Have an 02 A4 Quattro 1.8t 100k miles, Tried 3 thermostats, none will open at temperature I have replaced the flange, hose going from flange to heater core because it blew, resevoir. Checked the water pump by rotating the motor with a wrench while feeling inside the thermostat inlet for the water pump to rotate. head is not blown, not loosing coolant, no coolant in oil/oil in coolant, good compression, good water flow through radiator(disconnected and ran garden hose through at full pressure). The Temp guage inside the car will never go above half but the top hoses get burning hot, bottom hose to radiator ice cold, no water going through heater core(also not clogged, hoses to it ice cold and feel empty when squeezing), also even though temp guage inside car never breaks half the coolant temp warning goes off.

I know for sure the thermostat is not opening, water on top half is damn hot, water in lower half below the electrical connector(lower radiator hose) is stone cold, but I can not for the life of me figure out why, does anyone know what tells it to open? should I try the CTS's? both are green top. Any help would be greatly appreciated.

 

pull the thermostat out and boil it. make sure its not opening.

 

Did you ever figure out your thermostat issue. I have the exact same problem. Replaced thermostat, both temp sensors and water pump. Not getting any codes or cel's and temp gauge in middle, but still my coolant boils out of the expansion tank. I'm totally stumped.

 

My car is a 2003 A4B6 2.0 ALT nonturbo engine 215,000 kms.
This is the same problem that I'm having right now. A exact description.
i did replace the ( it's electrically controlled ) thermostat because it was running cold.( It turns out the plastic legs inside that hold the actual thermostat unit had broken off & it was loose & letting coolant past it )
I also replaced the top Rear coolant flange & either swapped the CTS sensor in it for another one, or a new one.( Can't remember )
I replaced the lower CTS sensor on the radiator flange, this operates the cooling fans.

The dash temp gauge never gets over 91°C, ever.
But the metal coolant tube near the top of the engine, with the bleed nipple on it gets fantastically hot.
The top radiator hose gets pressurised & really hard.
The fans hardly ever turn on. But they have when I squeezed the top hose when it was hot but not pressurised. It seemed to 'Burp' it. Almost like there's a air bubble stuck down there somewhere.

When the cars cold/warm & I take the resiviour cap off & run it at idle, not much happens there, but if I take it up past 3000 rpm, it might give a big 'Burp' & push a lot of coolant out of the resiviour, then release air, then the coolant that's still in there flows down untill it's empty. If I top it up after that, it will do the same again later on.

I have bought a vacuum operated Coolant flush system to use. On the theory that there's a air bubble I can't get out normally.
And then I read that they can ruin/ collapse the heater core on the B5 ,& B6's, which I find a bit hard to believe, but enough to think twice & worry about it.. I have a heater core I can experiment on though.

I think the next step, without using a OBDI scan, which I think I should do. Is operate the thermostat by using straight 12 volts & see what happens. Maybe check if the little heater cool inside it is open circuit somehow.
Or, replace the upper CTS & see what happens.
Vacuum bleed the coolant system.

What I can't understand is why the dash temp gauge never goes over 91°C. I read that they are not a actual temp, but a average.
Also, it does increase from cold exactly as I'd expect. Though, once it's at 91°C, the heaters quite warm, and then after a bit it gets to quite hot. Like, it's feels like that once the dash temp is 91°C, that coolant going thru the heater is still getting hotter after that.
That CTS is directly inline with the heater coolant hose.

Any hints, ideas, questions? I will come back & post again so the answer will be here at some point in the future.....

The fact the dash temp gauge never goes over 91°C is weird
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after edit, a while later, etc etc.
I changed the lower Coolant Temperature Sensor on the radiator hose near the radiator.
Topran part # 109 896 755. & It's fixed it.

Just after that I got a cheap OBD2 & download the Torque app & find out that the dash temp gauge reads 90°C when the real temp is approaching 80°C, and stays there untill the real temp gets up to 100°C & then might peak at 92°C while the real temp is 103°C.
Which is as high as the real temp has got.

So that's all it is, a vague vag gauge & probably a dud lower CTS, it's actually a thermometer sort, not a switch like most of the others have.
Actually it's interesting watching the Torque app, when you drive normally, it's 100°C, if you start driving it hard, overtaking or suddenly speeding up, the computer tells the partly electrically controlled valve thermostat ( it still opens & closes like a normal thermostat, it's just that it can be electrically operated to open as well ) to open up & cool the engine down to a lower temp, so it gets down to ~93°C & then rises to 100°C once you start driving normally again.

 

hi mate, thanks a lot for posting this, I found it after searching around to see if I find any similar issues.
I have the same car and the same issue. I replaced the thermostat and it blows the coolant out of the expansion tank. The only thing is that I haven’t replaced the sensors yet. The thermostat plastic things were broken who knows where they went.. I hope it won’t block anything inside the coolant system.
I was thinking to put back the old thermostat to check if I can bleed the system properly but tomorrow I will go and buy the two sensors. It either that and I get to spend a few more bucks or call the scrap yard and get 250€ for the whole car haha.. minus the write off so I actually end up with nothing.

one more thing.. I used to take off the intake manifold to get to the thermostat which is a pain but I actually found a post on pelican parts and you only have to remove the alternator. Plus lift the intake a bit to remove one of the screws that hold one of the pipes to the oil cooler.

————— Edit after problem is solved ——-
Plugged in vagcom cable and I had an error for the G62 sensor the 4 pin sensor from the back of the cylinder head (part number 059919501A), but after I deleted it never came back. I replaced both sensors anyways the 2 pin from the lower radiator hose near the drain plug and the 4 pin sensor and the water never boiled again. However I noticed that when I squeezed the big hose there was coolant going out from the overfill hole on the expansion tank and when I checked the blue cap it had missing plastic from the black valve inside. As a quick test I took the cap from my vw polo and car works great so far… until next time 😁

 

Hi, I guess your heating system got collapsed and need replacement. Sometimes there is a slight shortage of spare parts, I know. But overheating isn't healthy for any device. You can talk to https://baikalmechanical.com/ specialists that provide industrial heaters and cooling systems. I was buying cooling tower for my garage the other day. At least they will know what you need to replace exactly.

 

Hi guys, thanks for all the posts above. I am experiencing the same issue although slightly different. I have an Audi A3 Sportback 2.0L FSI normally aspirated. The first issue I experienced was the fans not coming on even though the vehicle was hot. I checked the upper hose which was hot but the lower hose was cold. I assumed the thermostat had failed in the closed position. I replaced it and experienced the same problem and then I bought another one and no change. I took the thermostat out and tested it in hot water while applying 12V and it opened as it should. I put it back on the car hoping the issue would go away. I then developed a coolant leak between the oil cooler and oil filter housing where they mate. I bought a new oil cooler and gasket but the vehicle still leaks coolant there and I suspect because of the pressure build up from the overheating is causing it to leak. I then went on to buy a new CTS just in case. I know this won't solve the issue because if the thermostat doesn't open then the fans won't come on. Does anyone have an idea what else to check? TIA.
Water pump was also changed.