Coolant temp fluctuation

[fliprio]

Evening all

Im currently having an issue with the coolant temp on the mk5 GTi, when going down hill or just pottering around town a bit the temperature needle just dips back from 90 degrees to about 80, perviously its been rock steady on 90 no mater what.  A bit of looking around on line sounds like its the thermostat playing up.

I went into the VW dealer today as it needs a service and cambelt/water pump, so I asked how much for the thermostat while they have it all apart and the coolant drained for the water pump.  They said its most likely the water pump as the impeller starts to move on the shaft so when its idling and a bit cooler the shaft is not pumping much coolant but when the coolant warms it grips and pumps coolant and hence the temperature fluctuation.  After I came away I thought a bit about it and surly this would cause less coolant when not throttle is applied and it would get hotter not cooler?

Any way to check if its the thermostat or not?  they still wanted £250+ to do it while the rest of the car is apart!  I know its a pig to get to but that does seem a bit extreme.

Anyone else had this problem solved by replacing the water pump?

[Stevepd]

Do a search for bypass thermostat.

[Wolf Burger]

I had this.

First change the coolant temp sensor. If this doesn't fix it change the main engine thermostat.

Temp sensor part number 06A919501A
Main thermostat 06F121111F

Will be very surprised if that doesn't do the job. 

[fliprio]

Do a search for bypass thermostat.

Its a dsg car as well, so looks like it has 2 bypass thermostats

How do you work out if its bypass or main thermostat?

VW said if it were the main stat it would never get up to temp at all, although the gauge is non-linear so even if it says 90, it might not be getting there

The main issue is that changing lots of bits costs a lot with VW and the main thermostat looks a pain to do....after spending the weekend doing the headgasket on one of my otehr cars im wanting someone else to take this one apart.

[fliprio]

I had this.

First change the coolant temp sensor. If this doesn't fix it change the main engine thermostat.

Temp sensor part number 06A919501A
Main thermostat 06F121111F

Will be very surprised if that doesn't do the job. 

I take it a full stat change solved yours rather than the cheap sensor fix?

[chris s]

Look for my post on here mate, if it sounds like the same symptoms then my money is on the main thermostat... Wouldn't waste your money on the temp sensor

[Wolf Burger]

I had this.

First change the coolant temp sensor. If this doesn't fix it change the main engine thermostat.

Temp sensor part number 06A919501A
Main thermostat 06F121111F

Will be very surprised if that doesn't do the job. 

I take it a full stat change solved yours rather than the cheap sensor fix?

Yes. Been perfect since. Use the search at the top of the screen and you will find many others saying the same thing.

[fliprio]

so general consensus is that it being the water pump is a load of old rubbish?

I just find it amazing the VW main dealer wants £94 investigation fee rather than just fix what I ask them to try first, might take it somewhere else as the age and milage its probably not worth having main dealer service history any more where an independent will do

[fliprio]

I got the actual coolant temp on the climate control display this morning, on a 6 mile run to work it got to 75 degrees when stationary and drops back to 70 when moving, gauge actual shows 90 when its at 75 and drops back to 80/85 when its at 70.  Was a bit cold this morning though

[skard]

I, like many others in threads you are reading, had similar symptoms and it was my stat, although the GTI has a different stat to the ED30, the behaviour is the same.

I got a trusted mechanic to fit it with a new cambelt and water pump, it turned out to be a whole job on its own.
I'd suggest finding a recommended indy as opposed to VW, but you are still looking at £150+, however running the car long term at a lower temperature is not going to do it any good.