2007 Q7 4.2 TDI Loosing coolant and running cold

Hi, newbie so forgive me if this has already been done to death.

I have a 2007 Q7 4.2 TDI. Love the car however there is this one niggle regarding the coolant. Since buying it (Oct) it has been steadily loosing coolant, at first it was an expansion tank per week. I changed the expansion tank cap and it appeared fine for a month then the coolant light came on again. Sure enough the level was below min so I filled it up again (G13). I then took it on a run, normal operating temp driving through town with no issues, hit the motorway and temp dropped to 50. This sounds to me like a stuck open Tstat, however the garage had a look and said that it looks new and can't see any leaks to justify the coolant loss. There is no water in the oil, no smoke from the exhaust, it never overheats and no loss of power. No obvious leaks anywhere either. This morning I ran it with the cap off from cold, nothing at first until I applied some gas, then water bubbled and was sucked in, topped up and did the same until no bubbles and a full expansion tank. Audi can't see me until mid Jan so I was just wondering if you guys had any idea as to what it could be.

I had a coolant leak on a 2006 3.0TDI Q7 a few years back, turned out to be a pinhole leak in the main radiator. It was hard to find and couldn't be seen without dismantling half the front of the car. The garage put a pressure test on the coolant system to help find it. The pinhole was on the back of the rad facing the engine and was likely a manufacturing fault rather than damage from a stone chip.

Thanks, it can’t be much as it’s not obvious. First garage just said it was the head gasket (no pressure test) as they couldn’t find a leak. Surely there would be other symptoms if the HG had failed?

Head Gasket would likely cause overheating rather than running cold.
I’d get a pressure test done before touching the cylinder heads.

That’s what I thought. It’s never overheated, just a gradual coolant loss. The TSat would explain running cold but it looks new. Think I’ll just keep an eye on things until the *********** takes a look

Update. Overheated due to dumping it’s coolant on Christmas Eve after driving 250 miles. Been in Audi for 10 days. TStat and expansion cap replaced. They said they could not find a leak but the system was pressurising. No gases in coolant, no coolant in oil, no blockages, full flush completed. Got it back to find a visible drip behind drivers wheel (directly under expansion bottle). Any ideas?? Audi don’t have any.

That sounds suspiciously like a head gasket leaking under hard load.

They won’t confirm it’s the HG. They don’t think it is so just want to work their way through the cooling system changing parts and costing ££ as they go

They simply can’t confirm it is a head gasket, you have to be decide based on what you can see, gut instinct and not a small amount of courage. Unfortunately being a V8 doubles the work as you will never be able to decide which it is and as it has probably done a good few miles the chains will need doing so it is an engine out job. Add to this that you MUST not reface the cylinder heads, so you fit exchange heads at £4438.54 a pair (including VAT) so it turns into a £10k job before you even blink, with a trade book value of at most £10k.... well it’s hardly surprising cars like this end up being broken for their spares.