Thermostat

[Jono8]

Ok, so drove to work today and monitored the temperature on the climatronic. Up to temperature in 9 minutes of slow driving. settled at 78 and seemed to stay at that most of the way (with variations of 1/2 degrees). Giving it some beans and going up hill made it go up to 82/83.

On the downhill section where i am coasting in gear for a mile+ the temp dropped to 71 ( this was the lowest i saw). I noticed the gauge drop slightly off the 90 mark as soon as the climatronic showed under 75 (so the the 12 O'clock point of the temp gauge is clearly weighted to fall at under 75).

As soon as i got back on the throttle, temperature is back up to ~78 again for the rest of the journey.

 

[Minstadave]

Sounds like you may have a stat that is stuck open.

 

[Conscript]

Personally, I would change the thermostat. Although the engine temperature will change depending on load/speed (which is normal), in my experience generally the temperature gauge will not budge unless it moves out of a particular range - I assume this is so it only indicates abnormal temperature fluctuations (such as you've experienced rolling downhill). Obviously such a condition will cause the engine temperature to drop, but a fully working thermostat should close slightly allowing the water to warm and the temperature to stay within that particular range. I wouldn't have thought that a modern engine would have trouble maintaining it's temperature, even when coasting for over a mile. Depends on the car of course, but the very fact that the gauge moves enough to be visible seems to indicate an abnormal drop in temperature.

I actually had to change my thermostat earlier this month. I was noticing the car taking a long time to warm up, and found that on the motorway, the gauge had dropped from it's usual position bang in the middle to a point about a quarter of the way down the scale. I changed the thermostat (which was a pain the rear - thanks Mazda) and the gauge is back in the middle again.

 

[Brookert]

Ok, so drove to work today and monitored the temperature on the climatronic. Up to temperature in 9 minutes of slow driving. settled at 78 and seemed to stay at that most of the way (with variations of 1/2 degrees). Giving it some beans and going up hill made it go up to 82/83.

On the downhill section where i am coasting in gear for a mile+ the temp dropped to 71 ( this was the lowest i saw). I noticed the gauge drop slightly off the 90 mark as soon as the climatronic showed under 75 (so the the 12 O'clock point of the temp gauge is clearly weighted to fall at under 75).

As soon as i got back on the throttle, temperature is back up to ~78 again for the rest of the journey.

I would say it is fine. If we are talking a good minute+ of coasting, there would be no heat be produced by the engine, so a 7 degree drop is actually pretty good. What engine etc do you have.

 

[Jono8]

I would say it is fine. If we are talking a good minute+ of coasting, there would be no heat be produced by the engine, so a 7 degree drop is actually pretty good. What engine etc do you have.

It is a 2006 Octavia VRS. 2.0TFSI engine.

 

[Conscript]

I would say it is fine. If we are talking a good minute+ of coasting, there would be no heat be produced by the engine

Well that's not quite true - even an engine at idle will be generating a fair degree of heat, and a mass of metal like an engine block doesn't exactly cool off rapidly.

 

[Jono8]

Well that's not quite true - even an engine at idle will be generating a fair degree of heat, and a mass of metal like an engine block doesn't exactly cool off rapidly.

But at idle the injectors are firing. As far as i understand it, when coasting in gear and completely off throttle then the engine wont be generating any heat as the injectors wont be firing?

 

[Conscript]

I've just seen that you mention it might be an alternator off job to change the thermostat. In that case, considering the temperature drop is only just enough to move the gauge, then perhaps it's not worth worrying about. I had to take the throttle body off and even then the thermostat was in a ridiculously hard to reach location when I changed mine, but then I was witnessing a much larger drop while on the motorway, so it definitely needed doing.

But at idle the injectors are firing. As far as i understand it, when coasting in gear and completely off throttle then the engine wont be generating any heat as the injectors wont be firing?

I think combustion still takes place, but the amount of fuel being used is minimal - will have to wait for someone better qualified to answer that for sure. Even so, there's still friction from moving parts and the fact that your engine block will retain a lot of heat in it's mass.

 

[Jono8]

I've just seen that you mention it might be an alternator off job to change the thermostat. In that case, considering the temperature drop is only just enough to move the gauge, then perhaps it's not worth worrying about. I had to take the throttle body off and even then the thermostat was in a ridiculously hard to reach location when I changed mine, but then I was witnessing a much larger drop while on the motorway, so it definitely needed doing.

Yeh if it was dropping whilst on the motorway then i would get it done tomorrow!

It does seem to run very close to the lower threshold of the acceptable operating temperature though (seen as the dashboard gauge drops off the centre at below 75 degrees (as indicated by the climatronic)).

 

[Brookert]

I think combustion still takes place, but the amount of fuel being used is minimal - will have to wait for someone better qualified to answer that for sure. Even so, there's still friction from moving parts and the fact that your engine block will retain a lot of heat in it's mass.

I'm going to do some google fu, but I'm pretty sure a fuel injected engine doesn't inject fuel when rpm is above a certain level and your coasting in gear. The force turning over the engine is the wheels when you are coasting.

My car (audi tt) shows no fuel being consumed in gear down a hill, but in neutral it will show fuel being consumed.

 

[Jono8]

I'm going to do some google fu, but I'm pretty sure a fuel injected engine doesn't inject fuel when rpm is above a certain level and your coasting in gear. The force turning over the engine is the wheels when you are coasting.

My car (audi tt) shows no fuel being consumed in gear down a hill, but in neutral it will show fuel being consumed.

Yes, this is what i have read as well, and likewise, the realtime mpg read out shows no consumption whilst coasting down said hill.

 

[Jono8]

I hate cars, i would prefer it if it had just broken and it had very clear signs of a stuck open thermostat. At least then i know it would be ~£200 well spent.

With this i just don't feel sure enough and would hate to see it behave exactly the same after changing the stat

Thank you for all your continued help on this though guys. I am still a little undecided as to what to do : /

 

[Brookert]

I hate cars, i would prefer it if it had just broken and it had very clear signs of a stuck open thermostat. At least then i know it would be ~£200 well spent.

With this i just don't feel sure enough and would hate to see it behave exactly the same after changing the stat

Thank you for all your continued help on this though guys. I am still a little undecided as to what to do : /

I would keep an eye on motorway temps. As soon as the temp starts dropping on the motorway, when at a constant speed, replace it. 9 minutes (I think thats what you mentioned) is fine to get warm.

 

[Jono8]

I would keep an eye on motorway temps. As soon as the temp starts dropping on the motorway, when at a constant speed, replace it. 9 minutes (I think thats what you mentioned) is fine to get warm.

Yeh, i will do. Not doing a motorway journey for a while, but i will be watching the gauge like a hawk when i do.

Yup, it has never taken longer than 10 minutes for the needle to hit the 12 0'clock position on the temp gauge. Normally quicker.

 

[Conscript]

I would keep an eye on motorway temps. As soon as the temp starts dropping on the motorway, when at a constant speed, replace it. 9 minutes (I think thats what you mentioned) is fine to get warm.

This. It sounds like the only symptom your experiencing is slightly reduced fuel consumption (which could well be something else anyway). Keep an eye on it, if the problem worsens then get it changed.

 

[Jono8]

This. It sounds like the only symptom your experiencing is slightly reduced fuel consumption (which could well be something else anyway). Keep an eye on it, if the problem worsens then get it changed.

Indeed. the fuel consumption thing could be all the rain and puddles. i think all of this week I've been driving through puddles/wet leaves my whole way to work and back. It has only, i think, dropped by say 1-2 mpg.

The car is filthy

 

[Jono8]

I have been thinking scientifically about this. if the thermostat was indeed stuck open, then surely the ambient temperature would effect the engine/coolant temperature more?

I have driven when it is 2-3 degrees ambient aswell as 12/13 degrees ambient in the past few days and the engine temp has always been 76-82 whilst driving (apart from the long coasting bit). i.e. the ambient temperature makes no difference.

is that right?

 

[shifty_uk]

Apparently this is a common issues on VAG engines? My A3s temperature gauge seems to be sitting at the quarter way mark and not getting up to full operating temperature presumably causing the noticeable drop in MPG. Going to get the thermostat replaced.

 

[Jono8]

Apparently this is a common issues on VAG engines? My A3s temperature gauge seems to be sitting at the quarter way mark and not getting up to full operating temperature presumably causing the noticeable drop in MPG. Going to get the thermostat replaced.

This is not the problem i am having.

In fact mpg wise, it seems to have gone up now to what it was before i started worrying about this a few days ago.