My 05 reg 1.9 TDI (4x4 version, about 136k on the clock, most of it over rough and windy Peak District roads) has been losing coolant for several months now. There's no obvious leak that I can see, and the car's been into the garage a couple of times without a fault being found. The warning light comes on roughly once a week, and I've been topping up with coolant to make sure it doesn't drop too low.

So now for the question: I check the coolant level in the morning, when the engine's cold, and the coolant is nowhere to be seen in the expansion reservoir (so well below the min mark!). I take the cap off to top it up, a whole load of pressure is released and the coolant comes bubbling back up to around about the min mark. Clearly I can top up at this point, up to the max mark, but that's not really much coolant compared to the amount needed to get to the min mark from an empty expansion tank.

I've searched through the forum and found numerous threads about faulty sensors (don't think it's this, given the level), slow leaks, water pump issues, porous cylinder heads and more. I don't fully understand all the different things so can anyone give me some guidance please? Is it normal for the coolant system to remain pressurised 12 hours after the car stops (and if so, how do I correctly monitor the coolant level)? And do I need to think about investigating further, or just accept that the car is getting on a bit, has gone a fair distance and so just top it up regularly?

Thanks for any help.

Like you say, could be due to lots of different things.

I can't comment about coolant pressure, but when the system is running fully pressurised (e.g hot engine idling), you should see the coolant in the bottle more or less in the same level when not pressurised, somewhere between the MAX and MIN marks.

I say sell it or fix it - you're taking a big gamble if you ignore it.

There could be something seriously wrong. Find a more competent garage to get it sorted or you might be risking cam belt failure and consequent serious damage to the engine.

My experience of my 1.9 TDi leaking without any sign of where the coolant was going (nearly a litre could disappear without a trace) was a leaking water pump. This resulted in a contaminated cam belt, which is not a good thing!

To find a leaking water pump the garage would have to have take the time and trouble to get the cam belt covers off and poke about inside and around the cam belt housing. Did they do this?

Has the oil any white residue? Or the cap? Sounds like head to me.

No white residue in the oil.

Selling the car isn't really an option (until it dies properly), it's really not worth much any more.

I'm due another service in a couple of weeks so I will specifically ask them to check the water pump and investigate the leak further. Anything else I should be prompting about, in relation to the leak? I have no (other) reason to question the competence of the garage and they have sorted a number of tricky issues for me in the past so I'm not keen to just move on without trying.

Coolant system becomes pressurised due to to the coolant expanding when it gets hot. When it cools down the pressure should drop again. If a coolant system is becoming more pressurised than heating can provide then I'd be thinking head gasket and pressure making its way into the coolant system from the cylinder compression. However this does usually show in other ways as well such as steam out the exhaust, brown sludge under the oil or coolant filler caps and when running with the coolant filler cap removed air bubbles coming up through the coolant. Can't think what else will add pressure to the coolant system?

Definitely no brown sludge under the filler caps, and I checked this morning - no bubbles coming up through the coolant with the engine running. I'm not sure about steam out of the exhaust, pretty difficult to spot when it's chucking it down! Definitely a lot of pressure in the coolant system though.

If you also have a leak in the head gasket and the cylinders are compresssing the coolant, you may when you remove the expansion bottle cap be able to smell the products of combustion.

Ian

Ah, didn't check that. But then my nose is so bunged up today I probably couldn't smell anything anyway. Thanks for the suggestion, I'll try it when I can breathe properly again!