Mk4 GTI coolant loss

[watusay]

Hey guys have an issue thats bafflimg me and I could use some help.
Have an MK4 GTI AUM. Car had been losing coolant and was easily identified by a little puddle or drops on ground. Was the radiator leaking from the corner close to the drain tap.
Anyways I replaced the radiator as well as the coolant bottle as mine had turned brown and all seemed well for a few weeks.

However after a spirited drive..when I got home and had the car idling..I noticed my temp guage very very slowly start to creep up a bit. I switched the car off and chexked the coolant level and it was below min but still had coolant in. I started the car and the moment I drive it cools down immediatley and wont budge and even when Im idling at the traffic lights it wont budge from 90.

I let the car cool down over night...in morning I check coolant and have to fill maybe 400mls. Use it the wbole day..not a single issue get home...temp is 90..let it idle for 1 min and the temp guage slowly creeps up.

Check coolant level and its dropped again to the min mark. Dont refill to max and drive car for a week and check coolant...still at the below min. Fill up to right spot and end of the day it drops to min and stays there.

I cant seem to find the leak anywhere as there isnt a spot of fluid on the ground, even when parked in the garage(my garage has cream tiles even so easy to spot). The water has to be going somewhere I just can't find out why and also the weird temp creep at idle. This has gone on for about 4 months now and csr still runs like a boss and doesn't smoke or overheat anywhere besides when i get home and ket it idle for a bit(must note it wont happen if i drive decently and chilled)

Water pump was changed around 35k km ago with timing belt etc. Metal impeller.
Oil was changed today, I think 4000km since all this started and didnt look to be contaminated with water.

Any any help would be greatly appreciated. Sorry for the long and confusing post

[Ashveer03]

Seems to be a problem with the new bottle/cap. Check for traces of watermarks around/under the water bottle to confirm the leak.
Did u replace the cap aswell? If so, fit the old cap onto the new bottle and check again. The cap probably not holding pressure hence sending some coolant out the bottles overflow.

If there was a leak elsewhere, most of the coolant would leak out which WILL leave the bottle empty.

Sent from my SM-G531H using Tapatalk

[panic-mechanic]

And the thrmostat works as it should and your fans run?

[watusay]

So gave it a few days and no , no tell tale waterspots or pink crust from the G12 apparent on the coolant bottle.

Panic not sure how to check thermostat but I do hear the fans kick in but ar random moments of idle and doesnt seem to be running when I have the temp creep up on idle.

Had a 300km trip I did today. Car performed flawless with the coolant at the Min mark and on return home was still at said mark. Seems to lose the coolant from Max to Min and yes its a bit confusing cause I for the life of me can not see a leak anywhere...Ive looked. The water pump doesn't seem to be leaking either and the heater works(I read that a failing water pump would cause the heater to not work correctly). Maybe Im making a noob mistake somewhere. I did do the radiator install myself

[panic-mechanic]

Thermostat is closed with the engine cold. It will stay closed until it warms up,to around 90 degrees. To check that keep feeling the top and bottom raditor hoses.the top will warm up and the bottom will be cold. When the thermostat opens the bottom hose will start warming up. If the thermostat is broken it will either never let the bottom hose warm up(broken in closed position) or if the bottom hose warms up from the start it is either not present or broken in open position.

[watusay]

I figured that may be the way to check but learnt its always better to ask. The car is doing just that Panic. While warming up to 90 degrees only the top hose is warm...a couple mins at 90 and the bottom hose starts to warm up too.

With my coolant at the low mark I figured a warning should be triggered. Sensor looked okay but decided to Disconnect and reconnect it just ro make sure. I see rhe sensor is damp inside...hmmmm No other traces of coolant loss apparent bur could rhis be my mystery leak?

[panic-mechanic]

It could be

[Sixty 9 GP]

I had a similar issue in the past, it was the seal of the Waterpump that was leaking, the heat from the engine used to cause the water to evaporate before it even started dripping on the ground.... climbing underneath the car, i eventually saw the water streaks against the block.

[Howie-WP]

I see rhe sensor is damp inside...hmmmm

Dry it and drive around and then remove and see if damp again. As far as I know it should not be damp on the inside, perhaps there is some sort of hairline crack that expands as the temp increases. Also, I don't think they are that expensive, perhaps just get a new one from GW?

[THE GAME]

I had a similar issue in the past, it was the seal of the Waterpump that was leaking, the heat from the engine used to cause the water to evaporate before it even started dripping on the ground.... climbing underneath the car, i eventually saw the water streaks against the block.

This one had me going for few months, until I jacked up and saw the water marks on the block
On my Golf VR6, water just disappeared and I wondered where,

getting parts together and get the pump changed soon,

[wickid3]

Hi, on my car it was the water flange on the side of the head, it drops a teeny tiny bit out.Drove it for a couple months like that before I had to top every day by this time it was much easy to find the leak.

Its better to just take it to a radiator shop and let them pressure test much quicker way to find the leak.

[panic-mechanic]

or if you are using G12 then take a blacklight and find the leak. It will flouresse under the black light

[watusay]

Thanks guys for all the replies. No its not the waterpump and yes Ive looked for all kinds of leaks, inside out.

This is what the sensor looked like. Bad pic but can see the drops on sensor.



Covered it with a piece of clear plastic and this is what it looks like



So pretty sure its the bottle

[panic-mechanic]

At the price of them just swop out the bottle already.

[watusay]

Definitely.....done and dusted. Lucky it didnt get pulled into the harness.
Will test and report back bit Im pretty sure this is it and no real traces because of evaporation

[watusay]

Sooooo..Guys changed bottle, and no more coolant loss, but, now I noticed that if I let the car idle for more than 2mins the temp starts climbing to lines below the red, and when I start moving it goes back down, and when I park off and check, the water is boiling in the reservoir to the point its coming out the lid(I didnt change the lid but it seems correct)...the fans dont seem to come on unless I switch the car off, switch it ack but on ready and not completely on and then the temps go down..

So not sure now if its the fans or something else?

[panic-mechanic]

back to my original question. Does the fans run and does the thermostat open.....

[ICE King]

back to my original question. Does the fans run and does the thermostat open.....

Panic hit the nail on the head initially, sounds like the thermostat is stuck closed hence the boiling, water is evaporating.

Faulting thermostats is common fault on these motors,

I suggest change the thermostat anyway, about 150 bucks for the most,

[watusay]

Okay so think Panic may be right, can I just double check my findings.

Took her for a drive now, a hard one, performed nicely and temp stays at 90, go home, idles for 2 mins and the temp starts going up, fans dont switch on. Switch car off. Pop bonnet, check reservoir, its boiling and water sounds like its been sucked in. Water level has dropped.
Check top rad hose, hot as fook. Check bottom rad hose, and believe it or not its just slightly slightly warm. Move hand further up to drain tap and its cold. And hose feels full. So Im guessing the water is not reaching this side as it should be as hot as the top rad hose no?


Switch car to ignition(not start) and only then do the fans kick in full blast and water temp starts going down to normal after 2 or 3 mins. water is now below min mark( its evapoting right as theres no leaks anywhere?)

So is it the thermostat?

P.S while i was down there I noticed the air temp sensor( the one that sits behind the fog grill came off its bracket and is loose) and is just hanging there. Will get it back in place tomorrow but would it cause any issues?

[Ashveer03]

A boiling upper hose and cold/warm lower hose is most likely a bad thermostat as everybody stated. I would replace it, u can test it if u like.
While doing that, run a hose through the radiator to eliminate any blockages within the radiator.

Outside air temp sensor - from what i know, is for the climate control, and it definitely wont be linked to your current problem in any way.

Sent from my SM-G531H using Tapatalk

[panic-mechanic]

Yes thermostat not opening. That will cause the fans not to run.
No outide temp sender is just a display on the dash. No affect on engine cooling.

[watusay]

Will it lose so much water though in just a few mins, it definitely disappearing faster than with the leak in the other bottle.

Also this morning, started her in the driveway and let her warm up, guess what, both hoses are now hot but the fans didnt come on at temp. But she stayed at 90 for 10 mins so not overheating this time around.


Freaking give up!! Sure this doesnt sound like a head gasket? Maybe have an air pocket ??

[watusay]

And now its doing something else. No if I top up the coolant...I literally just leave home and the coolant light flashes...check and its already below the min. But it stays there. Then when it idles it will creep up 2 notches above 90 and now the fans will kick in. Go back down to 90. Fans switch off. Creeps up again slowly. Fans kick in ...back down to 90 and so on.
Also if I on the AC the fans stay on and the temps stay at 90. Its like a bloody see saw. Both rad pipes are hot as well

[panic-mechanic]

Just change the damn thermostat.

[TDIFlava]

Sounds like the thermostat, see if you water houses are swollen up when engine is running my be that the head gasket has blow through to a water port had the same on a Renault with the coolant disappearing as you mentioned in your opening post. Could be that the radiator failed do to pressure in the cooling system I'm not a mac but just my 2c