Lets play a game: Thermostat vs Head gasket

[rollerton]

Okay. So I has the 4.2L. This is a new problem as of today. Engine runs awesome, tons of maintenance, nothing out of place or broken.

Start car, cold. Awesome, runs awesome. Tons of grunt, nothing suspicious. Warms up, coolant hits 1/2 hot...awesome. Still no issue. Except no heat.
Heater blows COLD. A/C works and blows COLD.
A Few more minutes if you drive it hard the gauge starts to climb... coolant and oil temps both continue to climb. I have found the coolant tank cap has failed, but I feel like this is not the problem.
Still no heat from vents on HIGH heat, lower heater core hose is HOT, upper is not as hot, but still hot. Now the reservoir is gurgling. Like the kind of bubbles that you can feel going 'blurb blurb' through the engine .
No errors, no misfires. Haven't done compression/ leak down yet, but suppose I need to.
TONS and TONS of air coming through the coolant tank as the temp climbs...both rad fans run high speed. Bleed heater core and it's only air...and more air and more air. Like a continual BLAST of air out of the bleeder.
It will reach 3/4 hot and that's too hot, but doesn't get hotter. Shut it down and the coolant (mostly water now) continues to bubble and boil.
It's hot.
Well fuck. Other than some pinging from the engine which seems fairly more than extreme lately there have been no symptoms.
Seriously...votes?
Don't say head gasket....but there is a shit ton of air in the reservoir when it warms up, and the reservoir is brand new. FWIW there is zero evidence of oil in coolant or vice-versa. Oil is clean, coolant is clean.

[JBX]

Whitish smoke after start-up? What does the air coming through the coolant tank smell like? Like Gas/exhaust? Maybe a coolant passage is blocked and you are getting localized boiling? If it is a head gasket leak, you are going to have one or two REALLY clean spark plugs and the related cylinders are going to be very clean too. You could inspect those with a mirror/flashlight. Maybe you could get lucky with just torquing down the heads?

Suck man. Best of luck. Right now, you are running mostly water and a faulty coolant cap=low boiling point. Maybe a new cap and proper coolant will resolve everything?

--jb

[Mad Cow]

You can buy a kit that tests coolant for exhaust gases, it does sound like a head gasket but I've also seen a very similar case be a bad water pump.

[PaperToast]

dont know about the 4.2, but i did find out from the dealer (i think) that the head gasket on the 2.7 doesn't fail except in overheated cases.
my vote is try a new coolant cap, cheaper than a head gasket job.

does the coolant level drop at all?

[csosnowski]

I have a suspicion that the system has air trapped at the thermostat and with the bad cap the system can't pressurize.

It also sounds like the thermostat is faulty as well. Since the heater core is behind and above the main cooling system, the core relies on the cooling system being pressurized as well as the water pump working properly to get the coolant circulated through it.

I had a similar problem on an a8l when the coolant bottle was cracked, however in that case there was coolant loss from the bottle. Same symptoms though until the cracks opened up.

Good luck!

[rollerton]

Yeah..I'm a little puzzled. It had not overheated, temp doesn't go past 3/4 and it just started yesterday at lunch.
There's no smoke not when cold or hot, exhaust smells clean not like coolant. I have noticed it leaving a little coolant drop or two after it's parked, I think because the cap was failing.
I've just never seen the heater core lose heat so quickly; when at first it started to run hot the heat was working and as the temp climbed the heat pretty quickly turned to COLD air, and as I slide the hose back it's just a continuous stream of hot air and then the gurgling in the reservoir. HUGE bubbles and oddly the coolant level doesn't drop much as the air is gurgling out of the reservoir.
It's NOT acting like it would after losing coolant: As in after doing a water pump, I typically fill the system and run it, coolant drops, fill some more until it stabilizes, close it up and run it, bleed the heater core if needed.
Instead the temp climbs, evens out at halfway, coolant level does NOT drop then it starts to overheat...and just air.

[Osmosis]

Tons of air coming from the coolant tank makes me think, sorry to say, head gasket. When I purchased my A6, the previous owner fixed the thermostat as it was bad and had very similar symptoms as you are having...minus the bubbles from the coolant tank. I suppose your water pump could be going out and some how cavitating and maybe pulling some air in the weep hole, if Audi's have them on the water pump. I think you can take it to a Midas or another local muffler/radiator shop and they will put a tester on the coolant cap/radiator cap and they usually do this for free hoping for your business. I would take it to a shop and see if they can test it for you. In this video, about 1/2 way through is the tester I"m talking about. https://www.youtube.com/watch?v=U0DdVQoBu4U

[Groucho]

I just got done with a similar problem with my 2.7T, not sure if it relates or not. Knew my after run coolant pump was needing replacement. Have been putting it off for a while cause it only caused me grief every month or so when I might have to add some coolant to the tank, plus this car still has the SAI and therefore I would almost definitely need to remove the intake to do the job.

Then, about 3 weeks ago, my wife called and said the car was puking coolant on the ground and was bubbling in the tank real bad. Hadn't overheated (to her knowledge), but we had to use two 1.5l coolant bottles (diluted 50/50) to fill it back up. Drove home fine--no leaks under car. A day later, same thing happened. This time, when I went to remove the coolant cap it almost split in half as the threaded portion was thrashed. I hadn't noticed the day before. Ordered a new cap and tank from genuineaudiparts.com, and after it arrived I installed it and waited to see. No more bubbling, but now the after run coolant pump was definitely leaking much worse than before. Ok, I give in. Time for the after run pump replacement.

Got the after run pump replaced and thought that would be it. Nope. The plastic "T" fitting just off of the front of the coolant tank busted and started puking all of the coolant out. Sonofa. Got a replacement, installed it and it's been over a week that my coolant level is exactly the same each day with no drips from underneath--car runs great.

I know these things happen, but I sensed that all of this sprung up from that damn coolant reservoir cap. For $13 plus shipping, it might be worth a shot. I'd h8 to have to worry about a head gasket at all.

[brokenwrench]

Most shops can test for exhaust gases in the coolant. If your coolant system stays pressurized after shutting the engine off then you don't have a leak to atmosphere (i.e. head gasket). If your coolant system loses pressure, maybe try pulling the spark plugs and check the cylinders for coolant. Of course, if it is a head gasket and you have coolant leaking in, you run the risk of hydrolocking the cylinders next startup.

[rollerton]

Ok. Well..put a good coolant reservoir cap on it and warmed it up, the heater core bled pretty normally and the heat works perfect. It will not overheat at idle, runs perfectly normal. But begins to overheat after about 1/2 mile of easy driving.
Now, it seems perfectly normal, no bubbles or anything, until you drive it.
And the coolant tank DOES stay pressurized after it sits...I hadn't thought of that, hopefully that helps clear the gasket suspicion.
I'm going to guess it's a partially failed thermostat. Feeling the upper radiator hose during warm up it stays fairly cool for what seems like a long time. I fact it feels like the coolant is VERY hot on that hose near the engine, but the hose is kind of cool near the radiator. So I'm thinking simple heat-sinking is heating that hose but coolant is not coming out of the engine much to flow through the radiator. Maybe a little.
Doesn't overheat at idle because the thermostat is letting just enough coolant through to stay @ temp, but when RPM's increase the t-stat is impeding the flow through the radiator. I think the broken cap was compounding the bad T-stat problem as it was over heating it was letting air in, tons of bubbling etc...I hope that makes sense.
Well. That's the hopeful solution at this point. I may pull plugs and check around before I pull the front end off I guess.

[Osmosis]

Take a drive to the radiator shop and see if they will test it for you. That test will most likely be free and give you piece of mind if it is a head gasket or not.

Good luck!

[glennda5id]

You can put a coolant pressurizer on it and check pressure when running. Rev it at idle. Pressure should not get above 6 -8 psi. If the coolant is not above 260 (boiling point) and it's bubbling you most likly have a HG failure.

Modern engines with MLS headgasket developer micro leaks that only leak under high pressure. Exhaust gas get pushed through the small leak during combustion and exhaust gas goes into the coolant. This over pressurizes the cooling system and heats up the coolant. You will not have coolant leaking into the cylinder so no white smoke. And it is more likly to occur under load while driving and not at idle because of higher combustion pressures underload. I would not be surprised if this has been going on for a while and damaged your bad surge tank cap.

Best test is gas analyzer in the coolant tank at shop or block leak detector kit available at auto parts store.

You could also have a crack in the cylinder head.

[rollerton]

So I got the Thermostat out. Looks perfect.


But.....? I've had it in a pot on the stove boiling for 15 minutes, doesn't seem to open at all. I know, not a perfect test- but that will usually open a thermostat from my experience.

[02 A6 2.7T]

So I got the Thermostat out. Looks perfect.


But.....? I've had it in a pot on the stove boiling for 15 minutes, doesn't seem to open at all. I know, not a perfect test- but that will usually open a thermostat from my experience.

Thermostat was failing. When I bought my S6 it was great for 2 weeks, and then I noticed the gauge climbing past half. Replaced the thermostat and timing belt components (58K miles on the clock) and it cured it...

[csosnowski]

Ok. Well..put a good coolant reservoir cap on it and warmed it up, the heater core bled pretty normally and the heat works perfect. It will not overheat at idle, runs perfectly normal. But begins to overheat after about 1/2 mile of easy driving.
Now, it seems perfectly normal, no bubbles or anything, until you drive it.
And the coolant tank DOES stay pressurized after it sits...I hadn't thought of that, hopefully that helps clear the gasket suspicion.
I'm going to guess it's a partially failed thermostat. Feeling the upper radiator hose during warm up it stays fairly cool for what seems like a long time. I fact it feels like the coolant is VERY hot on that hose near the engine, but the hose is kind of cool near the radiator. So I'm thinking simple heat-sinking is heating that hose but coolant is not coming out of the engine much to flow through the radiator. Maybe a little.
Doesn't overheat at idle because the thermostat is letting just enough coolant through to stay @ temp, but when RPM's increase the t-stat is impeding the flow through the radiator. I think the broken cap was compounding the bad T-stat problem as it was over heating it was letting air in, tons of bubbling etc...I hope that makes sense.
Well. That's the hopeful solution at this point. I may pull plugs and check around before I pull the front end off I guess.

That's a great picture!

So I got the Thermostat out. Looks perfect.


But.....? I've had it in a pot on the stove boiling for 15 minutes, doesn't seem to open at all. I know, not a perfect test- but that will usually open a thermostat from my experience.

If you boil the tstat it will open, no mistake. I test them at my shop that way.

[rollerton]

That's a great picture!
If you boil the tstat it will open, no mistake. I test them at my shop that way.

Yeah. Got a new one, boiled it and it opened before the water even boiled. Old one NEVER opens at all.
I'll take the $15 fix even if it takes 6 hours of labor.
Hopefully I get it back together without breaking anything.

[glennda5id]

That's a good sign. It should open at 190, well before boiling. I have done it before and they open. Hopefully bubbles were due to the coolant boiling and not exhaust gas.

[Sparkstack]

How many wagons do you have?

[idudditzR8]

Guessing this was a tstat problem? I've got the exact same scenario. Local shop seems to think it's head gasket but i'm going to do the tstat change myself and see if that fixes it.

[02 A6 2.7T]

Guessing this was a tstat problem? I've got the exact same scenario. Local shop seems to think it's head gasket but i'm going to do the tstat change myself and see if that fixes it.

Unless the timing belt was done in the last year, replace it along with the water pump and all idler pulleys at the same time ... you're already there when you get down to the thermostat ...

[rollerton]

Guessing this was a tstat problem? I've got the exact same scenario. Local shop seems to think it's head gasket but i'm going to do the tstat change myself and see if that fixes it.

Yes sir. Bad thermostat.

And I have another bad thermostat to do also. Luckily this one is failed open and I have a heated garage. Meh.

[julex]

Mine is also failing but in open position. I don't feel like taking apart the front for this (TB job is still a couple of years out) so last winter I simply used sheets of plastic to block air flow between condenser and radiator. Worked well. In summer it is fine.

[idudditzR8]

Yes sir. Bad thermostat.

And I have another bad thermostat to do also. Luckily this one is failed open and I have a heated garage. Meh.

Same car with a bad tstat after replacing? I'm getting a lot of bad news from mechanics telling me it's looking like a head gasket. I haven't had it in at the specialized indy shop yet, hopefully soon.

[rollerton]

Same car with a bad tstat after replacing? I'm getting a lot of bad news from mechanics telling me it's looking like a head gasket. I haven't had it in at the specialized indy shop yet, hopefully soon.

No, it's a DIFFERENT car. I am not sure if that's a good thing or not. You'd think the thermostats would be a little more reliable..good thing it's ONLY a 4 hour job to replace a $10 part. I think a head gasket is way less likely than just about any other problem. I've heard of a handful of head gasket failures on these cars, but typically you can trace it back to some cause.
One head gasket test, doesn't give you a super definite conclusion, but...get engine up to good operating temp and shut it off. Wait 15-20 minutes and crack the reservoir cap. See if it releases pressure. The theory is that the coolant system will bleed pressure off through the head gasket leak and the system won't be pressurized as it normally would. Again, not a perfect diagnosis- but it's free and easy.

[julex]

I found a fast and cheap solution to the multi-hour work of replacing stuck OPEN thermostat. It is called in-line thermostat housing. There are two options:

1) custom engine built "Racing" housing like Meziere WN00071 ~$80 when said and done with thermostat from GM motors: https://www.amazon.com/gp/product/B0...=ATVPDKIKX0DER
2) BMW E21 320i inline thermostat which is cheaper but you have to replace the whole unit if/when it needs replacement and doesn't look like racing part plus you need to blank out by-pass port too: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

There is a suitable straight section of upper radiator hose to place these inline. Right after the bend downwards where expansion tank merges with hose.

[idudditzR8]

I got it in to the Indy shop and they diagnosed the tstat failure. Should get it back today, very happy!! I was also happy when they told me these 3.0t's are rock solid and I should have no fear about the usual Audi horror stories I was getting from other mechs. They suggested the SC pulley and program mod for 75hp, guessing that might get me up to the 400hp mark. We'll see about all that later though. Thanks for all the info AZers, very much appreciated!