Help overheating no coolant loss

[DPs8]

Hey guys need some input. On my way in to work the other morning I got the warning to pull over and let cool down and temp gauge going all the way up. I pulled over and let idle and the temp came back down the coolant reservoir was full and puffing out. When I would drive again temp would go up but if I let it idle temp would come back down. I also noticed I didn’t have much heat coming out of the vents. I was able to nurse the car home and thinking the thermostat was stuck closed (with all the thermostat issues and as far as I know this car has the original I decided to replace it). The car is doing the same thing now with the new T-stat so I am wondering if the water pump is the culprit (I still have some heat coming out of the vents, no strange noises from the water pump, and there’s gotta be enough flow to keep it from overheating at idle. Is there anything else on these cars that would prevent coolant flow, anyone hear of a plugged radiator being an issue all of a sudden? Or should I just bite the bullet and replace the water pump? The car has 15 s8 135k mi

[TexasDfwS4]

I would guess the thermostat needs to be replaced. When the fail they fail closed.

[DPs8]

I relaxed the thermostat and am having the same issue. I’m wondering if I should go ahead and assume the water pump is at fault or could there be something else more likely in the system that would cause an over heat condition off idle. (Coolant warm up control valve, plugged radiator, something else in the system?

[DPs8]

Also on the s8 per the service manual the oil cooler has to be removed - is this the case or can you sneak the water pump out without removing the oil cooler ?

[qcrazy]

Classic switching valve failure. You can just disconnect (and plug) the vacuum line and the car will run perfect without it hooked up.

[DPs8]

Classic switching valve failure. You can just disconnect (and plug) the vacuum line and the car will run perfect without it hooked up.

Ok I was wondering if the switching valve would cause and overheat condition? Just curious If we are assuming the valve is stuck closed how would removing the vacuum line change the valves position ? If I activate the cylinder head coolant control valve on the rosstech the switching valve should actuate right ?

[qcrazy]

99% of the time the vacuum solenoid fails open. This means that vacuum is always applied to the switching valve, holding the valve closed.

Sometimes it sticks open sporadically at first, but eventually the vacuum solenoid will get stuck fully open. When this happens, the car will always overheat.

The classic symptoms of a bad switching valve is that at idle the car is perfectly fine. But any time you try to get up to speed, it quickly and instantly overheats. Pulling over and idling will then immediately bring the car back to normal temp.

These symptoms above indicate a bad switching valve. This is then almost always caused by a failed vacuum solenoid. (On newer cars the solenoid is actually a separate serviceable part, but not on the 4.0T).

I just went through this in August while on vacation, 6 hours from home. I had to fix this on the side of the road in order to get home. Pulled (and plugged) the vacuum line and was back in business within 30 minutes. Since getting home I have left it unhooked with no issues. I will not bother hooking it up ever again.

As far as the VCDS test, I don't think that is useful. There are no sensors on the part to indicate the actual position of the valve. You are only testing the wiring to the solenoid and the solenoid coil. As long as the wiring is intact, the VCDS test will pass fine as the car does not know the actual position of the valve or the solenoid.

[DPs8]

Awesome I will try tonight and let you know how it goes ! Thanks for the help!!!! 🍺

[DPs8]

Qcrazy you sir are the man! I disconnected the hose to the actuator on the switching valve and put a cap on the solenoid. A quick vacuum on the cooling system and top off and pretty quick after starting had much better heat out of the vents. Took her for a test drive and temp held at mid point like normal. Thanks for the help you saved many future headaches and cuss words!

[qcrazy]

That switching valve is just a failure waiting to strand you. Glad you got it!!

[DPs8]

Yea I couldn’t find much information on what it actually does or how it works. That’s one thing that drives me crazy with this car! Thanks again!

[qcrazy]

It is there to block coolant flow to help the car warm up faster, for emission purposes. I can tell you from having mine disconnected for a few months that it takes about 2-3 minutes longer to warm up than it did before. Barely noticable.

The 4.0T uses this valve. The new Audi's use a flap in front of the waterpump for the same purpose. And yes, those vacuum solenoids fail open too.

[hillbilly911]

"Qcrazy, you sir are the man!"

After taking my wife's 2013 AUDI A8L in for the oil strainer recall, the car started overheating not long after leaving the dealership. They charged me to replace a couple of turbo cooling line seals while they were in there; so they did meddle in the cooling system. I found the vacuum solenoid portion was broken off, and not actuating the cooling valve open. I manually pushed the lever in the down direction. That solved the cooling issue. Now trying to have them replace the assembly.

Thank you for the information in this thread. It really helped me tremendously!

[Slobo7x]

This is good info for anyone, do you guys have a location of where such valve is? Do you unplug the electrical connector as well?

Thanks

[qcrazy]

Center, front of engine. You can just reach the vacuum line with car put together (e.g. when standing on the side of the road). You just have to pull a couple of covers for access. Hard to reach anything else though. Leave the electrical connector attached or else the car will see an open circuit.

20221017_100039.jpg

[Dasquade]

Qcrazy helped me on this aswell ;).
For what it is worth, as 99% of the times it is the solenoid that fails and that doesn't throws an error, you can use vcds/obd11 to open/close the valve. It should make a very soft click noise (need to be standing in front your car with ear close to the location of valve, middle of car under the charge cooler brick). If you don't here the click, it is broken.
*yes went through a new Tstat, new waterpump internals, reservoir and line, etc....but in the end it was this solenoid.

[oripaamoni]

Good to know!

[Ratnik1990]

Qcrazy helped me on this aswell ;).
For what it is worth, as 99% of the times it is the solenoid that fails and that doesn't throws an error, you can use vcds/obd11 to open/close the valve. It should make a very soft click noise (need to be standing in front your car with ear close to the location of valve, middle of car under the charge cooler brick). If you don't here the click, it is broken.
*yes went through a new Tstat, new waterpump internals, reservoir and line, etc....but in the end it was this solenoid.

Can you please tell me what this valve is called in VCDS ?

Thank you for your time :)

[Dasquade]

Can you please tell me what this valve is called in VCDS ?

Thank you for your time :)

Sorry i use obd11, there it is called "coolant switching valve".

[Alabama]

Can you please tell me what this valve is called in VCDS ?

Check AZ thread "Overheating", post #24, where AZ member infinitereality wrote "One thing I did note (unless I misread it, ridiculously hot in Louisiana and drench in sweat at this point), was that there was an option to test Cylinder Head Coolant Valve (aka N489). It tested fine, but was interesting I could test it. " If that's the solenoid of concern, maybe that person can help you.

[mlee1999]

Sorry i use obd11, there it is called "coolant switching valve".

Hi, how did you test the valve using obd11? My RS7 started overheating last night, I'm thinking it's the thermostat but would like to rule out the switching valve solenoid if possible. I have an OBD11 and found some settings but couldn't figure out how to open and close it. Thank you.

[Dasquade]

Hi, how did you test the valve using obd11? My RS7 started overheating last night, I'm thinking it's the thermostat but would like to rule out the switching valve solenoid if possible. I have an OBD11 and found some settings but couldn't figure out how to open and close it. Thank you.

Bit late, but in obd11 it also is called "Cylinder Head Coolant Valve". You can find it under "output test" in the engine module. Engine kinda needs to be turned off, ignition on to be able to hear the soft click.

[vtraudt]

It is there to block coolant flow to help the car warm up faster, for emission purposes. I can tell you from having mine disconnected for a few months that it takes about 2-3 minutes longer to warm up than it did before. Barely noticable.

The 4.0T uses this valve. The new Audi's use a flap in front of the waterpump for the same purpose. And yes, those vacuum solenoids fail open too.

https://www.fcpeuro.com/public/asset...JPG?1685736460

HOW did you disconnect? I was out at the car when first reading about the switchover valve. In the front of the engine I found a vacuum canister in front of engine, center, slightly down, and a vacuum line going from the cannister to ??? (can't see). I pulled that one off at the ??? end. I put a screw into the line (coming from the canister).

Is THAT the correct method?

[vtraudt]

https://www.fcpeuro.com/public/asset...JPG?1685736460

HOW did you disconnect? I was out at the car when first reading about the switchover valve. In the front of the engine I found a vacuum canister in front of engine, center, slightly down, and a vacuum line going from the cannister to ??? (can't see). I pulled that one off at the ??? end. I put a screw into the line (coming from the canister).

Is THAT the correct method?

And since I caused all my issues by hosing down engine with gardenhose, I suspect the electrical part of that swtichover valve is actually bad.

a) Can the electrical solenoid be activated via VCDS OUTPUT TEST? What is the valve number or name?
b) I can't easily get to the electrical connector to unplug (to see if/what code would be generated. Currently there is NO CODE to be found.

Lastly: would a stuck/non working switchover valve explain all 3 symptoms I am having?

1) overheating (when driving, not at idle)
2) cold radiator (99 deg C engine coolant temp, 49 deg C coolant temp at radiator outlet, no fans running)
3) no heat

[Akakios]

And since I caused all my issues by hosing down engine with gardenhose, I suspect the electrical part of that swtichover valve is actually bad.

a) Can the electrical solenoid be activated via VCDS OUTPUT TEST? What is the valve number or name?
b) I can't easily get to the electrical connector to unplug (to see if/what code would be generated. Currently there is NO CODE to be found.

Lastly: would a stuck/non working switchover valve explain all 3 symptoms I am having?

1) overheating (when driving, not at idle)
2) cold radiator (99 deg C engine coolant temp, 49 deg C coolant temp at radiator outlet, no fans running)
3) no heat

You can test it manually once off but not aware of any specific output tests on it. It WILL throw a code though if it fails.

It's underneath the charge manifold, so in theory you may be able to reach down to disconnect, but it will be fun (use an inspection camera to have eyes on it while grabbing it . I had my car in service mode and there's still a limited amount of room.

https://www.youtube.com/shorts/4kd_EriXQvc

If it fails while the valve is open, you're clear. If it fails while it's closed, you're screwed.

This valve closes the loop to warm the engine up quicker and thus if it fails, you're creating a one way dead end for coolant flow.

[vtraudt]

You can test it manually once off but not aware of any specific output tests on it. It WILL throw a code though if it fails.

It's underneath the charge manifold, so in theory you may be able to reach down to disconnect, but it will be fun (use an inspection camera to have eyes on it while grabbing it . I had my car in service mode and there's still a limited amount of room.

https://www.youtube.com/shorts/4kd_EriXQvc

If it fails while the valve is open, you're clear. If it fails while it's closed, you're screwed.

This valve closes the loop to warm the engine up quicker and thus if it fails, you're creating a one way dead end for coolant flow.

I will try if I can reach the 'plunger' and move it. If stuck should plunger be 'in' or 'out' to keep flow to radiator open and prevent the overheating?

Would stuck switchover valve also explain NO HEAT? I don't think so. But also need to double verify the 'no heat' symptom.

[vtraudt]

Would failure of electric water pump or pumps explain 'cold radiator', and 'overheating' and 'no heat'?

[vtraudt]

I will try if I can reach the 'plunger' and move it. If stuck should plunger be 'in' or 'out' to keep flow to radiator open and prevent the overheating?

It looks like 'rod out' is "open"?

Is rod 'spring loaded' to return to open, and vacuum to suck it closed?

[Akakios]

It looks like 'rod out' is "open"?

Is rod 'spring loaded' to return to open, and vacuum to suck it closed?

Correct, so that if it fails, it should fail open. In my case, I put a brand-new part on it and it was a bad part from FCP so it's stuck open until I get it back into service mode. Takes a little longer to warm up, no big deal.

[vtraudt]

Update: got heat (took a bit when turning on, but came out normal/hot). Could have been air lock from loosing coolent.

I disconnected hte actuator hose form the solenoid, plugged the hose (put screw in) and capped the solenoid vac line.

Some driving yesterday, got coolant at engine to 95C, exit radiator only 50C. Will do more driving and logging temps.

[vtraudt]

Drove car very spirited today.
Started VCDS logger of various coolant related parameters, from after idle warmup to return to idle cool down. Saved the log, but instead of a full data set of the whole 20 minute log, only saved a 'snapshot', see below. Disappointing.

Took again very high temps (high 90s, parked and idle it even showed 101C) and 'out of radiator' stayed very low, too me indicationg the crossover valve had NOT opened (no flow to radiator).
But at some point, the radiator temps started to go up. Verified with IR gun on various componenents.
Fan speed was low percentage most of time, only after parking (idle) did fan kick in higher.
During long idle, fans came on once in a while.

Am I 'normal' again?

Saturday,22,March,2025,15:22:44:37142
VCDS -- Windows Based VAG/VAS Emulator Running on Windows 11 x64
VCDS Version: 25.3.0.3 (x64) HEX-NET2 CB: 0.4660.5
Data version: 20250301 DS365.0
www.Ross-Tech.com


VIN: WAU32AFDXFN020918 License Plate:
Mileage: Repair Order: a8 heatup coolant log



--------------------------------------------------------------------------------
--------------------------------------------------------------------------------


Address 01: Engine (4H0 906 014 J)

15:21:55
IDE00025 Coolant temperature 100 °C
IDE00192 Coolant temperature at radiator outlet: actual value 73.9 °C
IDE00194 Coolant temperature at radiator output: specified value 98.0 °C
IDE00380 Coolant fan 1: activation 10.00 %
IDE00381 Coolant fan 2: activation 10.00 %
IDE01384 Map controlled engine cooling thermostat: specified value 15.34 %
IDE02832 Coolant circulation pump: activation 0.00 %
IDE05327 Activation for electrical auxiliary coolant pump in HT circuit 0.00 %
IDE05329 Activation for electrical reserve coolant pump in NT circuit 0.00 %
IDE06828 Coolant recirc. pump 2: spec. value 100.00 %
IDE07068 Temperature sensor 2 for charge air cooler 59.3 °C
IDE07724 Coolant temperature at engine outlet: calculated 99.7 °C

[Akakios]

Drove car very spirited today.
Started VCDS logger of various coolant related parameters, from after idle warmup to return to idle cool down. Saved the log, but instead of a full data set of the whole 20 minute log, only saved a 'snapshot', see below. Disappointing.

Took again very high temps (high 90s, parked and idle it even showed 101C) and 'out of radiator' stayed very low, too me indicationg the crossover valve had NOT opened (no flow to radiator).
But at some point, the radiator temps started to go up. Verified with IR gun on various componenents.
Fan speed was low percentage most of time, only after parking (idle) did fan kick in higher.
During long idle, fans came on once in a while.

Am I 'normal' again?

Saturday,22,March,2025,15:22:44:37142
VCDS -- Windows Based VAG/VAS Emulator Running on Windows 11 x64
VCDS Version: 25.3.0.3 (x64) HEX-NET2 CB: 0.4660.5
Data version: 20250301 DS365.0
www.Ross-Tech.com


VIN: WAU32AFDXFN020918 License Plate:
Mileage: Repair Order: a8 heatup coolant log



--------------------------------------------------------------------------------
--------------------------------------------------------------------------------


Address 01: Engine (4H0 906 014 J)

15:21:55
IDE00025 Coolant temperature 100 °C
IDE00192 Coolant temperature at radiator outlet: actual value 73.9 °C
IDE00194 Coolant temperature at radiator output: specified value 98.0 °C
IDE00380 Coolant fan 1: activation 10.00 %
IDE00381 Coolant fan 2: activation 10.00 %
IDE01384 Map controlled engine cooling thermostat: specified value 15.34 %
IDE02832 Coolant circulation pump: activation 0.00 %
IDE05327 Activation for electrical auxiliary coolant pump in HT circuit 0.00 %
IDE05329 Activation for electrical reserve coolant pump in NT circuit 0.00 %
IDE06828 Coolant recirc. pump 2: spec. value 100.00 %
IDE07068 Temperature sensor 2 for charge air cooler 59.3 °C
IDE07724 Coolant temperature at engine outlet: calculated 99.7 °C

As long as the actual is below the specified, you are good to go.

There is apparently a way to activate it in VCDS without pulling the rubber boot off and attaching a vacuum source to get it to move.

[vtraudt]

There is apparently a way to activate it in VCDS without pulling the rubber boot off and attaching a vacuum source to get it to move.

Anyone know how it is called or what address it might be (in VCDS) to run ouput test on it?