Operating Temperature Not Reaching 90

[lukemk5gti]

I've seen over the past while that this is quite a common issue.

The coolant temperature should reach 90degrees after a few miles of driving from cold but when it fails it tends to only reach 90deg when idling in traffic or doing city driving.

I currently have this problem which judging by my symptoms I imagine is my main thermostat.

Note - I have a manual so don't have the DSG sensor

I had alignment booked in with VW on Friday so they said they'd take a look at the main thermostat for free and reported that it was working fine. This happened to be after congested city driving so when I arrived at VW my gauge was at 90deg.

I tried to explain this to them but I really got little to no help on the matter.

I don't want to shell out €400 to replace the main thermostat nor do I want to spend 6 hours under the engine trying to replace it myself until I can guarantee what the problem is.

Does anyone know how I can determine this? Or if it could be anything else?

[john87]

I'd like to know how they managed to prove that the thermostat was working fine as it's purely a mechanical device; it's not controlled electrically like other 'valves' on the car..

My money would still be on the stat being stuck in the fully open position 

[lukemk5gti]

I'd like to know how they managed to prove that the thermostat was working fine as it's purely a mechanical device; it's not controlled electrically like other 'valves' on the car..

My money would still be on the stat being stuck in the fully open position 

Good question but I actually was looking at this. With VCDS you can give the thermostat a code and tell it to open/close effectively. If it responds it's working, if it doesn't it's not.

It won't give an error if you scan as it doesn't provide feedback but if you tell it to do something and it fails you know it's broken.

[marko]

Faulty temp sensor??
Common on vw's

[lukemk5gti]

Faulty temp sensor??
Common on vw's

That was original thought but after some digging on the forum it would appear due to my symptoms I'm most likely looking at my main thermostat.

[jaye2992]

Anybody have a part number for the coolant temp sensor? Mine gets upto 90 finebut then occasionally drops and then goes back to 90....garage I got it from said they had changed the thermostat so hopefully not then. Unless they lied.... Thanks :)

[chris s]

I've got a temp sensor I never used it you want it...£15? I had the same issue with my car and it definitely sounds like it's the main thermostat to me

[jaye2992]

The one that goes on the intake side? Gets to temp fine but drops and then goes back to 90? Got a receipt for new thermostat so don't know whether to take it back to them or not hmmm

[chris s]

Yeah that's the one I've got spare, but to be honest it does sound like mine which was fixed when I had the main thermostat replaced

[jaye2992]

Cheers I will let you know if I need one :) probably will ring the garage tomorrow where I bought the car....they were meant to change the thermostat. Just been for a drive out and it was all good till going steady in 4th then it dropped to 70degrees and slowly went back to 90 haha.

[lukemk5gti]

I've got a temp sensor I never used it you want it...£15? I had the same issue with my car and it definitely sounds like it's the main thermostat to me

Did you have vw do this or did you do it yourself?

How did you/they diagnose it?

[jaye2992]

Yeah that's the one I've got spare, but to be honest it does sound like mine which was fixed when I had the main thermostat replaced

Is it a new one you have with a new seal? Thinking of changing it anyway as today to work and back it seemed fine. The garage are finding the parts invoice to check what they have had changed....

[jaye2992]

Changed the grey sensor today and everything seems to be the same....is it normal for the gauge to say 90 and then in the torque app the coolant temp says 77? Also is there only 2 coolant sensors on the gti? Sorry for all the questions!

[skard]

I had the same issue and fixed it by changing the main stat.
My Fiscon says ~75c+ when the cluster gauge says 90c.
If it fluctuates then I'd say it is failing.

[jaye2992]

Ah boooo, garage must not of changed it when i bought the car then......great! Just as i thought so best get it booked it for a change. Do you know if the green sensor is the one on the actual thermostat?