

[wilco184]

Hi Lauri,

I’ve since worked out it’s because the engine can’t keep water in the ‘block circuit’ as it should. The water in the part of the system should be held static by the control valve in the V up to 105 Deg C. For cars with this issue, water escapes back into the main coolant circuit.

It happens when the outside temp is 15 or above because EGR cooling is activated at 15 Deg C, and the main coolant circuit temp is lowered to 75 from 90.

I’ve had the thermostat and control valve replaced, but these haven’t been the issue.

If you’ve got VCDS I can give you some pointers of things to check.

Thanks,

Matt.

 

[7P_4Xmotion]

Hi Matt,

I'm seeing the same behavior in my CRCA engine Touareg. Could you elaborate more on your findings? This has been bugging me for a long time. I've asked the same question on clubtouraeg but no one has any ideas over there.

For me, when the outside temp is above 15 degrees C, the specified coolant temp is 70. Below this it jumps up to 95. The only exception to this is on a long trip recently when the outside temp was above 15, the commanded coolant temp started out at 70 but then went up to 95 at some point, even though the ambient temp stayed above 15. I've studied the relevant SSPs regarding the coolant system on these engines, and understand how the block circuit is kept at 105 where as the heads are kept at whatever the ECU specifies (normally 90). I just can't work out what triggers the ECU to want to run so cold.

Thanks.

 

[wilco184]

Hi Matt,

I'm seeing the same behavior in my CRCA engine Touareg. Could you elaborate more on your findings? This has been bugging me for a long time. I've asked the same question on clubtouraeg but no one has any ideas over there.

For me, when the outside temp is above 15 degrees C, the specified coolant temp is 70. Below this it jumps up to 95. The only exception to this is on a long trip recently when the outside temp was above 15, the commanded coolant temp started out at 70 but then went up to 95 at some point, even though the ambient temp stayed above 15. I've studied the relevant SSPs regarding the coolant system on these engines, and understand how the block circuit is kept at 105 where as the heads are kept at whatever the ECU specifies (normally 90). I just can't work out what triggers the ECU to want to run so cold.

Thanks.

Hi,

I've noticed that the specified temp will jump back up to 90 Deg C for me when the oil temp reaches 85 Deg C - no idea why this is. I can drive the car a bit harder to get the oil temp up and then the coolant gauge will sit happily at 90 Deg C for the rest of the trip.

I've you can post logs of measuring blocks IDE00025 (Coolant temp) and IDE03582 (Block circuit temp) then I should be able to tell you if the control valve is working properly or if there is a fault similar to my car where the control valve works but the issue is elsewhere - the temperatures follow each other differently depending on the cause.

Thanks,

Matt.

 

[7P_4Xmotion]

I know my control valve is working properly now, I had to replace it recently as it was getting stuck and not fully opening, therefore the block circuit was overheating. But that's all back to normal, I can see the block temp hit 105 then the control valve opens and the temp drops back down to under 100 within a few seconds.

I come across a Russian Touareg forum thread which this issue was discussed, but again no one knows why. https://www.touareg-club.net/forum/showthread.php?t=89886&page=9

 

[7P_4Xmotion]

And just today I drove for 4 hours with 1 stop, the whole time the commanded coolant temp was 70°C. Doesn't seem right, but there are no fault codes or anything like that which might cause this. The oil temp was well over 85 on occasions. I'm wondering if the ecu is running in some kind of protection mode after running hotter than normal on a previous occasion.

 

[7P_4Xmotion]

Wait a sec, is this you on the same site? https://www.touareg-club.net/forum/showpost.php?p=3155965&postcount=317

Can you please share the image on this page - http://www.touareg-club.net/forum/showpost.php?p=2617419&postcount=112
Apparently it's a document from VW saying this behavior is normal. I can't register with that forum.

I'm starting to think it's normal after all. I not only had a stuck coolant shut-off valve, but the solenoid that controls the valve was also faulty. This can cause the shut-off valve not to close and therefore the block circuit will never reach 105°C in summer.

 

[wilco184]

Hi 7P_,

Yeah, that's me on the Touareg forum - they're the only other people with a similar issue to ours that I've been able to find. I've attached the image to this post.

70 Deg C commanded is normal I think, reduced for better EGR cooling in warmer weather. I've seen similar behaviour on a Q7 with the CRCA engine, where the commanded temp didn't rise even with the oil temp over 85.

What does your needle do now? As long as the needle is at 90, all is well - even if the true commanded value is lower.

Thanks,

Matt.

 

[7P_4Xmotion]

Thanks very much Matt.

My needle is solid now, but I've had to replace a few things to get it that way. Sticking coolant shut-off valve, the vacuum solenoid that controls it (N489) and two temp sensors (G62 & G694).

So you still haven't been able to resolve your fluctuating temps? Did you change the vacuum solenoid as well as the shut-off valve? I'm sure you know this already but the shut-off valve closes when vacuum is applied, so if the solenoid is faulty it could keep the valve from closing the block circuit.

 

[wilco184]

That's great that you've fixed it.

No, my temps still fluctuate. I've only changed the shut off valve, though I have tried bypassing the solenoid so that the valve is permanently closed and the block temp still didn't rise much above the normal coolant temp, and I've proven both temp sensors by unplugging the solenoid, thus keeping the shut off valve open and verifying that both sensors read the same. I've also verified with a camera that the shut off valve is closing.

For my particular case, the differential between the block circuit temp and normal coolant temp decreases as engine speed increases, suggesting that water is flowing out of the block circuit but I can't work out how it's getting out!

Thanks,

Matt.

 

[7P_4Xmotion]

That's bizarre! I believe the only other path for coolant to escape is through the one-way bleed valve and back into the reservoir (besides the one-way valves for coolant entering the block). Part 28 in this diagram - https://webautocats.com/etka/audi/aq5/751/121410/

 

[wilco184]

Ah yes, I forgot to mention that the bleed valve has also been replaced, and I'd checked the old one which was working fine too.

Audi also replaced the one way valves for coolant entering the block a while ago at my request whilst the car was in for other work, though I've no way of confirming this was actually done but they were on my invoice. I did see the one on the left hand side (item 37 on your diagram) when it was out and all looked correct, but I've never seen the one on the other side (item 40). I'd like to check this one I've not seen, but it's difficult to get to.

Other than that, I can only suspect a defect with the block, heads or head gaskets.

Thanks,

Matt.

 

[Lukbla]

Hello, I’m from Poland, and I have similar problem with Touareg CRCA.
Will you found solution?

 

[wilco184]

Hi Lukbla,

No solution, I'm afraid. The only part is the 'block' cooling circuit that hasn't been replaced now is the head gasket, so I suspect the coolant passes between the two circuits at one or both of the head gaskets. The issue doesn't cause any problem when driving, just that the gauge wanders.

Kind regards,

Matt.