Coolant Leak After Highway Driving

Hello,
I'm having the following problem:

After several hours of driving on the highway (at speeds of 135-170 km/h), a stop of 5-10 minutes was made.
Continuing on – we hadn't even left the parking space when the Kühlwasser warning indicator came on.
The expansion tank had almost completely lost its coolant. Residual liquid was visible in the middle of the expansion tank's seam (between the min. and max. markings) – coolant was added, and driving continued. Incidentally, the coolant temperature gauge showed no unusual behavior (it consistently displayed the usual 90°C).
Then, even on short trips to the destination (total distance approximately 110 km), there were no more losses. (At the destination, the cap of the expansion tank was replaced, suspecting a faulty pressure relief valve!).

Highway driving (approximately 110-130 km/h). After 250 km, the first stop: coolant level OK. After another 5-10 minutes of driving, the low coolant warning light came on again. Pulled over to the right side of the road: the expansion tank was almost empty. There were signs of moisture or stains on the brake lines and components located below the expansion tank. The seam on the expansion tank was now dry. Coolant was refilled. During subsequent stops on the way home, the engine was not turned off: no further coolant loss was observed.
Incidentally, the coolant temperature gauge never displayed any unusual behavior (it always showed the usual 90°C).

Even during these short trips around the house, there was no more noticeable loss.

My initial suspicion was a faulty pressure relief valve in the lid, which would cause the water to boil and thus lead to coolant loss. Unfortunately, it probably wasn't that, since I had already replaced the lid with a new one, but the coolant loss persisted.
Given the moisture stains, I suspect a leaking expansion tank, a defective return hose underneath the tank, or a loose clamp.

What do you think?
And: How would you verify that?

Hi broesel,

(or)
Hello broesel,

(or)
Hey broesel,
Sure, here is the translation of the text from German to English:

'P.S. The car has approximately 230,000 kilometers on it.'

Hello,
Let's pressure test the cooling system; that should help us identify where the leak is.
Best regards,
Günther.

...or simply install a tire valve into the old coolant reservoir cap and carefully apply pressure. But please don't apply too much pressure, or the radiator might burst.

"1 bar would be a reasonable estimate, but without any guarantee. There might also be the opening pressure printed on the lid."

"There might also be a coolant additive available from Teilefuzzi that glows under UV light -> ask about it. It's useful for small leaks. Before (applying), verschandeln the engine."

@gwg:

Quote:

Hello, Let's pressure test the cooling system; that should help us identify where the leak is. Best regards, Günther

If he doesn't want to find the error himself, then why is he here, right?
We are not in a forum where you can just do whatever you want.
This forum is not intended for idle chatter or trying to appear intelligent.

Best regards, Rainer.

Hello,
The cause is likely the pressure buildup due to the reheating of the hot engine; somewhere, something isn't able to withstand the pressure. Possibly, the electric radiator fan isn't working.
Best regards,
Günther.


@Rainer

Sorry, I didn't mean it that way.

The repair manual specifies a pressure relief valve setting of 1.3-1.5 bar. Since the cap is brand new, it should function correctly and prevent excessive overpressure, right? I'll try the tip about using a tire valve. I'm going to attempt to install a T-piece with some tubing and a tire valve into the return line leading back to the expansion tank, so I won't have to 'drill' the cap.

I had already read about the glowing additive in forum posts, but unfortunately, it's not available everywhere.

Thank you for the quick responses!
greetings, broesel

Hello.

This looks like a classic case of head gasket failure – exhaust gases are being forced into the cooling system, which increases the pressure in the cooling system until it reaches the opening pressure of the cap on the expansion tank. This causes the coolant to leak out, and that's where the coolant stains originate. If you have an engine with the code AJM, then you've either won or lost (depending on the context).

Regards,

hb2000

@hb2000:
I doubt that's the case, because a head gasket failure would more likely lead to a continuous water leak.
During the first stop on the way back, the coolant level was still okay, but 5 minutes later the reservoir was almost empty. During subsequent stops with the engine running, the level was fine again. This doesn't seem consistent with a faulty head gasket to me.

Hi Broesel,

@hb2000:
I doubt that's the case, because a head gasket failure would more likely lead to a continuous water leak.
During the first stop on the way back, the coolant level was still okay, but 5 minutes later the reservoir was almost empty. During subsequent stops with the engine running, the level was fine again. This doesn't seem consistent with a faulty head gasket to me.

hello broesel

Hello,
When the engine is running, there is no heat buildup when it comes to a stop.
Does the electric radiator fan turn on?
Best regards,
Günther.

Hello broesel,

I don't want to discourage you, but I'd eat my hat if that's not the ZKD. The first time I had ZKD damage (at 210,000 km), I had exactly the same symptoms. Now, after two years of no issues (at 260,000 km), it seems like something similar is starting to happen again.

See: https://community.dieselschrauber.org/en/viewtopic.php?t=8172

Keep your ears open!

Regards,
Thomas.

Hello,
Are there any new 'insights'?
Best regards,
Günther.

Hello!

Does your heat exchanger happen to leak in the morning (when the engine is cold)? I had a similar problem and suspected a head gasket issue. I currently have G12plus coolant in my car. I believe it might be sealing (small) leaks, although that's just a guess. I noticed white residue from the coolant in two places (once at the heat exchanger connection, once between the turbo and the engine block), but there are no more leaks! It's possible that my VAG workshop (an old-school one) might have added radiator sealant to the engine without telling me.... Whatever the case, everything is sealed right now!

Hello,

I have now pressurized the system.
The pressure relief valve on both the old and new caps is functioning correctly, releasing between 1.3 and 1.5 bar (it starts at 1.3 bar and then opens further). I have installed a car tire valve into a T-adapter and connected this adapter to the return hose leading back to the expansion tank. During pressure build-up until the relief valve activated, the level in the expansion tank decreased (which is plausible because the hoses fill more fully due to the higher pressure). The pressure then slowly dropped over a few minutes to approximately 0.5 bar (possibly my cheap tire inflator attachment for the compressor isn't completely airtight). Without any pressure, the fluid level was back to its original level (so there was no noticeable visible fluid loss). There were also no signs of leaks anywhere, nor any traces on the ground under the car.
I replaced a cracked hose leading to the radiator in mid-2004.
Could it be that an air bubble has formed in the system?

To everyone regarding ZKD:
'In mid-2004, when I had a coolant leak, I installed a slightly longer return hose to test the head gasket. I disconnected the hose from the expansion tank and placed it inside the expansion tank below the water level: no bubbles were visible, even when accelerating.'
I will now repeat this test again to rule out any issues with the ZKD (presumably a specific component or system).

@AJM Chris: No, at least nothing came to my attention.

PS: I also haven't experienced any problems with my current job (mostly half-hour commutes to and from work).
Perhaps I simply need to wait until the loss also becomes apparent in normal operations.

The coolant loss only occurs at higher speeds and/or under high load, and then the pressurized coolant is forced out through the pressure relief valve on the cap of the expansion tank. Another indication supporting the theory of a faulty cylinder head gasket is gurgling and splashing sounds coming from the heat exchanger, which is located near the glove compartment inside the car. It seems to be a common issue with VW vehicles where the seals on the cylinder head are not designed to withstand continuous stress. Sooner or later, almost all TDIs with the engine codes AJM, ATJ, or possibly others, will experience this problem. My Passat has been running with this defect for 25,000 km now, and it has a total of 160,000 km on the odometer.

Hello broesel,
Then, after a longer, more demanding drive, try not to turn off the engine immediately, but observe it. After another drive, turn it off immediately and observe it again. Also, occasionally check the coolant reservoir.
These two observations hardly take any time and provide a lot of value.
Best regards,
Günther.

gwg wrote:

After a longer, more demanding drive, try stopping the engine ... immediately

.
Best regards from the turbocharger!

At the end of this section is a video; please pay attention to the coolant return flow.
Okay, I'm ready. Please provide the German text you want me to translate into English. Just paste it here, and I will give you the translation.

After a longer, more demanding drive, try stopping the engine ... immediately .
Best regards from the turbocharger!

Hello,
According to Volkswagen, this is permitted!
Otherwise, I wouldn't do it either; what I meant was the same 'behavior' as with the coolant leak!
Best regards,
Günther.