SQ5 BiTDI Coolant Temperature Fluctuates

wilco184

Registered User
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

New Member
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

Registered User
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

New Member
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

Registered User
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.
 
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