Coolant system over pressurising on long trips

[Chris_Golf7]

Hi Everyone,

I'm new here and I was hoping I could get some advice on an issue that I've been having with my 2014 Volkswagen Golf MK7 110 TDI HIGHLINE.

The issue started around 18 months ago. On long trips (250km+/ 2.5-3+ hours) the car would start to lose coolant.

The mechanic has looked over it and there are no signs of a leak from anywhere.

They thought it could be the EGR so that was replaced but the issue continued to occur.

The coolant expansion tank was also replaced but didn't fix it.

The intake manifold/intercooler was also removed and checked but there is no leak in there.

The mechanic also looked inside the cylinders and it doesn't look like coolant is getting inside.

It appears the coolant system is coming pressurising and pushing coolant out of the expansion tank but only after it has been operating for a long time.

I usually have to top the system up about 300-400ml after this happens and I've been able to drive the car for 30 minutes or so after the coolant alarm has come on without any issue. The car also does not overheat when the issue occurs it sits on 90 degrees.

Any advice would be most welcome.

 

[SRGTD]

I’ve seen a partly blocked heater matrix mentioned on some forums as a possible cause. Does your heater blow hot air into both driver and passenger sides of the cabin?

 

[Chris_Golf7]

Na. The air conditioner works fine.

 

[gixxerfool]

Na. The air conditioner works fine.

I think he means, when the heat is turned on does it blow hot air evenly temp wise out the vents? Does one side seem cooler than the other?

I wonder if your fans aren’t coming on high enough or if your cooling stack is clogged causing low flow. Could be a water pump starting to fail, although I think that’s unlikely since you have to drive so long to get it to occur.

 

[Chris_Golf7]

Aww sorry. Yes when the heat is on it blows evenly from which ever AC vents are open.

It's very weird as the issue has been happening for 18 months and it hasn't gotten any worse.

Could it be possible the fans aren't operating well enough and it is overheating the coolant enough for it to expand and overflow but not enough for the car to overheat?

It does seem to happen sooner during Summer as opposed to Winter.

 

[gixxerfool]

The heater core is a possibility. I would check your cooling stack and make sure you have good flow and cooling. You may want to consider changing the thermostat as well.

 

[Jose_Gti]

Water pump failing? Or maybe something as simple as the reservoir cap, sometimes under pressure it vaporize through the cap, no residue, everything works… could be….

 

[gixxerfool]

Water pump failing? Or maybe something as simple as the reservoir cap, sometimes under pressure it vaporize through the cap, no residue, everything works… could be….

This is a great point too. Cap is an easy and cheap fix.

 

[Chris_Golf7]

This is what it looks like after it has expelled coolant so it is very visible.

We replaced the coolant tank including a new cap and it kept happening so the cap was replaced again but it continued to happen.

Can you explain how a failing water pump or thermostat might be causing the issue?

I was thinking with the thermostat once it heats up and is fully open it does not have any other impact. Unless it doesn't fully open?

Thanks for all the advice so far.

 

[gixxerfool]

This is what it looks like after it has expelled coolant so it is very visible.

We replaced the coolant tank including a new cap and it kept happening so the cap was replaced again but it continued to happen.

Can you explain how a failing water pump or thermostat might be causing the issue?

I was thinking with the thermostat once it heats up and is fully open it does not have any other impact. Unless it doesn't fully open?

Thanks for all the advice so far.

View attachment 257577

Ok. So we can rule that out.

Both items would kind of be one in the same. Low flow/restricted flow. The thermostat could be opening, but not fully. Flow is good for the most part but the spring could be getting lazy after extended periods and partially shutting restricting flow.

If you have a water pump that’s slipping or not spinning fast enough due to premature failure, it could be restricting flow. Goes for the radiator or heater core.

One other, but I highly doubt it, is an early head gasket failure. It could start pumping exhaust into the coolant jackets and over pressurizing the system. I don’t think this is though.

The odd ball symptom is the length of time to produce this issue. Most of this should be repeatable at full temp regardless of time. So the question is, what about the drive time makes this lose coolant?

You could do a block test. Most shops will have the tester. It’s a tube with a bulb on top with a reactive fluid. It starts blue and changed to green or yellow depending on the concentration of exhaust in the coolant. If it stays blue, you’re good. Anything other than the original color would point to a head gasket, head or block issue. I don’t think it’s your issue, but it’s another data point.

 

[Chris_Golf7]

No evidence of a leak or trail anywhere on the car or after stopping it. The only area I've seen it is the coolant tank as above.

I have a friend who is a mechanic test the coolant after driving it for 2 hours and it showed up negative for carbon dioxide in the coolant.

 

[gixxerfool]

No evidence of a leak or trail anywhere on the car or after stopping it. The only area I've seen it is the coolant tank as above.

I have a friend who is a mechanic test the coolant after driving it for 2 hours and it showed up negative for carbon dioxide in the coolant.

Did he check for cool/hot spots in the radiator?

 

[Chris_Golf7]

No, Is that where you'd check for different temperatures to tell whether it is or blocked or not ?

 

[Jose_Gti]

Maybe too obvious but put the system under pressure and see where does it leak. You can rent the tool or buy it for cheap on eBay DAYUAN 28pcs Universal Radiator Pressure Tester Kit, coolant Pressure Tester kit coolant Vacuum Refill kit for Cooling System https://a.co/d/gUj9t6Q
What seems strange to me is, that it’s leaking overflowing from the expansion tank…
I have a GoPro that I’d put there and go for a ride, that way you could see how is really leaking.

 

[Chris_Golf7]

The other thing that was suggested is that it is the turbo-creating pressure back in the cooling system through the manifold. Not sure if that would be possible?