Engine temperature rising well above 90°C for a few minutes once the oil temp rises above ~75°C

[ithinuel]

Hello,

 

I noticed something new since the last service of our Rapid Spaceback (from 2017 1.4TDI 90HP DSG) got service early February.

We don't use it much so we only noticed it about 2weeks after the service so not sure how related it is.

 

In short, what happens is that when starting cold, the engine's temperature normally rises to 90°C and stays there until the oil temp catches up.

When the oil temp reaches around 70/80°C, the engine's temps also goes up. I have to remain gentle on the acceleration to keep it from getting too hot.

In the mean time the oil temp continues to rise up to ~100-110°C (depending on the road/traffic).

After a little while, the engine's temp goes down to its usual 90°C and nicely stays there until the engine's cut off. I am yet to find out what's causing the temp to finally go back down and identify what's causing this behaviour.

 

It only seems to do this from a cold start.

 

According to the garage, I'm due to change the cambelt because the car 5yo+ (although only 51000km).

 

Does anyone have an idea of what could be doing so and how I can verify if it is indeed the case?

[EnterName]

Hello!

Well I'm really not any kind of expert, but starting from basics, I assume your fluid levels are correct? (Oil & coolant.)

 

AFAIK, there's a system that closes off the coolant from circulating through the radiator while the engine warms up, to help the engine warm up. (Used to be a simple thermostat, now they're fancier, AFAIK.)

This means that on start-up, there's not much coolant circulating around the engine.

On the other hand, the oil has no such circulation inhibition, and so the full 4-5L of oil has to warm up to get up to temperature.

The coolant cools, and the oil lubricates. The oil needs to be within a specific temperature range to lubricate properly. (Something like 80degC through 120degC, with nominal a little under 100degC.)

The coolant helps the engine stay within the temperature range it needs to operate within.

 

From the symptoms you describe, from cold, the coolant is shut off from the radiator as it should be.

The coolant warms up pretty quickly, and the larger volume of oil warms up more slowly.

What I suspect is happening, is the system that allows the coolant to flow through the radiator to cool it down, is not working correctly.

So as the engine warms up and the oil warms with it, the coolant isn't getting the benefit of the radiator to cool it down when it should do, and the coolant gets unusually hot and the oil temp continues to rise also.

The fact that it subsequently sorts itself out makes me think the (or one of the) temperature sensors that enables the full coolant system is out of calibration, and requires a higher temperature than it should to kick into action.

 

As a stop-gap test, the next time you drive your car, as the temp gets too high, put the heater on max heat and max fans and see if that drops the temperature.

I'm guessing it will, and I'm also guessing that will prevent the faulty sensor from enabling the full cooling system, as it won't hit its out of range temp.

Which would back up my theory of the out of range temperature sensor.

 

Good luck! Let us know how you get on. 🤞

Edited 2 March, 2023 by EnterName

[Warrior193]

Welcome to the forum. ^^^^^ 2nd this. Depending on engine fitted, very likely to be the sleeve on the water pump staying closed for too long.

[J.R.]

100% what Warrior says.

 

Unless the 2.0 TSi does not have that system, in whiuch case ignore me!

Edited 3 March, 2023 by J.R.

[Warrior193]

5 minutes ago, J.R. said:

100% what Warrior says.

 

Unless the 2.0 TSi does not have that system, in whiuch case ignore me!

OP has 1.4TDI ?

[ithinuel]

@Warrior193Thank you
Yes it is a Diesel engine.

 

@EnterNameThat is (to some extent) what i thought it might be (something related to some by pass valve openning too late).

 

Am I correct in thinking that if it were a water pump issue, the engine would always overheat rather than only once oil temp reaches a high enough temperature but only for a little while ?

 

Is there a way for me to differentiate between a valve stuck while cold issue and a sensor calibration issue?

Is there a manual that would explain all this? (the owner's manual is pretty much useless when it comes to maintenance & diagnostic).

Do the temperature display use a different probe than the value controller?

 

If I had to guess, I'd assume that the ECU's using an hysteresis to control the valve and I'd look at what's the setting for the triggers but I have no clue about what tool to use the check that (maybe using some OBD2 device?🤷‍♂️).

[Warrior193]

Not sure with the TDI, but my understanding is that the sliding sleeve can stick closed occasionally, causing overheating until sleeve does open. I'm uncertain if this sleeve is part of the thermostat - or integrated with the water pump.

I believe the sleeve is purely to allow a faster warm up for the engine.

AFAIK, the sleeve is operated by water temperature only - Ie. not modulated by an external control source. 

Edited 3 March, 2023 by Warrior193

[EnterName]

12 hours ago, Warrior193 said:

Welcome to the forum. ^^^^^ 2nd this. Depending on engine fitted, very likely to be the sleeve on the water pump staying closed for too long.

I love it when someone can nail the problem in a sentence.

[EnterName]

1 hour ago, ithinuel said:

@Warrior193Thank you
Yes it is a Diesel engine.

 

@EnterNameThat is (to some extent) what i thought it might be (something related to some by pass valve openning too late).

 

Am I correct in thinking that if it were a water pump issue, the engine would always overheat rather than only once oil temp reaches a high enough temperature but only for a little while ?

 

Is there a way for me to differentiate between a valve stuck while cold issue and a sensor calibration issue?

Is there a manual that would explain all this? (the owner's manual is pretty much useless when it comes to maintenance & diagnostic).

Do the temperature display use a different probe than the value controller?

 

If I had to guess, I'd assume that the ECU's using an hysteresis to control the valve and I'd look at what's the setting for the triggers but I have no clue about what tool to use the check that (maybe using some OBD2 device?🤷‍♂️).

Warrior clearly knows way more about the specific operation than I do. I just have a rough idea of how the system works, but I'm glad we're all thinking along the same lines.

If you want to know more, this might be worth a look. I haven't read it, but the VAG Self-study series are useful.

https://procarmanuals.com/pdf-online-vag-ssp-640-audi-1-4l-3-cylinder-tdi-engine-series-ea288/

 

[Kostas123]

I had the sleeve issue on my 1.4 TDI. However, if the coolant pump is stuck closed, then how does the oil get cooled? The electric pumps seem pretty far away to pull any meaningful coolant thru the oil cooler...

[Warrior193]

There is a separate cooling system for the oil.

[Kostas123]

On 04/03/2023 at 21:39, Warrior193 said:

There is a separate cooling system for the oil.

but it still mainly runs from the main coolant pump, even with the auxiliary electric pump...

[ithinuel]

Thank you all for your messages!

 

I didn't know about the self study program, it's a great resource!

 

But either way, that's beyond my ability to fix so I'll get in touch with local garages and see what they quote me 🤷‍♂️