Coolant leak after a long highway drive

Hello,
I'm having the following problem:

After several hours of driving on the highway (135-170 km/h), a 5-10 minute stop was made.
Continuing on - we hadn't even left the parking space when the coolant warning light 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 minimum and maximum markings). Coolant was added, and the vehicle continued to operate. By the way, the coolant temperature gauge didn't show any unusual behavior (it consistently displayed the usual 90°C).
Then, even for short trips, there were no more losses at the destination (total distance approximately 110 km). (The cap of the expansion tank was replaced at the destination, suspecting a faulty pressure relief valve!).

Highway driving (approximately 110-130 km/h). After 250 km, first stop: coolant level OK. After another 5-10 minutes of driving, the coolant warning light came on again. Pulled over to the right: the expansion tank was almost empty again. There were signs of moisture or stains on the brake lines and the unit below the expansion tank. The seam on the expansion tank was now dry. Coolant was refilled. During the following stops on the way home, the engine was no longer turned off: there was no more water loss.
By the way, the coolant temperature gauge never showed any unusual behavior (it always remained at the usual 90°C).

Even during these half-hour trips at home, there was no (noticeable) loss anymore.

My initial suspicion was a faulty pressure relief valve in the lid, which could cause the water to boil and, consequently, lead to a loss of coolant. Unfortunately, that probably wasn't the problem, as I had already replaced the lid and was still experiencing the leak.
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,
PS: The car has approximately 230,000 kilometers on the odometer.

Hello,
Have the cooling system pressure tested; this should reveal the location of the leak.
Best regards,
Günther.

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

1 bar would be a decent signal strength, but there's no guarantee. The opening pressure may also be indicated 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 detecting small leaks. Before use, wash the motor.

@gwg:

Quote:

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

If he doesn't want to find the error himself, then he wouldn't be here, would he?
We are not here in a forum where you can just "get away with" anything.
This forum is not intended for idle chatter or trying to appear knowledgeable.

Best regards, Rainer.

Hello,
The cause is likely the pressure buildup resulting from reheating the hot engine; somewhere, something isn't able to withstand the pressure. The electric radiator fan might not be included.
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 be working, and excessive pressure should be automatically avoided, right? I'll try the tip about the tire valve. I'm going to try installing a T-piece with some tubing and a tire valve into the return line to the expansion tank, so I don'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 a blown head gasket – exhaust gases are being forced into the cooling system, which increases the pressure in the cooling system until it reaches the pressure relief valve on the expansion tank. This causes the coolant to leak, and that's where the coolant stains are coming from. If your engine has the designation AJM, then you've either won or lost.

Regards,

hb2000

@hb2000:
I doubt that, 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 then normal again. That doesn't fit with a blown head gasket for me.

Hi broesel,

@hb2000:
I doubt that, 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 then normal again. That doesn't fit with a blown head gasket for me.

hello broesel

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

Hello broesel,

I don't want to disillusion you, but I'd eat my hat if that's not a ZKD issue. The first time I had ZKD damage (at 210,000 km), I had exactly the same symptoms. Now, after two years of inactivity (260,000 km), something like that seems to be starting to happen again.

See: /viewtopic.php?t=8172

Keep your ears perked up!

Regards,
Thomas.

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

Hello!

Does your heat exchanger occasionally make a gurgling sound in the morning when the engine is cold? I had a similar problem and suspected a head gasket. I'm currently using G12plus coolant. I believe it might be sealing (small) leaks. I have white residue from the coolant in two places (one at the heat exchanger connection, and one between the turbo and the engine block), but no more leaks! It's possible that my VAG workshop (an older, more traditional one) might have added a coolant sealant to the engine. However, I wasn't told anything about it... Anyway, for the time being, everything is closed!

Hello,

I have now squeezed the system.
The pressure relief valve on both the old and new caps releases correctly between 1.3 and 1.5 bar (it starts at 1.3 bar and then opens progressively). I installed a car tire valve into a T-adapter and then connected the T-adapter to the return hose leading to the expansion tank. As the pressure built up to the release pressure, the level of 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 is not 100% airtight). Without pressure, the liquid level was the same as before (so there was no noticeable visible liquid loss). There are no visible signs of any leaks, and there are no traces of fluid on the ground underneath 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'm going to repeat this test now to rule out a ZKD.

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

PS: I also haven't experienced any problems with the current company (mostly half-hour commutes to and from work).
Perhaps I simply need to wait until the loss also manifests itself in the normal course of business.

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 expansion tank cap. Another indication supporting the theory of a faulty cylinder head gasket is a gurgling and splashing sound in the heat exchanger, which is located near the glove compartment inside the car. It seems to be a common issue with VW, 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 other TDIs, will experience this problem. My Passat has now been running with this defect for 25,000 km, and it now 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, check the coolant reservoir.
These two observations hardly take any time and offer 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 topic, there 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.

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

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