Do I need a thermostat? 14 Golf TDI

[hollyatv]

Looking for advice, all appreciated, details below.

2014 TDI Sportwagen, in Canada - got a check engine light, code was for engine coolant over temp. Cleared it, it came back and then went away on its own.

Car is reading outside air as -6.5 to 16 degrees Celsius on start up when it is 20-28 degrees.

The air temp will climb while coolant temp is rising, when it starts to get hot and the temp drops back down to 9 degrees.

Engine is not overheating but it’s climbing to 90 faster than I feel it should? My fuel mileage has also dropped, maybe over fuelling because my engine thinks it is cold?

 

[turbodieseldyke]

You need a scanner to read actual temperatures the engine cares about (intake air temp, coolant temp). The stuff on your instrument cluster is meaningless.

 

[MrCypherr]

The engine does care about the ambient temps so if its reading wrong, it can cause false readings else where.

When youre driving, you need to monitor the actual read out of the coolant temp with a scan tool. If it doesnt get above 80C, then I believe your thermostat is faulty. I had to do the samething as well. Coolant temps were always around 72-75C.

 

[Wilkins]

I assume by outside air you mean what is displayed in the instrument cluster. You are saying that reads an incorrect and erratic temperature. At the same time you are questioning if the engine coolant temperature is reading correctly and you seem to have had a reported over temperature problem while the temperature gauge didn’t report overheating.
Without looking at wiring diagrams I suspect you could have a grounding issue which is affecting the temperature signals. I doubt you actually have an overheating problem but that’s not impossible, the thermostat could have failed, it is about due for replacement. I have never heard of anyone having an over temperature code before.
As Tdd said it is very difficult to diagnose the problem from the information on the cluster. The car has ambient air temperature, intake air temperature and coolant temperature, plus fuel temperature, turbo inlet temperature, turbo discharge temperature and dpf outlet temperature all reported separately to the ecu. A proper scan tool will tell you what each of these are and I’m pretty sure the only one which is indicated correctly in the cluster is the ambient air.
Generally the temperature sensors work by reporting a voltage difference across them so if one, or several are not working correctly the problem could be somewhere in the wiring. Of course they could also be reporting correctly and you do have an overheating problem, but I think you can probably distinguish between-9 and 28 without an instrument. LOL.
Wiring diagrams are available online from Erwin https://erwin.vw.com/erwin/showStepByStep.do
Buy a 24 hr subscription and download everything possible for your car. It’s relatively cheap and amazingly good value if you are doing your own work. Ross Tech also has very good information in their forums, including how to read a wiring diagram.
good luck, wiring issues are a PITA