V40 low coolant issue

[Nabman]

I have a 2024 V40 1.6 Diesel. A few weeks ago we started getting a low coolant warning and then an overheating warning. Left the car with a garage who done a full head gasket pressure test, drained the system, refilled and cleared the faults. No issue with head gasket found. 5 miles later the warning came back on. Left the car back and this time they said the system was air locked and low on coolant. Replaced coolant etc and gave the car back. Same problem again, low coolant warning, high temp and now going into limp mode. Left the car back and this time they replaced the water pump and thermostat. Cleared all faults and on test drive the problem is still there, tested head hasket pressure again but test was negative.

Has anyone experienced this? When the car is cold, the coolant resevoir is sitting at normal levels but when it heats up the resevoir competely empties and the engine seems to retain the coolant. The level will not return to normal until the cap is released on the resevoit and the pressure gushes back to the resevoir. There doesnt seem to be any coolant loss that we can see. Also the heating in the car works intermittently.

I have been advised by the garage that the next step is to replace the egr cooler as it may be cracked. I dont want to give them the go ahead for this unless they are certain that it will solve the problem as it has cost me £750 already with the issue still not being solved. If anyone has any suggestions, please help!

[GrahamBrown1]

Head gaskets are not uncommon on these, how are they testing the engine?

I would pressure the system up preferably on a warm engine careful removing the cap. Pressure the system to just over a bar and leave for as long as possible.

Any pressure loss indicates a leak. You have had the water pump and thermostat replaced so you can only assume they are quality parts and are good. Exhaust gas pressurising the system is only via the head gasket or egr cooler. To prove the cooler is leaking you would need to strip down the egr to check for damp and signs of coolant.

To prove the head gasket, removal of the injectors and a bore scope may well reveal a leak into a cylinder.

I personally would be suspecting the head gasket from what I’ve seen of these.

[Courageous]

Have you ever seen mist on the inside of the windscreen in colder weather? That would imply a leak inside the car, heater matrix or hoses.....

[canis]

TLDR; Do not open the bottle when hot.

Originally Posted by Nabman

Has anyone experienced this? When the car is cold, the coolant resevoir is sitting at normal levels but when it heats up the resevoir competely empties and the engine seems to retain the coolant. The level will not return to normal until the cap is released on the resevoit and the pressure gushes back to the resevoir. There doesnt seem to be any coolant loss that we can see. Also the heating in the car works intermittently.

I'm confused. You either have a leak or you don't. Either the water is disappearing, or it isn't.

If you're losing water and having to add it regularly, then you have an actual leak that needs fixing. But if it's just disappearing and reappearing, then it must be going somewhere temporarilly, and that can only mean air in the pipes. There might even be bleed nipples on some pipes, look around for them.

Are you saying that:
1) Car is cold, coolant present at level, engine started.
2) Engine warms, Coolant disappears, expansion bottle is now empty.

This is where I get confused.

3) You open the cap (is the engine still hot, or even running?) and the water level returns to the bottle.

Right?

If so, what you need to do is stop the engine and allow it to cool. Do not open the bottle. In normal operation, the water heats and wants to expand but can't, creating pressure. This is important because the engine runs at a temperature higher than boiling point, but water under pressure can't boil. Opening the bottle releases pressure. When you release the pressure by opening the cap, the water can expand, boil, and force the fluid back out the only escape it has - the water bottle. And that's why your water level is returning when you open it.

What you should do is wait until morning. Then, open the cap, and refill to the level. The engine, having purged all it's air through the heater hoses and wotnot, will now be filled. Replace the cap and start the engine. Let it come up to temperature, and allow any air still remaining to gather in the bottle. Repeat this process daily until properly filled. You can still use the car.

In that regard, all fluids should be checked in the morning when cold BEFORE starting. It only takes a few minutes. Make it a rule for life. While you're waiting for your wife to get her shoes on, oil and water. Plus, if you do it before starting, you don't even need to wipe the stick!

The reason I'm saying all this is because if the water was getting burned off in the cylinders, or leaking into the upholstery drip by drip from the heater, or any other scenario where water is lost, then the water is lost forever. It doesn't return magically. And that's why I doubt you have a mechanical fault at all, and it's worth eliminating air using this method first because it doesn't cost any money or effort.

[Simmy]

another oldtimers tip try filling from the drain outlet on the rad.its worked for me a few times must be worth a try another possibility is a pin hole that only leaks when pressurised/hot

[Nabman]

UPDATE: Its definately NOT a leak. So we removed the coolant resevoir cap when the car was completely cold, started the car and kept the resevoir ful while the car heated up to full temp. The resevoir bubbled as quite a lot of air bled out. When the car was fully heated and no more air was bubbling out of the resevoir and we were confident that all air had been expelled we extracted som of the coolant to bring the level in the resevoir back down to between min and max, then we replaced the cap. For the rest of the day the car drove perfectly, no overheating and no low coolant light. The following day when the car had cooled the resevoir was full. Worried that we had overfilled too much coolant we slowly released the coolant resevoir cap and loads of coolant gusged out under high pressure. What could be causing this pressure? When the car heated up again through normal driving the coolant resevoir drains into the system again and the low coolant light comes back on.