Expansion tank cap

Ronnie Beck

Need some advice. I have had a few coolant leaks lately. Can somone answer these questions for me too.

1. Do expansion tank caps release pressure from the system?
2. Can they get stuck closed and cause the system to overpressurize and create leaks.

My water level drops when hot and rises when cold. I read in the manual it should rise to the max when up to running temp. I've had leaks from three seperate places in the last while. The leaks only seem to happen when the system is good and hot.

Thanks, I'd love to get to the bottom of this problem

On a T4 transporter. One of these caps.

mickdw

Yes, that cap has a pressure release system. The air is released up through the centre section and down and out all still inside of the screw thread of the cap. They can go faulty. Im no expert but I would have thought if the system was over pressurising that fluids might get lost out of the system. You appear to say that when hot the systems levels drop and then recover when cool without any topping up.
Is there any air trapped in your system?

Ronnie Beck

mickdw wrote: »

Yes, that cap has a pressure release system. The air is released up through the centre section and down and out all still inside of the screw thread of the cap. They can go faulty. Im no expert but I would have thought if the system was over pressurising that fluids might get lost out of the system. You appear to say that when hot the systems levels drop and then recover when cool without any topping up.
Is there any air trapped in your system?

Thanks for that mick. I might try a new cap. The valve piece on it is sort of discoloured like ther is residue on it.

I first thought it was the bleed screw leaking so I broke that over tightening it... Sealed it up so that it's no longer leaking. I would assume it's well bled if that been leaking.

The coolant level kept going down so I took off the under tray and found a leak from from the coolant temp sensor so I replaced the o-ring. It's no longer leaking but there is a leak from the flange that that sensor sits in. The one beside the water pump.

Now I'm not sure if it was the flange leaking all along or if it's just leaking through the weakest point as the cap is stuck shut??? Nothing leaks when the engines off and cold.

I had to open the cap after a long spin to top up with coolant. When I did there was a large release of pressure, alot more than I expected.

Nissan doctor

The cap only releases excess pressure, i.e if there is a fault causing the system to over pressurise(airlock, head gasket etc). It shouldn't be releasing anything under normal conditions but it could do if it was faulty.

Almost all VW's use the same cap so you could easily swop one over and see if you get any changes although the fact that you are springing leaks from a few places may well indicate that the system is actually over pressurising.

Ronnie Beck

Nissan doctor wrote: »

The cap only releases excess pressure, i.e. if there is a fault causing the system to over pressurise(airlock, head gasket etc). It shouldn't be releasing anything under normal conditions but it could do if it was faulty.

Almost all VW's use the same cap so you could easily swop one over and see if you get any changes although the fact that you are springing leaks from a few places may well indicate that the system is actually over pressurising.

Thanks for that ND. Must be an airlock so. Maybe I didn't bleed it fully when i put the coolant back in. No other head gasket symptoms. All the coolant disappearing can be found in the undertray.

It's hardly dropped at all this week. It's leaking alot slower now, I'm guessing because the sensor and bleed screw leaks are plugged and it's only the flange that's dripping a bit now. I ordered a new pipe with the bleed screw today and a new cap just in case it's faulty. Twas only a tenner.

Nissan doctor

Ronnie Beck wrote: »

Thanks for that ND. Must be an airlock so. Maybe I didn't bleed it fully when i put the coolant back in. No other head gasket symptoms. All the coolant disappearing can be found in the undertray.

It's hardly dropped at all this week. It's leaking alot slower now, I'm guessing because the sensor and bleed screw leaks are plugged and it's only the flange that's dripping a bit now. I ordered a new pipe with the bleed screw today and a new cap just in case it's faulty. Twas only a tenner.

Just keep in mind that a cooling system with any leaks cannot pressurise fully and therefore the coolant will be on the verge of boiling point as the fan kicks in which will lead to further damage if left for too long.