Hi guys really need your help with an issue I’m having with my car.

I was driving down the motorway last Friday and the temperature suddenly flew up into the red when at 70 with no prior warning then when I slowed down ready to pull off when slowing down the temperature started dropping as fast as it went up. When not under any load on the engine the car maintains its normal running temperature with no issues. When you accelerate hard or maintain motorway speeds the temperature flies up.

When I got home and the car cooled down the next day I checked the water and it’s not lost any water so its appears to have no overheated. I’ve not noticed any coolant leaks as it’s never lost water but it’s been pressure tested and that’s holding pressure.

I’ve heard of issues with the electronic water pumps being problematic so I’ve swapped the water pump for a new one the weekend and this has still not solved the issue.

Does anyone have any ideas what could be causing the issue ?

Without information regarding your engine and fuel type etc nobody will be able to help you.

What prior warning would you have expected of a rise in coolant temperature?

The VAG system fudges the temperature gauge reading to show 90°c over a wide range of actual engine temperatures, for example 70°c to 110°c, when the actual temperature rises or falls outside of these limits the gauge will swing to the real unfudged reading.

9 minutes ago, J.R. said:

Without information regarding your engine and fuel type etc nobody will be able to help you.

What prior warning would you have expected of a rise in coolant temperature?

The VAG system fudges the temperature gauge reading to show 90°c over a wide range of actual engine temperatures, for example 70°c to 110°c, when the actual temperature rises or falls outside of these limits the gauge will swing to the real unfudged reading.

Hi JR sorry I thought my profile would show my car information (noobie error). The car is a mk3 Octavia VRs 2.0l diesel 4x4 2016 model.

The car normally warms up to its normal running temperature and stays at 90 on the temperature gauge. But suddenly out of nowhere on a run it shot up to 130. Ever since this incident it’s not reading normal temperate readings now as in if driving around normally and sitting in traffic where it should climb it stays at 90 but put the engine under load and it’s goes sky high. I’m just concerned if it is actually overheating or not and if this is a much bigger issue elsewhere.

Until the recent new and enhanced user experience the forum would indeed have displayed your engine details, location etc beside your username.

Now we have to go searching for it which is very frustrating as you often find people have not given the info.

So for now its better to include the info in the posting, better than the title as that is not visible when reading the posting, hopefully the previous functionality will be restored.

If you have VCDS you can view the actual unfudged temperature sender readings (assuming the sensor is not malfunctioning) otherwise look at the oil temperature readings using the Maxidot display and see if they rise when under load.

9 minutes ago, Topgun433 said:

Ever since this incident it’s not reading normal temperate readings now as in if driving around normally and sitting in traffic where it should climb

It never did show those fluctuations, yes I agree it should climb but VAG temperature guages have not told the truth for 25 years now.

i would assume this is just a sender issue. Oil temp will give you a good indication of the state of things. Should be around 105C IMO at 70mph, with ambients above zero. Shouldn't go much above that unless towing or heavy load.

If I remember correctly when scrolling through the displays the oil temperatures were around 110ish.

I’m afraid I don’t have access to vcds I do have an obd reader but I’m not sure if this would give an engine temperature read out.

Something I should have added to my issue is that the car heaters have not been getting warm they have been stone cold (I did send it to a local Škoda specialist and they did a coolant flush and change a few months ago) I pressurised the system the weekend after changing the water pump and it did start getting warm but when the temperature went awol the heating went stone cold again. I’m not sure if the issues are linked at all or separate.

Where is the engine temperature sensor and is there a way to test it to rule it out ?

Sounds like you might have heater matrix issues... if blocked the car can cook the coolant (as JR says, it'll read normal until it suddenly shoots up). Could also be the coolant temp sensor or thermostat, but with the heating being dodgy I'd put money on it.

Would the best way of moving forward with sorting the issue out is flush the heater matrix out first ?

Do you recommend testing the temperature sensor before removing it ? Am I right in thinking it’s located on the side of the block underneath the air box ?

11 hours ago, Topgun433 said:

the water pump and it did start getting warm but when the temperature went awol the heating went stone cold again.

Almost sounds like a big lot of air in the system.

An air lock could stop the heater working and could well lead to overheating.

What Is it oil temp when it's overheating. That will be fairly tell tale.

I'd be thinking a thermostat issue if air is ruled out, or blocked radiator even.

The problem may have been there a while and masked by the flat spot in the temp guage.

There is a process you need to do to remove air from the coolant system. This can be done on VCDS or ODBeleven. You need to do in basic settings a function called "Fill Coolant Circuit"

1 hour ago, BlueWagon said:

Almost sounds like a big lot of air in the system.

An air lock could stop the heater working and could well lead to overheating.

What Is it oil temp when it's overheating. That will be fairly tell tale.

I'd be thinking a thermostat issue if air is ruled out, or blocked radiator even.

The problem may have been there a while and masked by the flat spot in the temp guage.

Before the issue first started it was at a local Škoda specialist who did a coolant change on the car and supposedly bled it to Škoda specs.

The oil temps if I remember correctly were between 105-108.

As I don’t have access to vcds or obd11 to try and purge the air out can the car be ran up to temperature with the expansion bottle cap off to allow the air to escape that way ?

2 hours ago, BlueWagon said:

Almost sounds like a big lot of air in the system.

56 minutes ago, Topgun433 said:

Before the issue first started it was at a local Škoda specialist who did a coolant change on the car and supposedly bled it to Škoda specs.

Now you tell us! Don't you think that might have been relevant information to have given at the beginning?

57 minutes ago, Topgun433 said:

can the car be ran up to temperature with the expansion bottle cap off to allow the air to escape that way ?

If that were the case then there would not be a bleed procedure.

Then you likely have air in the system as a result of their bleed. OBD11 isn’t expensive and is a very useful tool, if you fancy doing it yourself.

24 minutes ago, J.R. said:

Now you tell us! Don't you think that might have been relevant information to have given at the beginning?

If that were the case then there would not be a bleed procedure.

It was around 3 weeks ago they had the car in surely a potential air lock would have shown up sooner. It’s been running fine up and down the motorway since before then with no issues

Not necessarily. A) the pocket could have been trapped somewhere it was doing less harm and B) with the cold temperatures etc, the temp might have been within the 90 degree display range.

The trouble is that in most dealerships, they use a pressurisation system for getting rid of any air locks as well as using ODIS to tell it to purge the system too. People have mixed results with just allowing the cars to 'burb' themselves; it doesn't always work.

When my water pump was changed, I ended up needing to fill 1.5l within 100km... then again another 0.5l and again and again. I think 4 fills. Each time I checked and it appeared to be at the correct level. Then I looked again, and hey presto, low. I have no idea if they tried to bleed the system, I can only assume they kind of tried but it didn't really work.

I need to check mine again, it was low-ish when they the fixed the boost pipe and I've done a couple thousand km since them, some of that at high loads / engine speeds.

Keep a bottle of G12evo with you and make sure you aren't getting any leaks. It's worth getting yourself an ODBEleven dongle as you can get the car to perform the coolant purging process yourself.