Constantly topping up coolant, leak or more serious?

[MK3GTI_16Valve]

Hi all

Just got my 16V back on Wednesday, its been off the road for the last two months sorting out a few bits and pieces to get it through the NCT (mot). It passed anyway all good. However im getting about 40 min driving and the water temp light is coming on on the dash. Its taking about 2.5L each time to refill. I presume I have a air lock because there is no sign of a leak. If there is no leak then where is the water going for such a short spin? Could it be over flowing out the top of the expansion bottle as the pressure builds during the journey or do I have a more serious problem? Thought i read somewhere before that It could be a sign of head gasket issues Temp in the car is normal and no mayonaise on the oil cap. Going to try and bleed it after work today following rubs guide. hopefully its just an air lock but just wondering where all the coolant / water is going?

Cheers
Brian

[rubjonny]

gotta be going somewhere, check the rad over very carefully, its difficult to see the whole thing on a mk3 isnt it?  could have a cracked end and seeping out there. coolant flanges obv a good spot for leaks. you're lookin for white crusty deposits

[MK3GTI_16Valve]

Cheers

Will hopefully have an update before the end of the day, as you say it must be going somewhere

[MrBounce]

Try changing the expansion tank cap - only about £5 and it solved my issue 

[Len]

Check all the hoses first mate! But do it with engine hot and running as some leaks only occur under pressure!

[hamster245]

I just noticed yesterday whilst driving along that my coolant level light started flashing. Assuming the guages were correct, I was running at 90'c on the coolant temp and 78'c on the oil temp and it didn't get any hotter than that.

Had a look under the bonnet and noticed it had dropped below the min line on the tank, oil level and under filler cap are fine and no obvious signs of a leak from the visible pipes and when I restarted the car the light had gone out. I've topped it up with some tap water for now and going to keep an eye on it for the next couple of days to see what it does.

[MK3GTI_16Valve]

Sorted

Disconnected the hoses from the inner rad and found that to be completly empty. Filled that to the until i had a constant flow of water out of the rad and reconnected the hoses. Filled the system up and squeezed any hoses i could to remove any air from system. When i was happy that it was no longer airlocked put the lid on the expansion tank on tightly again and just monitored the car for a few minutes.

After a min or two water started to pour out of the vent on the side of the expansion bottle. Thought this was normal as i had filled the bottle above the max line. Thought it would settle eventually but it didnt. Unscrewed the lid again and found that there was no release of pressure when the cap lid was opened, system obviously wasnt pressurising. The seal on lid has shifted slightly, realigned it, refitted cap and hey presto water stopped coming out of the vent of the expansion bottle.

Only had the car back a few days, the only reason i didnt suspect the water coming out of the expansion cap was because i was intentially over filling it to be sure id had enough coolant to get to / from work. By the time id get home and look for leaks the water would have stopped coming out of the expansion bottle and the system airlocked.

Such a simple fix in the end