Cooling System Gremlins

[S2Roo]

Hi

I am having trouble with my cooling system. When the car gets warm the coolant level drops in the resovoir considerably, causing the RED autocheck warning light to illuminate. However when I release the resovoir cap the level rises enough to clear the autocheck warning. Once it has cooled it returns to the normal level. The car is loosing a minmal about of water over roughly an 8-10 week period.

On a previous service (Not whilst I owned the vehicle) Audi noted a leaky temp sensor, however I cannot observe any leaks on the system. I am intendin to get the system pressure tested, but have not got round to it yet.

Any ideas

I thought maybe one of the following:

Header Tank (resovoir) Cap valve sticking?
Air trapped in system.
Faulty water pump.
Head Gasket.

Hopefully not the latter, the car does not consume oil and I haven't observed mayo in the oil filler area.

Help would be much appreciated

[SteveH]

Now that is a strange one

Due to expansion, the water level should rise when the car is hot. Have you checked your hoses when the car is at running temp? Perhaps one of the rubber hoses is swelling under pressure, reducing the level in the tank.
If you're not loosing much water and the oil isn't creamy on the filler cap, I doubt it's the head gasket.
When you fill the system from cold try turning the cabin heater to hot and then fill the system until coolant comes out of the bleed screw on the heater matrix pipes.( up near the bulkhead) This could be the cause of the problem if it is an air lock. Also, some radiators have a bleed screw mounted on the top surface.

[JariP]

I've heard that sticking reservoir cap valve is rather common fault in Audis, but rarely noticed one. It causes pressure to build up and then makes the coolant to leak out somewhere from the hose connections. And as this leaking only happens when the engine is warm, it usually makes the leaken cooland to evaporate and leave no traces of that leak. And then the poor Audi owner is left to wonder WTF is wrong

[Beetie]

Just because its not mixing oil and water doesnt mean its not head gasket. It could be blown between the piston and the coolant system so exhaust gases could be getting in.
When mine went you could smell fuel in the header tank, but if you run the engine and hold an exhaust gas meter, such as an mot tester, it could pick up hydro carbons when the bottle top was undone.

[S2Roo]

Thanks for the replys ppl.

I have bled the secondary rad using the bleed screw on the top of the rad as you mentioned, but I haven't tried the bleed screws on the heater matrix pipes, so I'll give them a go.

I'll then try stickin my nose in the header tank to see if I can taste any toxic gases . Hope I dont taste any, as changin a head gasket would inevitiabily lead to other modifcations and posibilly a mortgauge!!

Any ideas as to how the header tank cap can be tested??
I have heard of these being a problem, I was told there are two different types, one with two valves and one with one??

Roo

[horvathg]

Roo,

I had the same happening. Then: since the car is approaching 60.000 I thought it would make no harm to change the timing belt and tensioner bearing and everything what might come. And right there, above the engine case, where you don't see anything when the front grid is at place, there was a rubber tube, thin one, I don't know its function, but it was dripping slowly on the engine case which was green frome the scale. Since the case is always hot, I've never seen any water drops under the car. But this was where I was loosing coolant water since I don't know when. New tube, no more water loss.

HTH

[Crawford_S2]

I am also loosing coolant No visable drips but found a puddle of coolant this morning

Just replaced the expansion tank cap the other day,didn't think that would work, too cheap Going to put it on the ramp tomorrow and have a good look.

[Martynf]

water leaks... the bane of me life.... top o the list is ALWAYS:

1) The two small bore pipes 1 the stat housing, 2 feeding the turbo water cooling pipe. Both leak.
2) Top hose splitting at the water manifold end
3) Top outlet of the rad broken (plastic rads only)
4) Hose clips not quite tight enough so water leaks past the clips
5) Afterrun pump maybe cracking
6) Bleedscrew weeping at teh bulkhead

All have to ruled out as they are VERY common!
Martyn

[Cobstar]

Had this problem with my car. Coolant was pooling in the undertray and then leaving a puddle on the ground but wasn't obvious where it was coming from. Turned out to be a hole in the radiator.