That looks very like exhaust is getting into coolant somewhere I am afraid. Could be egr cooler or head gasket. You could see if you could get a leak down test done.

As there is CO2 in system I doubt its the waterpump drawing air in.

On 11/02/2026 at 15:04, Alasdair1 said:

That looks very like exhaust is getting into coolant somewhere I am afraid. Could be egr cooler or head gasket. You could see if you could get a leak down test done.

As there is CO2 in system I doubt its the waterpump drawing air in.

I am still looking for a mechanic who can perform this procedure.

As I already wrote, the rattling/knocking noise from the engine that lasts 10–20 seconds during a cold start really bothers me. I think it might be coming from the pistons, so it may not be just a head gasket issue.

If I keep topping up the coolant and continue driving like this, what would be the next likely problem with the car? And how advisable is it to keep driving it in this condition?

25 minutes ago, Pikaa said:

I am still looking for a mechanic who can perform this procedure.

Bypass egr cooler.

I think if your oil is not milky it's more likely an egr issue than head. Not impossible to be head gasket but bypassing cooler is simpler Id say.

And cheaper to fix.

I think the knocking when cold may be oil too thick (change it for thinner oil). Or, worn bearings.

Engine likely quite worn out but if an egr cooler fixes water problem then keep driving it until it's done.

3 minutes ago, BlueWagon said:

Bypass egr cooler.

I think if your oil is not milky it's more likely an egr issue than head. Not impossible to be head gasket but bypassing cooler is simpler Id say.

And cheaper to fix.

I think the knocking when cold may be oil too thick (change it for thinner oil). Or, worn bearings.

Engine likely quite worn out but if an egr cooler fixes water problem then keep driving it until it's done.

Can the EGR cooler cause this even when the engine is cold? I mean, if I start the car after 24 hours, remove the cap, it bubbles immediately. If the EGR cooler were faulty, wouldn’t it need to be warm for the problem to appear?

1 minute ago, Pikaa said:

wouldn’t it need to be warm for the problem to appear?

Why?

I reckon coolant passes through it always.

On 12/01/2026 at 01:17, Pikaa said:

The EGR cooler was replaced because we disconnected the cooler and then, interestingly, it didn't consume water,

Don't discount the fact disconnecting the cooler last time stopped coolant loss.

Id see that as pretty good evidence the cooler is the problem.

Maybe new cooler is no good/poorly installed

On 2026. 02. 13. at 7:33, BlueWagon said:

Ne hagyd figyelmen kívül azt a tényt, hogy a hűtő leválasztása legutóbb megakadályozta a hűtőfolyadék veszteségét.

Szerintem ez elég jó bizonyíték arra, hogy a hűtő a probléma.

Lehet, hogy az új hűtő nem jó/rosszul van beszerelve

I was just about to take it to the shop to have the EGR cooler bypassed, but when I opened the expansion tank cap, this is what I saw...

Edited 17 February17 Feb by Pikaa

Is that oil or debris? If its oil then reckon its the HG. If its debris then it may be coming from intercooler.

Alasdair

2 hours ago, Alasdair1 said:

Is that oil or debris? If its oil then reckon its the HG. If its debris then it may be coming from intercooler.

Alasdair

It's oil, not debris, so it's starting to look like a head gasket issue for sure!

Reckon so. Cant think of anywhere else that the oil would be coming from especially with the bubbles in coolant in your video. Hopefully the head is good and a new gasket will solve the problem. Might be worth getting head checked for cracks if your removing it.

Alasdair

6 minutes ago, Alasdair1 said:

Reckon so. Cant think of anywhere else that the oil would be coming from especially with the bubbles in coolant in your video. Hopefully the head is good and a new gasket will solve the problem. Might be worth getting head checked for cracks if your removing it.

Alasdair

I just spoke to a mechanic and he will start removing the cylinder head next week! I will report back with the final result and what the problem was!

Thank you again for your persistent and detailed help! 👏🙏

You need a combustion Leak test as a bare minimum. Suspect oil is bypassing and getting into the cooling system....which means new cylinder head gasket most likely. Also do a borescope to check for coolant in the cylinders too (by pressuring the header tank and finding which one is leaking into the cylinders)

With oil in coolant and bubbles hopefully its easy to spot where its gone. As said get head checked for cracks in case its leaking there.

Alasdair

Have a read through this thread reference my cracked head.

Useless dealer couldn't diagnose and wasted a lot of the previous owners money making up faults...

6 hours ago, Cactusjack said:

Have a read through this thread reference my cracked head.

Useless dealer couldn't diagnose and wasted a lot of the previous owners money making up faults...

Thanks for your comment!

What kind of problems or warning signs did you have with your car that made you decide to remove the cylinder head? Has the car been running well ever since?

Only sign whilst driving was low coolant, so I was topping up every 200 miles or so. Never overheated, no smoke signs from the exhaust...

Extensive testing was carried out by a previous owner and a Skoda delarer but never found anything. Head gasket, and all coolers were replaced by them with a lot of warranty work but problem was never sorted. Previous owner quickly got rid and I bought it from a local garage during COVID so I didn't realise a problem until after the warranty ran out!

Sniff test showed exhaust mixing with coolant only after extensive use when hot... So off comes the cylinder head! No harm in getting it tested properly as gaskets need changing anyway.

All running smooth after head replacement with no further coolant issues, that was 4 years and 31k miles ago.

Thank you very much for your answer!

In the meantime, we have bypassed the intercooler as well, but it's not the cause of the problem (the coolant was still bubbling out of the expansion tank). We also connected a diagnostic tool, which came up with the following fault code.

That code I think is misfire on cylinder 4. It may be there that the headgasket is gone and your not getting proper compression/combustion etc. If the cooler has been bypassed then I would be 99% that its the head gasket or crack in head itself.

Alasdair

Unfortunately, there are two cracks on my engine block.

Edited 23 March23 Mar by Pikaa

Not good. Doubt it would be worth repairing even if its possible. Maybe pick up a second hand bare block and transfer parts from old engine or a second hand engine.

Alasdair

1 minute ago, Alasdair1 said:

Not good. Doubt it would be worth repairing even if its possible. Maybe pick up a second hand bare block and transfer parts from old engine or a second hand engine.

Alasdair

I won't fix it! I'll look for a used one!

I just posted that there is one, unfortunately..