A3 8P 2.0T FSI Coolant temperature problem

Hello, so I've been having this really strange problem with my 2005 A3. It started out with the car not even getting over the 50°C mark on the dash, replaced the thermostat and everything seemed fine. it would reach 90°C within 5 minutes of driving and stay there.

But eventually it started getting colder outside and I noticed the car would take longer to warm up. Right now we've got around -10°C out and it takes about 20 minutes of driving to get it up to 90°C. So I just assumed the thermostat was bad so I replaced it again but it didn't make a difference, I also did some investigation and found there was an inline thermostat going to the oil cooler or something so I replaced that too and it didn't make a difference either.

Now, something I've noticed is that if I turn off the heat inside of the car and leave it idling it will get up to 90°C in under 10 minutes but as soon as I turn on the heating it tanks down to about 60-70°C and will stay there until I start driving.

I am really confused by this whole situation since I highly doubt it would be the thermostat after replacing it twice and I'm simply out of ideas for what could be the cause.

Any input is appreciated.

splinter-cell said:

I complete agree. Engines warm up best when they’re actually working so idling a car isn’t the best idea when you need it to warm up. You should ideally start to gently drive the car as soon as possible after starting.

Also, having the heaters turned on does take heat away from an engine which is why it’s a good short term solution for when your car starts to overheat. This explains why it takes longer for your car to heat up in the extreme cold.

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I am completely aware of how a combustion engine works, have you ever sat in a car without any air coming out of the vents? If you had you'd know that the windows all fog up within minutes and therefore I couldn't have been driving the car during that test. Letting it idle to get warm once isn't gonna hurt anything.

You also don't seem to understand how a thermostat works, a thermostat's job is to set the minimum temperature of the coolant and therefore shouldn't allow circulation when it drops below that temperature (in this case 87°C) I know for a fact that what I'm experiencing is not normal behaviour, I have friends with diesels that reach their operating temperature within the usual timeframe even on colder days.

samisnake said:

How do you know the heater matrix isn't in circulation till the temp reaches 87? That's not true.
Not sure about the a3, but in some cars the matrix is in the cooling loop permanently, even if the blower motor is off. Can't see the a3 being different, and the tstat stops flow to the rad not the matrix anyway.

Turn the ac on if you don't want the windows to fog up.

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The heater is definitely in the loop all the time, I can feel lukewarm air from the vents way before I even get a reading on the temp gauge, but with the blower motor off I'm not really taking away any heat from it and it's only when I turn the blower back on that the temperatures start to drop. I have never seen this behaviour in any car before, once it's reached 90°C it shouldn't budge.

samisnake said:

Yeah shouldn't drop once it's reached 90.
I know you've said youve changed it twice, was that using oem tstats or aftermarket?

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Both were aftermarket, couldn't find anywhere online that sold OEM ones. When I replaced it the second time I inspected the old one and it wasn't leaky, nothing seemed out of the ordinary. I cut open the housing and took out the thermostat then tested it in some water on the stove and it opened at 90 as it should so I can't see how the thermostat could be the culprit.

SootySport said:

A3’s do seem to take longer than usual to warm up and at -10c doesn’t help much either. Have you checked the heater matrix is actually getting hot at anytime in the past? Maybe time for a new heater core as your is 15 years old now. Scale, sludge and rust particles do tend to collect in the matrix.

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While that is true I can't see why that would keep the engine from getting the temps up, if anything a clogged matrix would have the opposite effect.