Hi there fellow Audi fixers...

I've just bought a 1998 A4, 2.6 litre V6 front wheel drive Audi which has overheating issues, (hence the affordable price).

The radiator tank cap has a broken pressure-sealing part, so it doesn't hold any pressure. However, there seems to be another problem, possibly water pump or closed thermostat.

After filling it with water and getting the engine going (cap off) I can't see any signs of water movement in the tank, and after 10 sec or so the coolant begins to rise and slowly but steadily overflow.

I took the car for a very short and gentle test drive, about 1.5 km,watching the temp gauge (sitting on 90 deg Celcius) and the water was boiling over when I got back.

How can I confirm the problem is the water pump without removing the front of the car?

Thanks for any help.

 

I'm lucky enough to have not had any coolant problems yet, but I think two things are important here:
If you've got the cap off (or have a leak) it can't pressurize which lowers the boiling temp.
Also, if you or the person before you has been looking at the problem a lot, they've probably only refilled it with water (hopefully distilled but for these purposes it's the same as tap). With only water and no coolant the boiling temp will be a lot lower.

Put those two factors together and it won't take a lot to boil the coolant in the car.

That being said, neither may be true, both are just guesses. Best case scenario?* You just need a new thermostat or water pump. Since they're right next to each other you'll do them both anyway (as well as the timing belt + some other things related to it).

Worst case scenario? Blown head gasket and you'll have to pull the heads, replace the gaskets, and check for warpage.


*ULTIMATE best case scenario (but very unlikely): You just need to repair the leak/crack in the coolant system.



Edit: I have no idea if this is true, but I get the impression that since the coolant is filling up the reservoir the pump must be working. Would hot water rising to the top of the system be enough for it to fill the tank? I'm honestly not sure. Hopefully someone else can answer that.

 

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Thanks for the reply.
I should clarify that the only tank I can see is the clear plastic coolant tank with the blue radiator cap. The overflow from this tank runs straight onto the exhaust and then the ground.

I filled this tank with water, squeezing the top hose to try and remove air from it.

I'm using water until I get the cooling system repaired, too expensive to pour out on the ground.

Should I see water movement in this tank if the pump is working?

The pressure plug part of the cap was shattered, with bits of it around the flange - I'm wondering if a bit got into the tank, then down the hose and jammed the water pump...

I'm wondering if there is a way of pushing water around the system with a garden hose sealed in the end of a radiator hose to diagnose blockage (eg stuck thermostat, etc.)

 

Yep that's the reservoir or expansion tank. It often cracks due to old age.

It's generally the highest point in the system so all air should run there. There's also a high point in the heater core - make sure you remove the air bubble from there.

Yeah at this point you may as well fill with tap water, just don't leave it for TOO long. i.e. when you've figure this all out, drain it completely, flush with clean water, then fill with g12 and distilled.

In a working system, the coolant should flow out the bottom of the tank/reservoir and flow/dribble back in from the top (smaller) hose.

It's possible but it would have to be a fairly large sized piece, the water pump is decently sized. Not saying it can't, but that's a rare issue (I've never heard of it happening).

I think there's decent ways to determine if it's a stuck thermostat, but you may have to google because I have no expert in that.

Just to recap, are you saying that water is NOT flowing down out of reservoir? It just fills up with water coming back in?
It could be that you just have a few large air bubbles in the system that are restricting flow, i.e. an incorrectly burped system.

 

These thermostats are fail-safe, meaning they stick open when they fail. This would give you (eventually) a normal operating temperature at idle but the temperature would fall once you start driving. It's possible that the thermostat failed and stuck closed but it would be a less-than-common occurrence.

Have you checked the face of the radiator to see if it's uniformly hot? If not, you likely have a blockage that's compromising the ability of the car to cool itself. Likewise, do the fans operate as they should?

 

I got my hand down between the engine and radiator as far as I could without losing it (front of radiator is covered by Air Con radiator) but it was hard to tell if there were any differences.
I've bought a cap for the expansion tank now, but it seemed to me from the different forum answers that my problem is with the water pump not circulating the water, most likely due to the plastic impeller problem they often have. I removed the small return hose from the expansion tank and ran the engine - no water flowing out of it at all, until I lowered it a bit and the water level overflowed out of it.