over pressurized expansion tank, diagnosis help?
 

Not new to the world of Audi, however this is my first 1.8t. It's a 2001 5 speed 131k miles, and I've got a new issue with it.

When I first bought the car it had an issue with overcooling the engine, that is the oil and coolant temps actually dropped as I drove. I replaced the CTS and the thermostat, both aftermarket(silly idea?), and that problem was resolved. However I've got a new problem:

*slight overheating, temp needle moves past the centerline, but does not touch the line next to it. Only when driving on hills or highways.
*After a drive, the coolant tank will be over pressurized to the point that it will spill out if I do not have it tilted. It does not over pressurize when idling or holding a steady RPM. After a drive only.
* I sniff tested the tank for 20 minutes when I first bought the car and there was no exhaust gases in the system.
* No codes.
* System is fully bled.

I'm thinking it's a bad tank and cap, I don't see any leaks or loss of coolant anywhere. Maybe the aftermarket thermostat is bad? But then why would it keep the proper temperature in all other conditions?

Maybe the coolant flange? After I shut off the car I can hear a bubbling sound from around that area, however I do not see any traces of leakage. Perhaps it's letting air in somehow?
Cracked block?

Perhaps a failing waterpump?

Get back to me, thanks.

Okay, I hooked up a scan tool and noted that the idle temp is about 99C, on a drive it rose to 107C, then dropped to 95-99C as it idled. I didn't try driving any further after that, but I need a vehicle.

Now I'm wondering if the aftermarket thermostat is the culprit, or if the waterpump isn't doing its job.

I don't think it's a blocked radiator, water flows freely when pouring from the tank through the drain petcock.

I even thought that I may have had the thermostat in backwards and ended up pulling the alternator to take another look. It was on correctly.

Any ideas?

Hey,

Did you get the car scanned with a vagcom? As the pressure of the coolant increases, the mixture gets pushed into the reservoir which may leak if you have faulty tank and cap.
I would recommend inspecting the tank and reservoir cap very carefully for cracks in the plastic and rubber seal. You could try to replace the cap, if the rubber has deteriorated.

Have you considered getting the cooling system pressure tested? If you have access to an air compressor, then you can pressurize the cooling system with the engine cold and look for a leak.

No, just a generic OBD-2 scanner hooked up to a laptop using an ELM-327 USB, not quite as good as vag-com, or the Snap-On verdict I used to have at work. However, its readings are quite accurate. I can borrow a radiator pressure tester possibly tomorrow to see what I can find. I do have an air compressor too.

Although I'm leaning towards the possibility that the "Tough1One" thermostat i bought may just be defective and not opening at 190F when it is supposed to, or maybe even a clogged radiator not cooling the water enough.

Although with the old thermostat that was sticking it never overheated, it actually overcooled. I want to rule out the waterpump as the issue never occurred with the old thermostat. I've been reading up on how the impeller may have come loose from the shaft, and doesn't circulate enough water? I have full heat in the cabin with the car running, and with the old thermostat the car never overheated once.

any other ideas? The O-ring on the cap is fine, but there is a very micro-crack on the lower half of the expansion tank. I do not see any loss of coolant at all, until i open the hot expansion tank to let the air out, which makes the coolant bubble out.

How can I test for flow from the waterpump on these cars?

Solved!

Went to the VW dealership 20 miles out and bought an OEM thermostat. I tested the aftermarket and the oem one, and clearly the oem one opens up more than the aftermarket.

Oil temps stayed down, Temp needle never went over the middle.

I also went ahead and drilled an 1/8th inch hole in the thermostat itself to help bleed the system quicker. I'll report later if the issue persists, but I doubt it will.

I'm happy you solved the problem! Cooling issues can be tricky especially after replacing a thermostat. I would try to return the defective thermostat for a refund, if possible.

It was only $8 Bucks and I tossed the receipt so i'm not too worried about it, I'm just going to mark that up a lesson learned about delicate cooling system parts and where not to buy them.