Hi all,

My '97 A6 wagon is doing some weird things. After a long trip with it, coolant spews out from the expansion tank cap. How is that much pressure building up in my coolant system?? Could that be from a bad head gasket?? Also, on the last trip, the engine oil temp spiked to max and there was a nasty sounding metallic vibration or ringing coming from underneath the center of the car somewhere.

I recently replaced the heater core and thermostat.

Any help or input is greatly appreciated. Thank you in advance.

 

How recently have you changed those couple of things?
Maybe when reassembling everything you didn't "burp" properly the system. As you refilled with coolant, did you open up first the heater bleeder? As the coolant starts flowing there you close it up and then you have to loosen up the main rear pipe bleeder until you have a steady flow.
Either you had air pockets throughout the system that didn't allow proper coolant flow in the engine block (really hot oil you said?), or maybe your water pump ("metallic vibration coming from underneath the center") is sending you a message.
When was the last time when you changed the water pump? (or better said, did you ever change it?)

 

I'm like the third or fourth owner of the car, so the water pump could be a possibility. As for the heater core and thermostat, I replaced them within the last few weeks.

 

Where is the main rear pipe bleeder located?

 

You should be able to see it on the pipe, right next to the 2-prong coolant sensor. There's a "jungle of wires and wire connectors there (the coolant pipe has a support bracket on it there for a couple of connectors), but you should be able to see it.
It is like a 6-8 Allen head little plug-bolt. (EGR Valve area - so rear of the driver side bank)

You can see it here:

or here (top right in the pic):


Please disregard the "text" on the pics...LOL

Well I'm editing to share a few personal thoughts:
After I finished the work I did on the motor a couple of months back (including TB,WP, T-stat, head gaskets, etc), and refilling with coolant, I didn't bleed it properly. I only bleed it at the heater hoses, behind the firewall. I couldn't really get to the main pipe bleeder , because I didn't have the proper reach (Allen wrench or socket bit). Well, as it was getting hot, the fluid was bubbling in the expansion tank and was reaching the top of the tank; the engine side was HOT as hell, while all the front hoses were COLD. I knew that there's no proper circulation.
What I did was to empty again the system, start all over again and remove/eliminate that support braket on the main rear pipe and just re-route or leave those 2 connectors it holds suspended. That gave me full acces to the bleeder, with a socket bit and extension. Emptying again the system was a good occasion to flush it again with water, a couple more times thoroughly. I even flushed the heater core (pouring water through the feeding hose, and have the return loose into a bucket).
After bleeding in the proper sequence (at the heater core first, then at the rear main pipe), I got lots of air coming out at the main pipe bleeder. It was like the geysers at Yellowstone...LOL. After I got a steady flow of coolant, I tightened it up and shortly after the T-stat opened up and the coolant started flowing throughout the whole system as it should, and the fans kicked in.

If I were you, I would try to rebleed the system first. If that fails and you still overheat, then probably you should look into the water pump, if you get that metallic or strange noise.

good luck
(sorry for the long intermission/thought sharing stuff)

 

If your 1997 A6 wagon is quattro (AWD), then you have the rear and center differentials and the propeller shaft at the middle of the car. The center differential fill hole is above the front propeller shaft CV joint heat shield, which is above the catalytic converters. The middle of the propeller shaft also has a bearing, which may wear out.
If the exhaust pipe hangers fail, you could also get some loud creaking noises. You could also have bad motor or transmission mounts.
Have you ever changed the ATF or checked the front differential? These older automatics with four nuts holding the pan have to have a filter and ATF change every 45,000 miles. However, they usually only need about 3 to 4 liters of Audi ATF, since not all of the old ATF may be drained.

 

Are all these related to overheating and spewing coolant?

Edit: Oh, nevermind. You must be relating to the metallic noise

 

Since the coolant spews out of the expansion tank, you should check that both radiator fans turn on when hot. The fan motors are wired in parallel, so if only one fan motor is running, then the other is a dead motor or there is a wiring problem. If both motors do not run, then you may have a bad 50 amp fuse, a bad two speed fan switch on the bottom of the radiator, bad low and high speed fan relays, bad fan resistor or bad wiring. The 50 amp fuse is not listed in the owners manual and is hidden behind the drivers footwell panel.
If both radiator fan motors turn on, then you could have a bad head gasket, which leaks exhaust gas in to the cooling system.
The front of the radiator may get lint like material blocking the cooling fins in the area behind the air conditioner condenser coil.
When you changed the thermostat, did you remove the timming belt from the camshaft or crankshaft gears? The belt may have slipped a few teeth.
Did you have the overflowing coolant before the thermostat and heater core were replaced?

 

I did remove the timing belt from the camshaft pulley to install the thermostat, but I marked both pulleys and the belt with white-out so I wouldn't screw up the timing.

The coolant did not overflow out of the expansion tank before the heater core and thermostat were replaced.


So get this....My dad decided to take it to the store the other day despite the aforementioned problems. lol.....And apparently, he had no trouble whatsoever. No metallic sound, no coolant overflow, no nothing.....Which leads me to believe it IS an issue with coolant flow, since he only took it about 4 or 5 miles. Although, does that still explain no metallic sound??? Could that be the water pump making those sounds with no water getting to it? What is the path the coolant takes en route to the water pump?....For that matter, what is the complete path of the coolant in this 2.8? It is a quattro by the way. Thanks for the pics chefro.

 

I bled the system just as you described, chefro, and there seems to be good flow now throughout the system. (i.e. all coolant hose are getting very hot) However, the coolant is still pushing through the expansion tank's pressure hole (and cap I think) after a long trip. The metallic sound was no longer present when the system had coolant in it, so I'm guessing that was the water pump making the sound. There was also some steam/smoke rising from the passenger side of the engine after the test drive I just got back from, so I'm thinking my head gaskets are fried. (The engine oil seemed a little thin too) Any thoughts?