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6N1 Coolant leak and a stained spark plug

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So i've been losing coolant for the past couple of weeks - have to refill every week or so.

Two years ago I've replaced the entire thermostat housing which was rotten and the plastic was breaking off at the ports. However I can't find a leak now, and also can't correlate the leak with heater usage. So naturally I'm fearing an internal leak. I've taken the plugs our (unrelated) and wandered if the No. 2 plug (second from the right) is any indication of antifreeze in the engine block. What do you think?



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The threads of that plug are contaminated with engine oil, this is due to the plugs being recessed into the cylinder head where a small amount of oil quite often collects.

If this oil is not soaked up in a cloth before removing the plug(s), the result will be as in your example, it is also possible that some oil will drain down into the cylinder(s).


Are you sure that the carpet isn't wet near to the heater?


Is there any signs of water vapour in the exhaust gas WHEN THE ENGINE IS GOO AND HOT? If so, that could indicate a internal leak (Ignore any water vapour before the engine is really hot, this would likely be caused by condensation.


Other indications could be traces of water 'bubbles' in the oil on the dipstick. A build up of gunge (generally referred to as 'mayo' inside the valve cover (under the oil filler cap) may indicate an internal leak, but again this could also be caused by condensation, particularly if the car is used mainly for short journeys - You may not even have that problem in your part of the world.


If you are quite sure that there are no obvious leaks, it might be a good idea to clean the valve in the expansion tank filler (or simply fit a new cap)


If you still have problems, I would suggest that you ask a local trusted garage to carry out a 'sniffer test' - This involves sampling the air in the expansion tank for traces of hydrocarbons.

A compression test might also indicate a problem.

The presence of hydrocarbons and/or one or more cylinders with low compression pressure may indicate a failed head gasket.



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  • 2 weeks later...

Wow! Thank you for the detailed answer. Top Notch.

And sorry for the late reply.


In short:

Yes no carpet is wet and no smell of the antifreeze anywhere. I went ahead and paid a visit to a local garage (not a VW specialist though). They say the oil is clean, the exhaust is fine with no traces of vapor. They checked compression (both cylinders and coolant pipes, as far as I understand) and all tested ok. I don't believe they had conducted a sniffer test however.

Their final ruling was... the expansion tank and its cap. I find it odd since it feels tight and no traces of the green antifreeze anywhere around there. However I did order a new tank and cap and also noticed on the exploded views in the online diagrams that there might be an o-ring that I'm missing. Maybe you can help me decide on that last thing by looking at the attached picture.



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The sealing "O" ring is in the centre of the cap and looking at your picture




it appears to be there, but the caps are well know as a problem area.

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Alright then. Let's hope the other tank and cap will sort it out. Many thanks


By the way since I bought the other one used - is there anyway to check for sealing? Also how can I clean the inside of it in case it doesn't arrive in mint condition? I would like to see the fluid from the outside for a change



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You can buy a proper test kit to test the caps and it also will do a pressure test of the cooling system, they are available on eBay (UK) for around £40 but if you have a tyre pump you can test the cap for sealing and check that it will vent pressure, just make up some rubber pipe to connect to the bottom of the expansion tank and connect that too the tyre pump, pressurise it too 1bar (14.5 psi) and see if it holds the pressure if that's OK increase the pressure and at 1.5 bar (21.75psi) the cap should then vent the pressure.


I don't know what spare parts prices are like in Israel ? but in the UK a brand new coolant expansion tank is only £9 so cleaning one out is not something I would do, but a handful of dry rice and give it a vigorous shake that should break up any big bits of muck, empty that out then ordinary clothes washing powder and warm water should shift anything else.

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The part costs 30-80 GBP here, depending on condition. I ordered one on ebay.co.uk actually for 15 GBP and it indeed arrived in poor condition. But the cap looks good at least.


 I'll try that cleaning technique. Are you sure using dry rice is useful? From what I can see the tank consists of a separate inner compartment connected by tiny holes - I think only liquid can pass through those. 


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  • 2 weeks later...

Good luck. 🙂

If it's not too late, try running a fingertip down the inside of the old tank's neck, around the area that the o-ring makes its radial seal.

I happened to try this recently and felt quite a significant 'groove' where the o-ring had pushed into the plastic over the years.  This was on a 15 year-old 175k mile car.  It seemed to me that if this groove kept getting deeper, it must eventually cause a leak, especially if the o-ring is old and stiff.

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