Coolant Leak from Thermostat Housing

[OllieVRS]

Hey guys,

Noticed my coolants been slowly dropping over the past few weeks, and I've been having to top it up every week or so. There would be a coolant drip if parked facing downhill, but not if facing uphill  .

I did some investigating and found coolant around one of the upper coolant pipes on the front of the block, connecting to the thermostat housing. I used my cheap endoscope to take a picture:



Is it safe to assume to is the source of the leak? Here's a diagram of the cooling system, and where I think (based on me digging with the endoscope) it's coming from. Is it worth replacing just that seal or spending €65+ replacing the whole thermostat? From my research part 5 on the diagram below is N90365302 and the whole thermostat is 06F 121 111 F.



Cheers 

[LC5F]

Hi fella - it may be a cheap endoscope, but it provides very good images

The work involved to get at the leaking pipe you are not far off being able to replace the thermostat - so yes plan to replace that as well.
It seems odd for the seal to let go, all those parts are held firmly together with no real movement, it could even be a crack in the thermostat body.

Plus also get a new coolant sensor, you can't replace this with the thermostat in the car - trust me you don't want to be going back in there any time soon.
Plus get new O ring seals and just in case a new clip for the sensor.

It's pretty awkward to get to the thermostat, it would help if you have strangely small hands like trump.
Pelican parts have a good write up: https://www.pelicanparts.com/techarticles/Volkswagen_Golf_GTI_Mk_V/31-WATER-Replacing_Your_Thermostat/31-WATER-Replacing_Your_Thermostat.htm

90% of the work is on top of the car - belt off, tensioner off, alternator off, charge pipe from IC to throttle body off.
The hardest parts I found is the lack of space to get at the small nut on the back of the alternator and spline/triple square bolts in image 18 & 19, I messed up, forgot it was spline, using a torqx rounded them out... and then one of them sheared.

Sounds like it is currently manageable, if you can keep on top of the coolant top ups, you could hold off till the weather picks up.

[pudding]

Probably not a bad idea to budget for the whole housing in case it's cracked.  It could just be the O-ring got pinched or has just worn out/gone hard or something, but as above, with the faff involved getting at the darn thing, I would just replace the whole unit.

[OllieVRS]

Bought a genuine thermostat and seal from VW for €67. Inc VAT.

Just after spending 4 hours getting the alternator out, and then another 4 hours separating old seemingly moulded-together coolant pipes, I've arrived at the final boss.

The bottom hose on the thermostat won't budge, the one that should be the easiest to pull off since it's only held in with a metal clip.

Here's where I am so far:




Here's the hoses that won't separate, right below the red arrow:




Any tips for getting it out?

[LC5F]

You are referring to the part with the grey coolant sensor in it?
I seem to recall they don't really separate- better to just take it out together.
If your looking at the Pelican Parts link, last photo its attached shows them all together - undo the pipework below and it should come out.

[Octoparrot]

I think I kept mine connected to the thermostat, disconnected it at the other end and then did it onthe bench where I could get a good grip on it.

[OllieVRS]

Yeah @LC5F that's the one.

I think I kept mine connected to the thermostat, disconnected it at the other end and then did it onthe bench where I could get a good grip on it.

Good idea, I'd have to remove the intercooler pipe and then separate that coolant pipe at the radiator. Hopefully that's less of a nightmare than imagine it being.

I've found a video (around 9:30 into the video):

Where the bloke simply hits it with a chisel pry bar and a hammer!? Idk if doing that would end well for the plastic pipe

[OllieVRS]

I followed @Octoparrot 's suggestion, and it worked a treat! 40 min later, every thing is now separated.





The previous person who did this job used some sort of gasket sealant while putting the thermostat in.

Is this necessary? None of the guides said it was, I thought the o-ring seal is good enough of a seal in of itself.

Picture of the old thermostat with a bit of sealant still on it:

[Octoparrot]

I never put sealant on mine and it's never leaked, just thoroughly clean the mating face on the block  and around the lip of the hole with some sand paper and it should be ok.

[LC5F]

Glad you are sorted.

[OllieVRS]

Apologies for not having have replied earlier, I was making sure it wasn't leaking anymore over the past few days as I waited for the coolant level to balance out.

Long story short it's finally fixed. Over the past few days of my driving it, I've been monitoring the level and topping it up, less and less every time until today, where the level hasn't changed  . Changing the thermostat took 11 hours total (mostly due to stuck together coolant hoses).

But I did get a heart attack after I refilled the coolant and ran the car for the first time. I had no idea you had to run the fans to fully circulate the coolant.

I idled the car till it got to 90 degrees, with the heater off. I was wondering why I didn't have to put in as much coolant as I had drained out.

I proceeded to then drive it out and around the block, with the fans now on, when suddenly I got a red flashing and beeping light on the dash  . I almost had tears in my ears thinking it was low oil pressure and that it was time spend €€€€ fixing the engine again until I realised the expansion tank was completely empty.

Out of panic I'd turned the engine off without even looking at the symbol on the dash (before realising it was the low coolant symbol). It took a litre or two more coolant before it finally stopped dropping.

End of emotional story, here's to hoping I can drive for a while without any problems