1.9TDi (AFN) coolant loss - poss head gasket failure, opinions required

[Mockenrue]

Hi all

My A4 is losing coolant and I'm not sure why. I've searched this and the other forums exhaustively and read plenty of similar stories, but none are quite the same and it seems that people seldom update their threads once they've got sorted. So, I'm asking for help!

Car is a '98 1.9TDi quattro Sport saloon with 185k miles, AFN engine. I'm doing between 350 and 450 miles a week and I'm having to top up the coolant once a fortnight or so; it's usually one or two centimetres below the 'min' level so the loss isn't great but it's consistent. It appears to be venting from the bottom of the expansion tank as there are telltale splash marks below. When the engine has cooled there is still pressure in the tank and when I release it the coolant level rises slightly. The car is running perfectly normally, there's no white smoke on startup and there are no signs of contamination, i.e. no sludge in the tank and no mayo on the filler cap or dip stick. The radiator itself was replaced last August.

The independent I use took a look at the car and this is what they found (taken from the invoice):

CHECK FOR COOLANT LEAK, NO EXTERNAL LEAKS FOUND BUT COOLANT BLOWN OUT OF HEADER TANK AFTER ROADTEST. BOTTOM RADIATOR HOSE COLD, SUSPECT THERMOSTAT. REPLACE THERMOSTAT & RUN UP TO TEMP, OK. ROADTEST AGAIN COOLING SYSTEM STILL PRESSURISING UP UNDER LOAD. SUSPECT HEADGASKET/INTERNAL LEAK.

As well as the thermostat they replaced the tank cap which made no difference either. I'm not convinced they really know what the problem is so I thought I should run these symptoms past the experts on here before I shell out on a head gasket replacement.

Just to confuse matters further, shortly after I started losing coolant I started finding it difficult to demist my windows. After researching this I discovered that a leaking heater matrix is the likely culprit, which could itself be brought on by pressure and/or air locks in the cooling system. My carpets are bone dry and there's no sign of there ever having been any water in the plenum chamber so I don't think the drain holes are blocked.

I'm not particularly mechanically minded when it comes to the business end of things but I'm willing to learn, so any opinions and/or suggestions from the experts on here would be most welcome. I assume I need to do a compression test next?

Thanks in advance.

[brollah]

How does the car run, temperature wise? Does it sit stablely at 90C once it's warmed up? If so, the thermostat hadn't gone, although if the garage had changed it, it's possible they've actually created an air leak rather than cured one.

It has been known for a heater matrix to leak without flooding your car, so it certainly is something to explore, especially as you're finding excess moisture in your car.

Certainly doesn't sound like a blown head gasket, although there could be a slim chance of it being very early days of it's failure.

[kenney]

Coolant leaking from the header tank sorry to say is usually a sign that the head gasket is leaking.Take the vehicle for a hard run check the pressure on the upper and lower radiator hoses,if they are rock hard,the gasket is leaking.

[Mockenrue]

Thank you both.

brollah - the car runs up to temperature perfectly normally just like it always has, and it's never overheated. Once it's warmed up it remains at 90C constantly. Likewise the heaters work OK, no problems there.

kenney - head gasket was my initial thought but I'm getting conflicting information, not helped by the fact there's no white smoke or water from the exhaust and no coolant in the oil.

I know the water pumps leak by design when they're worn and seeing as the cambelt is due very soon I'll change it then. Someone else has suggested getting a compression test done on the cylinders, as low readings will point to a blown gasket. I'll do as you suggest re the hoses.

[kenney]

A TDI with a minor head gasket leak will not overheat unless driven very hard.A compression test will tell you nothing,unless the head gasket is really blown,and then you would notice that on the engines performance.

[fourringsrus]

Sorry to mention the "B" word but I had a BMW straight 6 showing these exact symptoms, the head gasket had gone and the head itself was cracked but at no time did I encounter poor starting or running issues,she just blew the expansion tank about a mile from home after a 2/3 mile run.
I know we are talking Audi here but thought it might be worth throwing in this little bit of experience

[brollah]

Consider me corrected then, lol. Sounds like a diesel trait alright, I've seen about 3 head gaskets go on subaru boxer's, they react completely differently to a blown gasket

[shark_90]

Fancy seeing you here!

[Dochkaian]

Hi,

I have exactly the same problem in my A4. Losing cooling water by the expansion tank (after being run) and limited heating from the H/X.

But thanks . . . . it looks like the head gasket will need to be replaced.

Out of interest was your fault confirmed as the gasket.

[mikeybutch]

Found a small crack in the tank of an avant bought 2 weeks ago .Very little sign of the leak but luckily it was in the front but I would not have thought to remove the tank and check all round because i have never heard of this and assume it was faulty from manufacture 04 and gradually revealed itself now .A new tank is only £12 so I thought it was worth passing this in the hope that it gets you out of jail!