Engine underheating - I've tried everything I can think of!

Gm95

Registered User
Hi guys,

So, I've owned my A3 just over a year now and I'm slowly catching up on things that haven't been fixed or been neglected by previous owner. One that has alluded me though is the engine not reaching the correct running temperature..

When I first had it, it would get up to 60c and stay there regardless of what temperature it is. I first off thought start simple and changed the temp sensor. This made no difference so moved on to the next step and bought a genuine Audi Thermostat. Drained all the coolant, changed this over and filled with the best coolant I could (which was about 6 hours of swearing and hurling abuse at the car). Drove it around and it got up to 90c and stayed there, happy days! Fixed, right?!

Now, I've realised that if the outside temperature is above 10c, the engine will be slow to heat up but it will go up to normal 90c and stay there. If the outside temp is below 10c, it will take an age to get to 90c and if it's below 5c outside. It won't go past 70c.

This morning, I drove to work (which is a 20 mile drive, mostly consisting of motorway) whilst it was reading 2c outside and the car never read above 65c or so. It seems to start rising in traffic but then when moving it will stay still or go down again.

I really am out of ideas here as I can't think of what else to change?! I saw on another forum that the A3 has an inline thermostat as well if it's DSG. Mine's a manual but could it have another stat somewhere that I'm not aware of? Or something else that would control the temperature like this?

Thanks in advance for any help!! I need to get to the bottom of it before i'm shivvering in traffic on the mornings! :thumbs up:

Car is an'05 2.0 TFSI Quattro A3
 

LEE69

07 170 DSG
The temp going down when driving and up when stopped is the stat.
Maybe you just got a faulty one?
Even my diesel will hold 90 in this cold weather
 

ch1z64

C'mon you's bhoy's in Green
VCDS Map User
Silver Supporter
It might be worth getting someone with a Vcds cable to plug in to car once you get it to the correct temp and monitor it live to see what changes it ie outside temp , have you checked all pipe wotk is unblocked , just that when you mention that below certsin temps outside it doesn't go to Max temp inside which makes me think there is a valve closing or a blockage restricting more heat which in turn is not allowing you to get to Max heat .
 

Gm95

Registered User
Thanks for the comments so far guys. I did some testing on my lunch break out in the car park.

I started the car from cold, turned climate control completely off and timed it - Took 15 minutes to reach 90 at 3c outside. (Seems perfect to me)

Whilst heating up, felt around the coolant pipes around the engine and all were hot so doesn't appear to be any air lock.


However, when turning the climate control back on to my normal 21c, the engine temp started tanking, all the way to around 65/70c after about 5-10 minutes. So seems to be related to the heater matrix taking too much heat? I can't think of how to fix this?
 

Gm95

Registered User
So did a further experiment on my drive home by driving the whole way with the climate control unit completely off.

The car heated up to 90c within around 10-15 mins as you'd expect a normal car to do so and stayed there. This was the same drive as I did this morning where the car wouldn't get above 65/70c with the heater on.

The CC seems to be drawing way too much heat from coolant loop, so much so that it's hindering normal heat up or even cooling the engine! Does anyone have any idea if there's something in place to prevent or control this that may not be working in my case?
 

Retroman

Audi A3 2010 Sportback 2.0 TDI 170 (CBBB engine)
I think it is extremely unlikely that a succession of thermostats are ALL faulty. If the temperature gauge is showing a lower temperature, then it has to be one of 4 things:- 1) Thermostat stuck open 2) Wrong thermostat - for example the A38P 1.6 TDI has one (according to Haynes) which starts to open at 92C whereas the 2.0TDI is 85C (aftermarket stats for mine can show any of the following 87C, 88C or 89C :sly:) 3) the calibration on the gauge is suspect (these things are not necessarily totally accurate) or 4) A coolant sensor (models have either one or two) is giving erroneous/inaccurate information to the instrument gauge. On certain BMW Minis for example, the coolant temperature sensor is a known problem area which often goes faulty and lights up the overheating warning light on the instrument panel and switches on the electric fan - on a cold engine. If you plug the car into most code readers it should show the coolant temperature (as measured by the coolant temperature sensor(s)) and if that looks OK (along with the fact that you have a new OE correct stat) I would "keep calm and carry on" :highly amused:
 

Gm95

Registered User
I think it is extremely unlikely that a succession of thermostats are ALL faulty. If the temperature gauge is showing a lower temperature, then it has to be one of 4 things:- 1) Thermostat stuck open 2) Wrong thermostat - for example the A38P 1.6 TDI has one (according to Haynes) which starts to open at 92C whereas the 2.0TDI is 85C (aftermarket stats for mine can show any of the following 87C, 88C or 89C :sly:) 3) the calibration on the gauge is suspect (these things are not necessarily totally accurate) or 4) A coolant sensor (models have either one or two) is giving erroneous/inaccurate information to the instrument gauge. On certain BMW Minis for example, the coolant temperature sensor is a known problem area which often goes faulty and lights up the overheating warning light on the instrument panel and switches on the electric fan - on a cold engine. If you plug the car into most code readers it should show the coolant temperature (as measured by the coolant temperature sensor(s)) and if that looks OK (along with the fact that you have a new OE correct stat) I would "keep calm and carry on" :highly amused:


Thanks for taking all the time to give me that much info! However, whilst I agree completely with you, on reevaluation. I think this is different to what I actually thought.

The car will heat up perfectly without any issues, regardless of outside temperature providing the climate control unit is switched off. This makes me think that the temp sensor and stat is working as it should.

The issue only starts when the Climate Control unit is turned on. If it's switched on when the engine is still heating up then it never will heat up. If the CC unit is turned on when the engine temp is at 90c then it will start lowering until it's cold again.

It gave me a fun game this morning on the commute whilst it was -2c. Drive for 15 minutes without any heat, get heat for 10 minutes, repeat. :thumbs up:
 

Gm95

Registered User
I was wondering if there was some value in the menu below could be off?

https://www.audiworld.com/tech/int5.shtml

Or, if there were some sort of mechanical stat/flaps that should regulate how much heat the CC unit can draw from the coolant loop that may be broken in my case?
 

Gm95

Registered User
How did you bleed the cooling system after changing the stat?

Filled with coolant to max line, started the engine and left it to reach operating temp with the cap off. Then topped it up.
 

SootySport

Registered User
Filled with coolant to max line, started the engine and left it to reach operating temp with the cap off. Then topped it up.
You did that right. May be the heater matrix it choked up with sludge, it is 13 years old. Try flushing the heater matrix with high pressure water, doesn’t always clear blockages but if you do notice an increase in warm air then this maybe the problem. Only way to forward them is to buy a new heater matrix.
 

VagTdiMan

Registered User
The temp going down when driving and up when stopped is the stat.
Maybe you just got a faulty one?
Even my diesel will hold 90 in this cold weather
Hi Lee,
Just joined the AS forum & was interested to see your reply in this post.
My PD tdi is suffering with seriously poor MPG ( 40 ) & my temp guage was working randomly, but mostly NEVER.
I changed the sensor, topped up the G12, left car running for 15mins or so while squeezing the fluid pipes to try & remove air etc.
Guage now works all the time, but never reaches normal (90) mark & goes down when driving.
When starting from cold the top coolant pipe warms up as does the one at end of fuel rail that goes to the sensor and in car heater system.
The lower pipe that goes to the thermostat remains cold for quite a while, but gets warmer once guage reaches about 80.
If i put thee heaters on or driver the car, the guage goes down.
Should i replace the nuisance to get to thermostat ?
P.S. the car had been standing for over a year before i got it.
Lucky for me, i still have my old tdi which needs a bit of work for MOT but always returned 60mpg on a run & always stayed steady at 90 whatever the weather was like.
regards,
Forum newbie: Win
 

Gm95

Registered User
Update to this:

I have changed the stat for a second time to a new one and the issue still persists (was just as much of a pig to do as the first time). I was hoping that the first changed stat would be faulty and this one would fix it but no luck. Still observing the same behaviour, where it heats as normal when the heater is turned off but will go back down to 60/70ish when the heater is switched back on. Leaving the CC unit on re circulation appears to not take as much heat but still stops the coolant getting to 90. I really am out of ideas now... :sos:
 

Nessy

VW + Audi mad
Update to this:

I have changed the stat for a second time to a new one and the issue still persists (was just as much of a pig to do as the first time). I was hoping that the first changed stat would be faulty and this one would fix it but no luck. Still observing the same behaviour, where it heats as normal when the heater is turned off but will go back down to 60/70ish when the heater is switched back on. Leaving the CC unit on re circulation appears to not take as much heat but still stops the coolant getting to 90. I really am out of ideas now... :sos:

I can’t suggest much but on my old B5 A4 the climate control panel had a secret menu that you could access which gave read-outs of live data eg actual coolant temp etc, actual road speed, and loads of other figures...
I don’t know if such a secret menu is available or accessible on the A3 8P CC panel as I haven’t needed to know , but I’d try and ascertain that your coolant temps are really as being indicated by your gauge....
When the gauge temps go down does the air from the heater vents also get cooler???
 

Nessy

VW + Audi mad
Hi Lee,
Just joined the AS forum & was interested to see your reply in this post.
My PD tdi is suffering with seriously poor MPG ( 40 ) & my temp guage was working randomly, but mostly NEVER.
I changed the sensor, topped up the G12, left car running for 15mins or so while squeezing the fluid pipes to try & remove air etc.
Guage now works all the time, but never reaches normal (90) mark & goes down when driving.
When starting from cold the top coolant pipe warms up as does the one at end of fuel rail that goes to the sensor and in car heater system.
The lower pipe that goes to the thermostat remains cold for quite a while, but gets warmer once guage reaches about 80.
If i put thee heaters on or driver the car, the guage goes down.
Should i replace the nuisance to get to thermostat ?
P.S. the car had been standing for over a year before i got it.
Lucky for me, i still have my old tdi which needs a bit of work for MOT but always returned 60mpg on a run & always stayed steady at 90 whatever the weather was like.
regards,
Forum newbie: Win

My A3 TDI is the 4th diesel Audi I’ve had over the last 25 years and they all without exception ran bang-on 90 degrees C , come rain or shine, summer or winter with a healthy thermostat.
Fit a new thermostat (Genuine only) and I bet normal service will be resumed......
 

Seanchadwick2k3

Audi A6 C6 2.0tdi 2011 Sline Special Edition
My TDI A6 is doing this also, heaters get nice and warm and stay warm but engine temp fluctuates, driving in traffic is no problem but as soon as you hit the motorway it drops right down.

Had the same issue on my last Mondeo which had two thermostats. I haven't attempted to fix mine yet but hoping to when I next service it so tuning in and hoping you find a solution before I come to tackle it!
 

Mike B

Registered User
The only thing that controls the temp is the stat on a water cooled engine backed up by the rad when it opens.The heater controls only draw from the matrix when needed acting like a mini rad that's why the temp drops if the stats not staying closed when it should in cold weather..The CTS will not itself affect the way the engine heats up only makes it run correctly.This is a very strange problem which must be down to a non working stat made so difficult by the time and effort needed to change it.Wax stats are such simple things but so hard to check if working correctly even from new but it would be surprising if a genuine one was faulty.Not easy but perhaps check if when warmed up to 90 check if the rad itself is in any way getting warm.It should remain stone cold even after driving a few miles in winter.Then if the gauge starts dropping check again and most likely there is warmth in the rad indicating that the stat.. is opening too soon or leaking coolant into the rad.A temporary fix would be to devise a safe way of blanking off part of the rad in cold weather to stop the heat loss .This needs care and is risky if you don't watch out for overheating but it should allow some heating to work.Not to rub salt in but my wifes golf has this engine and the heater is almost too powerful for our winters never needs to go over half way.My final thought is maybe you can find out what the specced temp is on the stat you fitted.Perhaps it just needs a stat with a higher temp rating that wont open too quickly.In my early days it was necessary to fit different ones in winter and summer but of course it only took a few minutes
 

pato89

Registered User
I have same problem on 2010 a3 2.0 tdi...
Everything is fine until i turn on heating it starts to drop temp...

Sent from my SM-A520F using Tapatalk
 

pato89

Registered User
Is it dangerous for engine not to change stat ??
And what stat are we talking about it seems these 2.0 tdi cr and maybe older 2 stats...

Sent from my SM-A520F using Tapatalk
 

Mike B

Registered User
No danger lots seem to drive around without noticing .Don't know the 2.0tdi so you need to check or get both changed
 

Seanchadwick2k3

Audi A6 C6 2.0tdi 2011 Sline Special Edition
Does anyone know what brand of thermostats are used and where is best to buy? Do you buy the stat alone or stat and housing?
 

aqibi2000

Registered User
I had a failing MAF, make my car take 40 mins to warm up coolant to temperature. Borrow one from a friend to test. Easy swqp
 

Gm95

Registered User
I can’t suggest much but on my old B5 A4 the climate control panel had a secret menu that you could access which gave read-outs of live data eg actual coolant temp etc, actual road speed, and loads of other figures...
I don’t know if such a secret menu is available or accessible on the A3 8P CC panel as I haven’t needed to know , but I’d try and ascertain that your coolant temps are really as being indicated by your gauge....
When the gauge temps go down does the air from the heater vents also get cooler???

Wow! Loads to comments here that I've missed. I did look around for the secret menu key combination to see if that could give me any clues. Any key combination online didn't seem to work with my climate control unit though unfortunately.

Also, the air coming out of the vents does seem to go cooler as the engine temp dips as you would expect and then heat back up when in traffic and the engine temp increases.

I've now changed the stat twice with genuine parts. First time from parts request on this site and second time from going to my local dealership and ordering it there. Both times, it hasn't made any difference.
 

Gm95

Registered User
Is it dangerous for engine not to change stat ??
And what stat are we talking about it seems these 2.0 tdi cr and maybe older 2 stats...

Sent from my SM-A520F using Tapatalk

I did read whilst researching for mine that the 2.0TDI has two stats. Mine is the 2.0tfsi so unfortunately just the one as I thought I was onto something there!
 

Gm95

Registered User
I had a failing MAF, make my car take 40 mins to warm up coolant to temperature. Borrow one from a friend to test. Easy swqp

Oh, that's interesting. I have been meaning to swap the MAF at some point as I have another issue of 'hiccuping' for a few minutes from a cold start that I thought the MAF would be responsible for. I have no idea how it would relate to engine temp, unless you know the science behind it? Either way, I'll try anything at this point!
 

Mykl-s

Registered User
I'm having a similar issue with my car in the colder weather. I'm planning on changing the thermostat when I get a chance in the next week or so, but as you mentioned that the thermostat didn't fix your issue, I was wondering if you'd worked out a fix?
 

Seanchadwick2k3

Audi A6 C6 2.0tdi 2011 Sline Special Edition
If your going to try replacing it for what it's worth I'd get the thermostat from Audi. In my experience aftermarket ones don't open at the right temperature but others have used aftermarket and been fine but I personally will be getting mine from Audi.
 

den911

Registered User
Hi, i doubt this is not related to your issue but i just thought it was worth a mention,if not for anyone else that may read it,anyway i have just serviced my 1.6tdi cabriolet and in order to do it i had to remove a full steel undertray that fit from front of bumper right to the back and beyond the sump housing in order to remove it i had to grind a few bolts of and not having replacements i drove the car minus the steel undertray for a couple of days and noticed the car would reach 90C when under load on motorway but when i hit the 30mph roads the needle dropped as though the fan had kicked it but it hadn't to around 65/75 i found this strange as it did it a few times upon driving around then it would settle at 90 ...I will move on now as i picked up some new bolts to fit the steel undertray and decided to try and fit it on my own which i managed to do and then went for a drive until it reached 90 i then hit the 30mph roads home for a few miles and prior to fitting the tray it would have dropped were as it didn't ,now unless i got lucky and it was not related i don't know but i reckon that steel undertray is there for a reason and maybe heat control could be it...Sorry about the long winded story but you never know if your car shipped with one and it is now missing maybe just maybe it might be adding to your issue ...
 

VagTdiMan

Registered User
I'm having a similar issue with my car in the colder weather. I'm planning on changing the thermostat when I get a chance in the next week or so, but as you mentioned that the thermostat didn't fix your issue, I was wondering if you'd worked out a fix?
Total Success, Changed thermostat & sensor, car now stays steady on 90 as it should. I used a Meyle stat
 

den911

Registered User
Glad to hear your sorted now,i too have a simalar issue just changed temp sensor and thought all was ok earlier but sadly not ..I have a thermostat change in the morning using a oem one were its already in the housing ...
 

VagTdiMan

Registered User
On the 1.9tdi like mine, the stat is a bit of a nuisance as you need to remove the alternator, so it took about 45 mins, but no big deal. After i done mine, a friend asked me to help do his & it honestly took 15mins & top up G12 time.
Good luck Den, it should be easy
 
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