Faulty Thermostat? Audi A4 B8.5

[The Shaikh]

I have an issue with my engine overheating that is a bit puzzling.

I have no coolant leaks, coolant is still topped off in the reservoir and the cooling fans both operate.

When I drive the car for more than 30 mins, the temp gauge rises slightly just above the 90 degrees mark and the fans kick in. The car still operates with no CEL or any other lights on the dash but its a bit sluggish when that happens.

Can't upload image for some reason.

The coolant pipes have water circulating in them and the water pump is working.

Did a pressure test and no leaks.

Changed the coolant housing and the lid based on some expert instructions and still does the same thing.

Car:

2013 Audi A4 B8.5 1.8T CJE

History:

Car has had the thermostat replaced under warranty at AUDI Sandton just after I got it. After about 2 years it went to a private mechanic who changed the water pump and thermostat as the water pump was dead and making a loud screeching sound. Its been over 13 months now then this issues has started. The strange part is, when I switch on the heater inside the car, the fans go off and the Temp gauge goes back to 90 degrees and everything if fine for as long as the heater is on.

if I switch off the car and back on while its hot, then the temp stabilizes and I can continue driving but after 30 mins or so, then the fans kick in again, Temp gauge rises and goes back to feeling sluggish.

I have run diagnostic tests multiple times and nothing comes up.

Thoughts? Comments?

[The Shaikh]

Picture of temp gauge when its "Hot"

[mk2-phreak]

What's VCDS reporting the coolant temp at when the gauge shows 90? Your mech may have fitted the wrong temp range thermostat

[The Shaikh]

What's VCDS reporting the coolant temp at when the gauge shows 90? Your mech may have fitted the wrong temp range thermostat

Engine speed: 728.00 /min
Normed load value: 10.2 %
Vehicle speed: 0 km/h
Coolant temperature: 94 °C
Intake air temperature: 51 °C

[Callum229]

Are you experiencing any coolant loss at all(even over months or so)?

Sounds like the engine is compensating and trying to keep the car cool(sluggish).

How is the fan sounding like, is it sounding like a jet taking off in front of your car?

Colour and state of the coolant?

Do you have the receipt of the thermostat fitted? (most thermostats for VAG are rated for 87 degrees anyways)

When you say it is overheating, does it go over 100 deg, or does it stay underneath that value?

I would suggest to first run a coolant flush, then replace with g13 coolant.

If that does not work, then remove the thermostat and run without it, check if car then overheats, if it doesn't overheat, then replace thermostat(must be rated for 87 deg).

Then if that does not work, get someone with a thermal camera to check temp of the pipes or use VCDS and check the two values or more of the coolant, they should not vary by much as one sensor is by the head and one by the radiator, check that both values are within 10 degrees or so when it is overheating, if it isn't close, then you either have a blockage or a sensor is faulty.

try not to drive the car much, as you risk damaging the engine.

[The Shaikh]

Are you experiencing any coolant loss at all(even over months or so)?

No coolant loss even or leak. Stays constant just below max, like mm below

Sounds like the engine is compensating and trying to keep the car cool(sluggish).

How is the fan sounding like, is it sounding like a jet taking off in front of your car?

[i]The fan sound ridiculous loud I can hear them in the car, But once the car is switched off then they slow waaaay down and run for about 3-5 minutes then go off[/i]

Colour and state of the coolant?

Coolant is pinkish... I think that is the original color.

Do you have the receipt of the thermostat fitted? (most thermostats for VAG are rated for 87 degrees anyways)

Not anymore

When you say it is overheating, does it go over 100 deg, or does it stay underneath that value?

Stays under the value

I would suggest to first run a coolant flush, then replace with g13 coolant.

If that does not work, then remove the thermostat and run without it, check if car then overheats, if it doesn't overheat, then replace thermostat(must be rated for 87 deg).

Then if that does not work, get someone with a thermal camera to check temp of the pipes or use VCDS and check the two values or more of the coolant, they should not vary by much as one sensor is by the head and one by the radiator, check that both values are within 10 degrees or so when it is overheating, if it isn't close, then you either have a blockage or a sensor is faulty.

try not to drive the car much, as you risk damaging the engine.

I will try to remove the thermostat and test as you suggested but its part of the Waterpump housing.

Thanks for the pointers.

[Callum229]

Yep sounds like it is "overheating" or there is a blockage somewhere.

My cars fans used to stay on even after I switched off for a few minutes because although the temp on the gauge showed 95 to 100, it was "overheating" and kept the fans on for a while even after I switched off.

Run a bottle of radiator flush, it does sound like a faulty thermostat though as it seems to be remaining closed or there is a restriction somewhere in the cooling system. Maybe that faulty water pump left some debris inside the cooling system, clogging it.

Either way, I would suggest removing thermostat, testing it in hot water, making sure it functions, reverse flow the water in the cooling system(using hosepipe) incase something is stuck, run a bottle of radiator flush thereafter and you should be fine.

It could also be a coolant temp sensor, and like in my car, they can be swapped from the radiator to the engine head where there is another sensor. I think it's only the sensor on the head that feeds the readout on the instrument cluster, so if swapping it, and your temp reads normal, then you need a new sensor.