Dodgy thermostat?.....

[Nessy]

Right, I'm beginnig to wonder if my thermostat is beginning to stick open?......
I'm sure that the temp gauge never used to budge off exactly 90 degrees C no matter how hard or gentle I drove but recently after long downhill coasting and periods of gentle throttle use the gauge is going down to beween 85 and 90.......its definitely moving!
Am pretty sure that the car is returning 1 or 2 mpg less than it used to as well!
The coolant temp sensor was replaced about 3 years ago so don't think that that is the cause of the changing coolant temp readings.......
Could these symptoms indicate a thermostat on its way out or am I barking up the wrong tree totally?

Thanks!

 

[aragorn]

sounds like its possible.

The guage reads 90 at any temperature between about 75 and 105ish, so if its dropping from 90 at all it means its cooled off quite a bit, or that the CTS or its wiring is dodgy.

I guess you need to get vagcom on it and watch the CTS output while driving, see what happens, Craigs S4 seems to have some wiring issue and the guage randomly drops to 0 and comes back a few minutes later. It also logs a CTS related fault code.

When the stat was faulty on our 1.8T it never reached 90 unless sitting idling in traffic. When driving regardless of speed it stayed below 75, and on the motorway it'd drop most of the way back to 60.

 

[g60leigh]

Just had mine replaced last week as it was stuck open, but mine was right down on 0 all the time driving and only going to 90 when stood idling for a long while

 

[Nessy]

Thanks for the replies gents, had a thermostat go on my old 80 TDI years ago and that manifested itself as a gauge that would only get to 75 and no more........
The fact that the A4 gauge is altering its reading , (admittedly by not much) is what was puzzling me though, will try and get my VAGCOM copy lead out and see if the freeware version aloows me to test the temp sensor.
If it doesn't I think I'll try and change the thermostat as this issue is beginning to get on my nerves,lol.....................

Cheers.

 

[Nessy]

Think that I may have found the problem........
Began reading the Haynes on finding where the thermostat is etc and came across the cooling section where it describes the radiator as having a belt-driven fan with a viscous coupling and a separate thermostatically-controlled electric fan.
If I read the Haynes correctly the viscous coupling should only engage the blades of the fan to turn (and so cool the radiator) when the coupling is hot ie above 75 degrees, yet when I started the car this morning from cold the viscous coupling fan was merrily spinning away??
It has been taking longer for the engine to warm up of late as well,which until now I had put down to the arctic weather we had been having, lol ,so can anyone confirm my diagnosis of a duff viscous coupling?

Thanks.

 

[H2o-uk]

well funny you should mention, as I have a 2.6 V6 and my fan on the end of said viscous coupling spins merrily away as soon as I start the engine up and not just once its warmed up. I wonder if I have a similar issue as I've been noticing some weird temp fluctuations recently, I too have been blaming the weather.

 

[aragorn]

the viscous fan will spin with the engine cold, just not very quickly.

Its also easy to stop with your hand (althgouh dont try that unless your sure the coupling isnt siezed)

Regardless of the fan speed, even with the viscous and electric running full bore, the coolant shouldnt drop below 90c, thats the thermostats job.

get vagcom on there and see whats happening, it could even be a faulty guage...

 

[Nessy]

well funny you should mention, as I have a 2.6 V6 and my fan on the end of said viscous coupling spins merrily away as soon as I start the engine up and not just once its warmed up. I wonder if I have a similar issue as I've been noticing some weird temp fluctuations recently, I too have been blaming the weather.

Weird isn't it?
It wouldn't make any sense IMHO to have a fan cooling a radiator from 1st moment on start-up from cold (though I think this is how cars ages ago had them, am sure my old 1960 P4 Rover had this!!!!), so think that there must be something wrong with my viscous coupling.
Only thing I can't get my head round is this....if the coupling is knackered then yes it will increase the time taken for the engine to warm up (so similar to a stuck open thermostat in that respect) , but why would a incorrectly operating viscous coupling cause the running temperature on the gauge to go down?
I came back from work last night over a very mountainous route and by the time I had come down one particular mountain (no throttle, just braking over the entire descent) the gauge was reading 80 degrees!
Something is amiss, is it a combo of thermostat AND coupling which are faulty, or just one....
Must admit to being quite puzzled.

 

[Nessy]

the viscous fan will spin with the engine cold, just not very quickly.

Its also easy to stop with your hand (althgouh dont try that unless your sure the coupling isnt siezed)

Regardless of the fan speed, even with the viscous and electric running full bore, the coolant shouldnt drop below 90c, thats the thermostats job.

get vagcom on there and see whats happening, it could even be a faulty guage...

Mmm, the fan was spinning at a hell of a rate with the engine stone cold, was using my nice new Maglite torch to see if the viscous fan blades were turning as it was a bit dark in there and accidentally caught the torch on the fan outer ring.
Glad it wasn't my fingers as it was spinning with quite some force, lol!
So i'm fairly confident my coupling has had it.
Looks like I might have a combination of faults then, am off work Tues so think I'll delve into the thermostat issue further, thanks!

 

[yendall24]

hi all to test the coupling start the car from stone cold get yourself a rolled up newspaper and gently push it against the outer edge of the fan blade if it stops easily then the coupling is ok if it shreads the paper it is stuck on

 

[Nessy]

hi all to test the coupling start the car from stone cold get yourself a rolled up newspaper and gently push it against the outer edge of the fan blade if it stops easily then the coupling is ok if it shreads the paper it is stuck on

Have to confess that I haven't yet been brave enough to try this,lol but think have sorted the problem.
Did a search on here for viscous coupling and came across various posts that said as long as the viscous fans weren't too noisy (which mine isn't) then all is OK.
Apparently when they go wrong they become really noisy (the shogun effect) so turned my attentions to the thermostat again.
Replaced it this morning and the gauge now reads 90 degrees all the time.
Have a feeling that the oil temperature gauge is now reading higher too (it now also sits steadily at 90) , but cannot be certain as I've never really taken THAT much notice of what it read before,lol.....

 

[yendall24]

the oil temp rise is to be expected higher coolant temp means higher oil temp i know what they mean when say noisy when its stuck on you get a sort of wooshing noise when you rev the engine but to be certain all is ok the newspaper test is best

 

[Nessy]

the oil temp rise is to be expected higher coolant temp means higher oil temp i know what they mean when say noisy when its stuck on you get a sort of wooshing noise when you rev the engine but to be certain all is ok the newspaper test is best

Yes, I guessed that if the old thermostat had been playing up then it might have been causing the oil temp to be low as well.......
Another thing I did today was figured how to get the climate control display to show the engine coolant temperature , went for a drive and the coolant temp once warmed up stayed at a constant 90 as well.
Hope that is the end of my gauge woes now , will try the newspaper thing next time the engine is cold and have got the bonnet open , thanks............