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Coolant leak and something odd?

everson38

Active Member
May 15, 2017
361
24
walsall
Hey guys New to the forum. I am posting on behalf of a friend so apologies if don't have all info.

A 2.0 Seat Leon FR and it has a coolant leak we could see it leaking for somewhere that seem to be the lower radiator hose connected to the radiator side. The thing that was odd was, the reservoir was just on the min line, but when the pressure was relieved on the expansion bottle and fully removed the lid, the expansion bottle filled back up? (I know shouldn't remove lid whilst cars on / under pressure)

Does anyone know what would cause this re filling back of the expansion tank?

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RUM4MO

Active Member
Jun 4, 2008
6,086
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South Scotland
Normally I'd suggest that there is some trapped air in the system somewhere, that for some reason, has not managed to get purged back up into the air space at the top of the reservoir, but -----?
 
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BoomerBoom

Active Member
Jun 1, 2018
355
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I think it's more the law of physics, by removing the cap while hot you've reduced the system pressure and allowing the gasses in it to expand (the liquid volume stays the same).

If the coolant been in awhile then there shouldn't be much dissolved gas in the system, but your leak could be sucking air in when cold.
 
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Lozzy15

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Mar 24, 2015
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But to the point where it's below min and then back at max??

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That is quite an increase for sure. I've had them go from around half to max before when the engine was boiling hot. If you've got a leak though and air is getting into the coolant system, you've then got more air in there that can pressurise and therefore expand when the pressure is released so it does make sense. Have you managed to fix the leak yet?
 

everson38

Active Member
May 15, 2017
361
24
walsall
That is quite an increase for sure. I've had them go from around half to max before when the engine was boiling hot. If you've got a leak though and air is getting into the coolant system, you've then got more air in there that can pressurise and therefore expand when the pressure is released so it does make sense. Have you managed to fix the leak yet?
So to ensure I have my science right. The leak is causing a loss of fluid which is also sucking in more air, this extra air once the lid is removed during hot/warm engine is escaping in a sense.bleeding it self and allow the coolant to fill up those air pockets and return to normal level? Not too sure if he has the leak fixed yet. Will need to check in.

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Damo H

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Oct 3, 2012
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Aren't you supposed to look at the coolant level when the engine is off, but warm?

Edit ignore me:

From the Manual:

"Checking coolant level
– Park the vehicle in a horizontal position.
– Switch the ignition off.
– Read off the coolant level on coolant expansion tank. When the engine is cold, the coolant level should be between the marks››› Fig. 319. When the engine is hot, it may be slightly above the upper mark."
 
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everson38

Active Member
May 15, 2017
361
24
walsall
Aren't you supposed to look at the coolant level when the engine is off, but warm?
Well I've always been told never open expansion tank when car is hot and I guess running (except maybe when bleeding a system). I was doing something else and my friend opened the expansion without me seeing but said it filled back up, now due to myself never opening an expansion due to worry of scolding. I've just never seen / heard of it filling back up, so was trying to find out if it's normal, bigger issue? What it means etc

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Damo H

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Well I've always been told never open expansion tank when car is hot and I guess running (except maybe when bleeding a system). I was doing something else and my friend opened the expansion without me seeing but said it filled back up, now due to myself never opening an expansion due to worry of scolding. I've just never seen / heard of it filling back up, so was trying to find out if it's normal, bigger issue? What it means etc

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Suspect as others have said, the pressure has kept the coolant in the system. Undoing it as relieved said pressure. I've seen a reputable mechanic do it. When asked, should you do that, their response, I would not expect 'jo public' to do it for danger of scolding but nothing mechanical. However the engine had cooled down enough for him not to worry about it to much, but to be safe he did it very carefully/gingerly with gloves and a cloth.

AS for the manual it doesn't say anything about damaging the engine unless there is nothing in the expansion tank.

"● The cooling system is under pressure. Do not unscrew the cap on the coolant expansion tank when the engine is hot: risk of burns!"

https://www.seat.co.uk/datamanual-manual/leon/my20_w28/en-uk/LEON_07_19_EN.pdf

Whats weird, is my coolant is G13 for sure (marked on the expansion tank), but the above manual says it should be G12Evo...

Edit - 07/2018 manual says G13 coolant.
 
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BoomerBoom

Active Member
Jun 1, 2018
355
124
A pressurised liquid will have a higher boiling point, so the coolant could be at 105 degrees. This is fine until you depressurise the system by loosening the cap. This causes the coolant to instantly boil, increasing its volume and eject steam/coolant through the hole you've created.


For how not to release a pressurised system.
 
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everson38

Active Member
May 15, 2017
361
24
walsall
Thanks to everyone who has commented . So finally in regards the coolant refilling 8n the expansion bottle when it was opened / we pressurised...this is normal? I know we have a leak somewhere but just want to know that's not a job of fix.leak and then fix this rising and falling of coolant.

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Damo H

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Oct 3, 2012
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A pressurised liquid will have a higher boiling point, so the coolant could be at 105 degrees. This is fine until you depressurise the system by loosening the cap. This causes the coolant to instantly boil, increasing its volume and eject steam/coolant through the hole you've created.


For how not to release a pressurised system.
What a muppet. Yeah if its already steaming.

Sadly on these cars you can't go off of the guage in the car, it always shows 90 degrees. I've had an OBD reader showing between 75 and 115 degrees coolant temp and the cars just shown 90. In fact I have a sneaking suspicion the coolant temp is just a timer for how long the engines been on.
 

everson38

Active Member
May 15, 2017
361
24
walsall
Speaking of coolant temp. Does any9ne know what the operating temperature is for these cars?

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andycupra

status subject to change
its normal. if you had a leak you probably have some air in the system, you are just releasing the pressure and some of the pipework has probably relaxed a bit also.
You should fix the leak and run it up to temperature with the expansion tank lid off and top up as required to purge. Then let it cool down WITH LID OFF - so when cool the system is at normal atmosphere pressure.
 
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LouG

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Dec 1, 2017
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Nelson, New Zealand
It can be anywhere just either side of boiling point for plain water. My recent cars temps are very stable at around 90C, my bike runs at 71 and goes to 85 - 90 in slow traffic.
 
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Damo H

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Oct 3, 2012
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It can be anywhere just either side of boiling point for plain water. My recent cars temps are very stable at around 90C, my bike runs at 71 and goes to 85 - 90 in slow traffic.
Is that going off on OBD port reader, or what the dash tells you? Dash isn't accurate for coolant.
 

LouG

Active Member
Dec 1, 2017
1,328
476
Nelson, New Zealand
Is that going off on OBD port reader, or what the dash tells you? Dash isn't accurate for coolant.
This is other cars. The Toyota also had torque pro running. It was always stable. Which reminds me, I'll hook up torque pro to the Cupra.
See how it goes over summer.
PS. If the temp gauge isn't at all accurate, why does it show the temp increasing? Mine doesn't always show 90.
Not forgetting that there are three temp indicators, 4 counting oil, and they all read from the ECU.
 

Damo H

Carbon Snob
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Oct 3, 2012
3,813
2,247
Car Length In Front
This is other cars. The Toyota also had torque pro running. It was always stable. Which reminds me, I'll hook up torque pro to the Cupra.
See how it goes over summer.
PS. If the temp gauge isn't at all accurate, why does it show the temp increasing? Mine doesn't always show 90.
Not forgetting that there are three temp indicators, 4 counting oil, and they all read from the ECU.
It’s well documented on this forum that the temp increases then just sits at 90 degrees when the engine is in it’s normal temp range.

In France it was at 90 degrees even though the OBD port reading was saying it was only 75. It got as high as 115 and still the the display read as 90.

Oil temperature display matches the OBD port always.
 

RUM4MO

Active Member
Jun 4, 2008
6,086
402
South Scotland
The reason for "90" being maybe 75C > 105C is that while the normal coolant temperature gauge is an analog device - it is being supplied with a signal from a digital source, so it will never move smoothly across its scale as it would if supplied by an analog more jump in steps - but as we tend to watch the road while driving we never notice that so this system works well enough as that gauge is only meant to be an indicator and not an accurate temperature gauge.

Another thing is to remember, if the car is filled with the correct coolant/water mix, even at ambient pressure, the boiling point will be quite a bit higher than 100C - though that does not mean that opening up a hot pressurised coolant system is a good idea.

For what it is worth many people comment that their cooling system level changes dramatically between cold and a hot engine, none of my cars have ever behaved like that, though maybe I don't also only end up to continuous nose to tail traffic.
 
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