Hope someone can help , I have a 2015 VRS diesel and it was building pressure and blowing coolant out the expansion tank , took it to a mechanic and he said he replaced the water pump but the coolant is dropping to almost empty until I open the cap and it returns. What puzzles me most is I’m certain that two hoses entered the firewall just to the right of the expansion tank ,one hose directly over the other yet when I picked up my car they are gone all I see now is a slit in the fire wall , when I asked the mechanic what happened them he just laughed it off and said the split in the firewall was for a left hand drive. I’m 100% certain the hoses were there . Can somebody please confirm this and provide a photo if possible.

Many thank

13 hours ago, Missyoctober said:

took it to a mechanic and he said he replaced the water pump but the coolant is dropping to almost empty until I open the cap and it returns

So did the mechanic fix the issue or is it still building pressure.

13 hours ago, Missyoctober said:

yet when I picked up my car they are gone all I see now is a slit in the fire wall

I doubt a mechanic would steal Hoses.

Are you sure they aren't on the other side of the firewall? As in the passenger side.

Also, wouldn't there be open connections where the Hoses went onto? Can you see that?

Maybe google the engine bay or look for cars for sale with a pic under the hood

The car is still building pressure When I was with him yesterday to voice my concern about the low coolant in the expansion tank he opened the top and the coolant came flowing back in so much some went out to hole in the bottom of the tank . He told me to keep driving that there was nothing to worry about. He did a test for the head gasket and he said it was fine so there has to be something else causing it. I just can’t be throwing money at the problem hoping something will work. As for the hoses that I think were going into tue firewall I wouldn’t be accusing him of stealing them , maybe he bypassed something or other , see photo of opening in firewall at the location I think they were at.

18 hours ago, Missyoctober said:

he opened the top and the coolant came flowing back in so much some went out to hole in the bottom of the tank .

That doesn't make sense....the coolant flowed into the tank but went out the hole in the bottom....

Do you mean it came out the cap?

What did the mechanic say about the coolant levels??

18 hours ago, Missyoctober said:

maybe he bypassed something

Does your heater still get hot? If so, then he hasn't bypassed that.

Not really understanding why you think he bypassed something but won't admit it.

Test the heater I guess. That will confirm.

As for the low coolant, I'd be asking the guy why it's so low.

Maybe it is just a bit of trapped air comming out.

Is it still low now?

What happens if you opened the cap now.

8 hours ago, BlueWagon said:

or

When coolant builds up the spring in the cap will allow the coolant out the little hole on top and it exits the bottom of tank see photo below.

The heater is working fine only difference I see in that is when everything is closed of (fan and vents ) hot air keeps coming out .

The coolant is still saying in the system and when I release the cap it comes back in. It was on the low mark this evening then when I got home it was up to normal level then I loosed cap and it came gushing back in , see video. Thanks

The coolant is in a closed system, which by design builds pressure. The coolant in the system can and does go past the boiling point of water, which is what in part causes the system to pressurise. As everything gets hot, the coolant is given a vessel to expand in to - the expansion tank. As you de-pressurise the system by unscrewing the lid on the expansion (or header) tank, the fluid, which is already under pressure, will naturally want to expand into it. This is normal, and the water will be very hot, if not boiling.

Although my Golf's not diesel here's a picture of what I think is the same area on mine:

If you heater is still working, then no pipes to the heater matrix have been removed. The heating pipes go through the bulkhead to the right of the above photo - about a foot closer to where the battery is. Here's a picture of them on mine:

Hope that helps a bit.

Gaz

I understand the coolant expands , but why would the low coolant warning come on ? When I check it there’s little or no coolant in the tank until I open the cap and it returns and the warning goes off. This has happened while I’m on the motorway and I’ve had to pull over and wait to open the cap. I’ve never had this issue with the car or any car before . This only started a few weeks ago

If the correct coolant filling procedure was not carried out there may well be air locks in the system.

Mine took a couple of weeks to settle down after my water pump was changed with top ups needed for the duration.

On 17/07/2025 at 21:18, Missyoctober said:

why would the low coolant warning come on ?

Because the tank is full of air....not coolant.

The prongs probably detect coolant conductivity which the coolant is alot more conductive than air.

On 17/07/2025 at 21:18, Missyoctober said:

When I check it there’s little or no coolant in the tank

Because the tank is full of air, air likely come out of the cooling system as the coolant circulation happens.

On 17/07/2025 at 21:18, Missyoctober said:

until I open the cap and it returns and the warning goes off.

When you open the cap, the pressure of the air is released to the atmosphere, so the coolant comes back into the tank. The air pressure in the tank had been holding it down in the engine and other pipes.

Coolant tank now has coolant in it, so warning goes off.

23 hours ago, Gammyleg said:

If the correct coolant filling procedure was not carried out there may well be air locks in the system.

I'd say this is the issue.

I'd keep topping it up as needed

Question:

Does the coolant tank show low in the morning when the car is cold?

Next time you get home, have a look at the level, see if it comes up overnight as the air in the tank cools and contracts a bit.

I'd say there is an airlock somewhere, but where? That is the question..... Without understanding the layout, I would try and find the highest hose clamp, undo it and fill the cooling system from there with the engine running and cold.

My BMW E38 740 Sport with 4.5 litre V8 takes about 30 minutes to fill and bleed the cooling system properly. There is a bleed valve on the expansion tank that MUST be higher than the heater hoses at the back of the engine. This is difficult as the bleed valve is actually a lot lower when the car is level so, it MUST be done on ramps; and the engine has to be cold, i.e. with the thermostat closed and the heater on full heat to make sure the matrix is filled properly. If the stat opens you have to go away, make a cup of tea, watch the cricket for an hour and start again... Quite a rigmarole and it sounds like VAG havn't fitted a bleed valve to save money.

Edited 18 July, 202518 Jul by SwallownAmazon

14 minutes ago, SwallownAmazon said:

My BMW E38 740 Sport with 4.5 litre V8 takes about 30 minutes to fill and bleed the cooling system properly

The Skoda is likely the same but requires connecting to VCDS or similar to sequence the process correctly.

I think it can be done without a VCDS but it may take time to fully purge the system of air which is probably what was happening with mine over the two week period, and may be happening with the OP's car.

Assisted by the illuminated low coolant warning light, i'd wait for the system to fully cool down, remove the cap and then refill to the max level. After a while the warning light would cease to come on anymore and the problem went away.

It's worth noting that the default coolant level in my expansion tank is 'Min'. If I were to top it up to 'Max', within a day it would again default back to 'Min' again and stay there.

Edited 18 July, 202518 Jul by Gammyleg

Thw three most important factors for a successful bleed are, I believe:

System should be cold.

Heater should be at Max heat

Engine should be running to circulate the coolant.

Right this has nothing to do with how it’s been bled or anything like that and there are known issues with these engines and coolant loss. Search this forum or Facebook for “my gtd blows cold”

So first things first, do you have heat from both the drivers vent and passengers vents in the car? If very uneven it’s likely a blocked heater matrix. The matrix is next in line to the egr cooler on the coolant circuit so when the engine does a dpf regen the coolant is a lot hotter in this part of the circuit, if the matrix is partially blocked it vents coolant back to the reservoir which usually over pressurises things and dumps coolant out the overflow of the reservoir. Check for white coolant staining under your reservoir.

Next thing it could be is a leaking charge cooler. Also very common on these. Easy to check with a endoscope camera through the temperature sensor.

The other thing it could be is a leaking egr cooler. Again quite common.

The hoses going to the matrix in the engine bay are on the back right, not behind the turbo, so your mechanic hasn’t removed them.

On 18/07/2025 at 16:56, SwallownAmazon said:

Thw three most important factors for a successful bleed are, I believe:

System should be cold.

Heater should be at Max heat

Engine should be running to circulate the coolant.

8 hours ago, T07 said:

Right this has nothing to do with how it’s been bled or anything like that and there are known issues with these engines and coolant loss. Search this forum or Facebook for “my gtd blows cold”

So first things first, do you have heat from both the drivers vent and passengers vents in the car? If very uneven it’s likely a blocked heater matrix. The matrix is next in line to the egr cooler on the coolant circuit so when the engine does a dpf regen the coolant is a lot hotter in this part of the circuit, if the matrix is partially blocked it vents coolant back to the reservoir which usually over pressurises things and dumps coolant out the overflow of the reservoir. Check for white coolant staining under your reservoir.

Next thing it could be is a leaking charge cooler. Also very common on these. Easy to check with a endoscope camera through the temperature sensor.

The other thing it could be is a leaking egr cooler. Again quite common.

The hoses going to the matrix in the engine bay are on the back right, not behind the turbo, so your mechanic hasn’t removed them.

The heater has the exact same temperature on both side of the car .

When I checked it cold this evening before the drive home the coolant was just below the minimum mark dove 30mins home and checked and the coolant is way over the max mark , sometimes after a drive it can be low but when I open the cap it comes gushing in . It doesn’t seem to be losing coolant just sometime the underside of expansion tank is damp. As far as I’m aware the mechanic checked the charge cooler. The way the car is now I just couldn’t take it on a long journey. I

From Mk1 Octavia TDI experience under very much similar circumstances, what likely has happened is that the engine has not been refilled with coolant properly and has been driven, overheating head gasket. This only results in exhaust in coolant when the engine is under load. Static coolant pressure test does not detect it, as the pressure is insufficient (<1.5bar vs tens of bars coming from the cylinders via leaky gasket). To confirm in later stages of the problem, warm up the engine, find a long enough uphill road, accelerate in high gear (ideally ~1min), and then usually either you get low coolant warning or the coolant temperature goes berserk. Another telltale symptom is slow loss of coolant into the engine when it is very cold, but you'd need to wait till winter for this. In any case, most likely this is an error on part of the workshop and unfortunately it is very likely that the head has to come off, preferably in another workshop :)

In my case, after a timing belt / water pump job by a dealership in late summer, unfortunately I only noticed the slow loss of coolant in winter, then experienced coolant being expelled in late spring. Replaced the head gasket at an independent and all was well for then next 5 years. Dealer who originally bodged this ignored me completely, so I ignored them for all my future car purchases...

Edited 21 July, 202521 Jul by dieselV6

1 hour ago, dieselV6 said:

From Mk1 Octavia TDI experience under very much similar circumstances, what likely has happened is that the engine has not been refilled with coolant properly and has been driven, overheating head gasket. This only results in exhaust in coolant when the engine is under load. Static coolant pressure test does not detect it, as the pressure is insufficient (<1.5bar vs tens of bars coming from the cylinders via leaky gasket). To confirm in later stages of the problem, warm up the engine, find a long enough uphill road, accelerate in high gear (ideally ~1min), and then usually either you get low coolant warning or the coolant temperature goes berserk. Another telltale symptom is slow loss of coolant into the engine when it is very cold, but you'd need to wait till winter for this. In any case, most likely this is an error on part of the workshop and unfortunately it is very likely that the head has to come off, preferably in another workshop :)

In my case, after a timing belt / water pump job by a dealership in late summer, unfortunately I only noticed the slow loss of coolant in winter, then experienced coolant being expelled in late spring. Replaced the head gasket at an independent and all was well for then next 5 years. Dealer who originally bodged this ignored me completely, so I ignored them for all my future car purchases...

The mechanic is a VAG man , he did several pressure tests and he’s certain it’s not the head gasket , what he did say when I went back after a few days to query why the coolant was rising and falling he said to drive it for a another week and he didn’t rule out the charge cooler even though he did a probe test.

Unless he pressurized cylinders to 100bar on a warmed up engine (I doubt it), he cannot diagnose partially overheated head gasket failure, as exhaust goes into coolant only under high engine load with a warmed up engine. While it could still be something else, all symptoms described so far point to what I had described, together with mechanic's reassurances on how everything is tickety boo with head gasket and pressure. Anyway, just my opinion, time will tell if I am right. If it is the head gasket, you will experience increasing loss of coolant, including coolant expulsion in large quantities after long periods of hard engine work (uphill runs for example). If/when it gets worse, the problem will become obvious, but of course then it will be some months since the water pump job and no recourse. Not the first engine damaged in this way and not the last. HTH

Exact same problem with mine, could it possibly be since at the same time that I got my tb+wp replaced, i got a pipe replaced that had burst from another incident and all the coolant had drained out. Could the mechanic possibly had not bled the coolant properly and there’s an airlock?

On 14/07/2025 at 23:13, Missyoctober said:

Hope someone can help , I have a 2015 VRS diesel and it was building pressure and blowing coolant out the expansion tank , took it to a mechanic and he said he replaced the water pump but the coolant is dropping to almost empty until I open the cap and it returns. What puzzles me most is I’m certain that two hoses entered the firewall just to the right of the expansion tank ,one hose directly over the other yet when I picked up my car they are gone all I see now is a slit in the fire wall , when I asked the mechanic what happened them he just laughed it off and said the split in the firewall was for a left hand drive. I’m 100% certain the hoses were there . Can somebody please confirm this and provide a photo if possible.

Many thank

7 hours ago, DanP6 said:

Exact same problem with mine, could it possibly be since at the same time that I got my tb+wp replaced, i got a pipe replaced that had burst from another incident and all the coolant had drained out. Could the mechanic possibly had not bled the coolant properly and there’s an airlock?

Very hard to get to the bottom of it , I just bypassed the charge cooler and the problem is still there so next I’ll by pass the egr cooler and if that doesn’t sort it I’m looking at a new head gasket.

27 minutes ago, Missyoctober said:

Very hard to get to the bottom of it , I just bypassed the charge cooler and the problem is still there so next I’ll by pass the egr cooler and if that doesn’t sort it I’m looking at a new head gasket.

My friend who’s just recently qualified as a mechanic believes there’s simply just an airlock, if this doesn’t work then I’m in the same boat as you 😅

Don't guess on this one. You can get kits to determine if combustion gasses are getting into the coolant. For me; an airlock would be an occasional 'burp' whereas a cylinder head gasket would be a constant series of gasses plus you can smell the combustion gasses in the coolant header tank.