Lkirkpatrick

Jack88

Well, it depends. If the car has bubbling in the expansion tank when it is up to temperature enough that the thermostat should be open but not overheating, generally that means that yes, a head gasket has failed. You would also be able to smell it if it was dumping in that much exhaust into the cooling system. It doesn't always result in white smoke from the exhaust or milky oil. Still, my first guess is a stuck thermostat. Old school diagnosis of the upper radiator hose being cool to the touch after the thermostat has supposed to have opened may be a good way to confirm. Unfortunately, replacing the thermostat is nearly all of the same labor as the timing belt job, making for an extra hour or so to include that with the repair. Given most of the cost of the timing belt job is the labor, I'd say have that done with it if it is even remotely close to that service. I always recommend replacing the thermostat with that service because of how cheap they are, how hard they are to access otherwise(it's an extra 5 minutes if you are already doing the timing belt,) and that they aren't an uncommon failure point.

Also, I'm not sure I would trust this shop. $400 thinking it was a bad coolant expansion tank seal? First off, that's a $60 part with no more than 1 labor hour to change, and secondly, I find it hard to believe they could misdiagnose that so badly and then be accurate at a head gasket failure diagnosis. So that makes them incompetent or they are trying to take advantage of you, either way, probably best to find a different shop. Diagnosis by throwing parts at it benefits the shop but can absolutely murder your wallet. Besides, a second opinion on a nearly $5000 repair is never a bad idea.

Lkirkpatrick

ltooz_a6_a8_q7

It's beneficial to understand the cooling system instead of listening to grease monkeys diagnoses and replace good parts.
- Too much air in the system will cause overheating. Bleed the system correctly.
- if head gasket is bad, most of the time, the coolant will be in the oil and white smoke would come out the exhaust.
- when in doubt, check the thermostat, if it stuck close, the radiator and hose will be colder than engine hose.
The worst thing for an A8 owner to be in is Not knowing the car and not doing the work him/herself and the bank book is not unlimited.
Cheers,
Louis

MP4.2+6.0

I come out the same as Jack (and others) basically, on all his various points. Could be head gasket, but failure rate on 4.2's statistically is insignificant. Rarely diagnosed and very rarely ever confirmed. Thermostat is always a much earlier place to suspect.

Also, having done the head job on a C5 A6 4.2 for unrelated reasons (internal engine damage leading to oil burning), my reaction to the quote is basically, that's ridiculous. Like Jack said, the thermostat unfortunately is about one step labor wise just below the timing belt job. The head removal is same as timing belt removal, plus removing intake, dropping front of exhaust on affected side, and pulling head. Parts specific to the head removal are not very meaningful cost wise (most of it is the timing belt replace stuff). So honestly another grand or max $1500 above a timing belt job is about as high as I could see it for the labor. Timing belt jobs, even very full ones with lots of rollers and tensioners replaced w/ OE parts, don't cost $4500-$1500 max = $3,000. Only with a complete head rebuild could I see approaching the number; which rarely would be done for basic blown head gasket unless it needs internal inspection for cracks or some such. As another metric, even in my home garage with ramps only, no lift, but a good number of hand tools, it's about two full days solo; lift would definitely help on the exhaust pull stuff plus the fluid draining. Timing belt and related pretty complete component swap out would be rounded to about long one. Maybe another hour the next day for either job for final fluids check/top up, no leaks, etc.

One other diagnostic for head gasket: if you have a water to cylinder leak, besides the steam out the exhaust (which is harder to see in warm weather), you typically find one spark plug literally steam cleaned. It is also reliable old school how you figure out which side the motor it is on even if you have a steam cloud on a V6 or V8 or whatever. Plug pull is not expensive, and a steam cleaned one is pretty darn determinative. Again, if you are anywhere near a tune up anyway, just replace them at same time. If none come up steam cleaned, feel more confident going down thermostat and then timing belt job path if some other pointers are also toward thermostat.

dvs_dave

jandreas

The D3 A8 cylinder head gasket (starting I think with MY04? - its documented in the Audi Mechanics training manuals) was optimized but not necessarily for the better, some of the spots are not as wide. If your problem is the cylinder head gasket (the now pressurized coolant tank gives it away, the spark plugs won't necessarily show anything...) than a $4500 quote is not that out of line, regardless of how much it sucks. You have to completely pull the motor. And then there is the potential cost to machine the cylinder heads. Its actually cheaper to just replace the engine with another from a junk yard (less than $1000USD nowadays)

MP4.2+6.0

Hmmm.

Plugs have worked for me diagnostically on Audi's for head gasket blows (never happened on a newer one) a bunch of times.
See picture. In car. 2000 C5 A6 4.2; tighter fit than D body generally, and also belt drive. Post 2006 cam chain drive would logically be motor pull, but not belt ones. I didn't even have to pull front clip to do that particular one, and done in home garage with only ramps. Again, head was off for internal motor issues, not the gasket or cooling.



Head machining: at most surface planing in normal course, and gasket blow should be one side only. $150 ish maybe, from doing the C5 head just in case. Easy to go down slippery slope toward a lot of engine rebuilding, but straight blown gasket does not typically require more, especially if other prudent stuff is done connected to timing belt job.

Back to altitude as post responses started. 4.2 not known for blown gaskets. Never say never, but... Instead, thermostat in particular should be eliminated as much as possible. That is a known many Audi motor area, 4.2 belt drive included. Plug pull may help spot head gasket, and importantly which side of the V in that case. Likewise and per Jack's early comments, during warm up when you have coolant gauge at mid point but not yet overheated or where fans kick on (have A/C compressor off to test), then if already bubbling, not a good sign for head gasket integrity. On the other hand, with relatively inexpensive heat guns, easy to take localized readings around radiator and contrast to motor, thermostat, hose, etc, areas to help support a bad thermostat or other cooling side diagnosis.