Mysterious over heating issue

[leenowell]

Hi All.,

I am having overheating issues and am hoping you will be able to help me as we are now car-less 😞

Test

Starting the car from cold, leaving it idle I tested the temp gauge readings against the coolant temp via OBD with the following results.

After approx 2 mins temp gauge 90 - OBD around 30

During next approx 15 mins, temp gauge stays at 90 and OBD gradually rises to around 80

Temp gauge starts to rise beyond 90 and OBD also rises.

Temp gauge gets to 3/4 and OBD hits 88 - red coolant warning comes on

Temp gauge continues to rise but OBD hovers between 88 and 90  (I only left it for a minute or so just in case).

 

Other things of note

1. At the end of the test I felt the hoses. Rad top hose is hot, the bottom hoses and those going in to the thermostat are cold

2. I have been having an issue with a slight leak where I can't find where it is (around 100ml loss over a few days) this seems to cause air in the system as the heater goes cold and bleeding the heater / the bolt on top of engine fixes it.

3. The temp gauge behaviour is also consistent with driving the car (I haven't done a OBD test whilst driving)

4. the cooling fans on the radiator does not get activated

 

The test above to me would indicate a temp sensor issue (assuming the temp gauge and the computer are using different ones) but the hose temp issue would seem to indicate thermostat issue.

Anyone have any idea whats going on?

 

Thanks in advance for your help

Lee.

[Magnet]

Hello Lee, 

I think you are diagnosing the issue yourself, and if this was my car, I too would be first renewing the coolant temperature sender. Personally, I would not use an aftermarket one. 

Fingers crossed that will sort the gauge discrepancy. Next on my list would be be to replace the coolant reservoir cap (again with a VAG one) to ensure the system remains correctly pressurised. This may be the source of your slight coolant loss if you cannot find any external evidence of a leak. 

I’m from the school of doing one thing and test, otherwise you will not know what corrected the fault.

Last component ( if necessary) would be the thermostat. Most certainly VAG only.

Kind regards,

Gareth. 

[leenowell]

Thanks very much Gareth for getting back to me.  Is the coolant temperature sender only used for the gauge and there is another one for the computer?  I did a quick google and it looks like it is at the back of the engine.  Curiously, I have another issue where there seems to be an oil leak in that rough area and and is squirting oil around the place (there is a complicated set of small rubber pipes which seem to be covered in oil).  I wonder if this is related?

 

thanks

Lee.

[Dan3222]

You could get one of these to help source coolant leak.

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F230723938073

 

[Magnet]

Hi Lee,

They are normally separate from the ECU coolant temperature sensor, and the two often have two different colour plastic connector heads. Sorry, but I can’t be positive as far as Audi is concerned. If they are separate then the gauge sender usually has two wires, and the coolant temperature sensor sometimes three. Best bet would to to wander into the parts section of your main dealer and they will have a diagram which will help with location, and clarify one-doing -two -jobs or not. 

Oil leak? Sorry Lee, I can’t offer any suggestion with this. 

Kind regards,

Gareth. 

 

[leenowell]

Thanks very much both.  I have changed the coolant temperature sensor and looks like it has solved the problem.  Temp gauge goes up steadily in line with the OBD value and stays at 90.  OBD registers temp once hot bouncing around the 105 to 113 mark.  Assume this is normal?  I'm sure saw somewhere that the ECU assumes up to 110 as normal and registers the gauge at 90.  Only other things to add

1. When I looked at the OBD error values after fitting it a new error saying the coolant temperature senor had a high voltage and the known engine mixture error.  After a while I thought I would try to clear the errors via the OBD app and they both had disappeared.

2. Looks like the oil leak it squirting out of the dip stick top.  I noticed that some of the pipes I had to remove to get to the sensor had a load of crud in them (not fully sealed but plenty there).  So, I am thinking my oil issue is a breather pipe issue - will raise another thread on that.

Thanks all... glad I didn;t have to replace the thermostat as that seemed a fair bit more involved.

[Dan3222]

Thanks for taking the time to post your resolution, it’s good to hear people fixing their own issues rather than relaying on a garage. 

[S5SUPERCHARGED]

I have the Audi S5 supercharged. Driving on a cold day temp gauge hit 90 that is normally the running temp. Sok  as it hit 90 fans kicked in went up to a 100 so I turned off. Thought was water pump it wasnt faulty thermostat.

[Magnet]

Hello David,

Not sure why you would have suspected the water pump based on the symptoms you describe. You didn’t change it did you? 

Kind regards, 

Gareth.

[S5SUPERCHARGED]

It was faulty thermostat sorry not water pump