Loosing coolant, no leak anywhere

[g_berserk]

It has happened twice since I bought the car in august last year.
It's a Cupra 265 DSG, with a pristine service record, only owner, no visible damage anywhere.

The thing is that in three separate occasions during wildly different driving conditions the MDF has popped a red light out of nowhere indicating low coolant levels.

The first time I limped back to the garage, let the car cool for an hour and topped with purified water since I didn't have anything else at hand.
I bought a quarter of coolant in the dealer as a precaution and thing behaved normally for a couple months, then the light came back again.

Once again I limped back home and waited 10minutes. Opened the reservoir cap and levels rised back to max what gives???????

Today on the way to work the same thing happened and I stopped in a side road, turned the car off, and carefully started to open the reservoir cap.
Levels came back to max, and then started to overflow so I closed it quickly.


I have checked the car multiple times from all possible angles and no leak is to be found, oil is not contaminated and the temperature gauge is always perfect.

From what I read it may be air trapped in the system, but I want to hear some opinions and ideas on how to purge the system.

Thanks

 

[LouG]

Check coolant level cold regularly, once weekly. See if it changes. Coolant level in the overflow tank rises when hot.
If the level doesn't change, you have a sensor issue. You're unlikely to have a air lock if the system has no leaks and hasn't been drained.

 

[WaveyDaveyGravy]

This is a fairly common issue. You can't see the leak as it is caught by the tray under the engine. Usually the water pump. Do a search on Golf R coolant leak. Mine is only 8 months old and it's started doing it.

 

[Upthere]

This is a fairly common issue. You can't see the leak as it is caught by the tray under the engine. Usually the water pump. Do a search on Golf R coolant leak. Mine is only 8 months old and it's started doing it.

This.

Common with S3's as well.

 

[ZK_FR150]

Think I've got a similar problem, its already been in the dealers but they couldn't find anything.
I'm not sure if they just did a pressure test and that was it.
I think the coolant drains quicker if the heaters been on for a while so it maybe related to the heater matrix or something.

Gonna get it booked in for a more thorough test

 

[andycupra]

reason it rises when hot and you undo the cap is that the coolant and air expands when hot.
I cant comment in the Mk3 but in my seats the level changes from min at cold to max when hot. Therefore if the level is mid, or at max when cold it will go above max when hot. (normal)

If you undo the cap when hot (not normally recommended and is a high risk task - so if you do this do so with caution and consider loosening to release pressure before completing removing) - this releases the pressure, air escapes and the level can rise a bit as any pipes that have expanded relax.

The actual level when cold can also change dependant and ambient conditions. there used to be loads of threads this time of year as its gets cold as people would get warnings when they first start the car as the cold level would be lower than usual due to general ambient conditions. (lots of people cleaning prongs or replacing expansion tanks when a top up fixes it in 95% of cases)
In your case it reads like you can get an alarm while driving? and therefore generally when the engine/coolant is up to temperature? so it would not be this issue.

Systems are now 'self purge'.. ie you no longer have to heat the car up and blead off the top of the rad like a home radiator..
However, when replacing coolant you should still:
Fill up coolant, LEAVE CAP off/loose
start engine,
monitor and fill up as required
allow engine to reach normal operating temperature and leave running for a few mins.
then stop engine. ALLOW TO COOL, all with the cap off/slightly open.
when cold, the level will have dropped a bit, top up as required, then fit cap and should be good.

in your case I think it would be prudent to do a level 'reset'. There is some danger that when removing the cap when hot then replacing the cap when the car cools it could then lead to negative pressure in the system, (with rubber pipes contracting) this can then lead to an artificially high reading when cold, you then top up.. and could lead to high or low levels..
to purge /reset the level, when car is cold note the level, then remove the cap, (does the level change? - if so this indicates the level was not set correctly, air lock or a potential leak when hot). leave for a few mins, then top up or remove coolant to get the level in the middle of the indicated range.
With cap still off/loose, start the car and allow to reach operating temperature (with CAP off/loose). you should see the coolant circulating into the expansion tank when at temperature. leave for a few mins, then turn engine off and allow to cool. (leave cap on top of tank but not done up) only when the car has cooled for a good period, recheck the level and fill up/remove as required. then replace cap. level is now correct.
If its an air lock causing issues then this should correct this.

If you then continue to get issues, then there is an issue with level sensor, air lock (less likely) or a leak. some leaks only happen when hot, especially if the level was too high as there is no air gap to compress when hot)

This also offers the opportunity to inspect for any leaks, as in this case if there is a leak its likely to be very small, (your not filling the coolant up regularly) so having the car run at temperature for a period while static may help detect any leak.

If the coolant is not circulating with cap off then perhaps water pump..

 

[LouG]

If you remove the cap when hot, wrap it with a towel and turn to the first detent only to release pressure. Better still, use a stick and push on one "ear" of the cap from a distance.

 

[g_berserk]

Thanks for the replys
I always lose the cap with a rag slowly letting the air escape slowly first, then completely remove and with one feet ready to run the other way

Did the reset level procedure yesterday after a third light in a single day and today the car seems normal.
Will be monitoring it closely every night when it's cold, and I have to take it for service in a few weeks, so if things continue to go abnormally I'll ask them to do a through inspection while they have it in the garage.