Morning peeps,

been out for a drive for awhile and when parked up I thought I could here a slight hissing noise,, open bonnet and I could here air leaking from the expansion tank ???

when I grabbed the top radiator hose it was solid and I could hardly squeeze it,,,

is this right surely it should not have that much pressure in it,,

what could be the likely symptoms that could cause this. 
not losing oil no sludge on cap or in oil 

could a stuck thermostat cause this?? It still only sits at 90degrees 

thanks peeps 

The maximum pressure is set by a valve in the cap on the expansion tank. Try another cap.

Common reason for air/gasses escaping from the cap valve is a failing head gasket that allows cylinder gasses to reach the cooling system and pressurise it.

Check the coolant level when hot by looking at the outside of the tank and also when cold by removing cap.

1) Is the hot level very low? 2) does the cold coolant level rise significantly as you take off the cap?

 

 

5 hours ago, pikpilot said:

The maximum pressure is set by a valve in the cap on the expansion tank. Try another cap.

Common reason for air/gasses escaping from the cap valve is a failing head gasket that allows cylinder gasses to reach the cooling system and pressurise it.

Check the coolant level when hot by looking at the outside of the tank and also when cold by removing cap.

1) Is the hot level very low? 2) does the cold coolant level rise significantly as you take off the cap?

 

 

Cheers for help mate,,

the coolant level was level with the top of the cap when cold,, I checked it this morning and there was next to no pressure in and the coolant level was normal. When I got in to drive home the the red temp light came on so I topped it back up when I got home. Probably a cup full but I probably lost that releasing the pressure. 
 

how the help can one day it have so much pressure then 2 days later it be normal 🤣🤣. I’ll be keeping an eye on the level and I think I’ll go at weekend to garage and see if they can do the sniff test and see if exhaust gases are getting in the coolant 

How long have you had the car?

 

Are you overfilling the coolant and not allowing for it setting it's own level.

 

Bottles have MIN / MAX on them and not FILL TO THIS LEVEL.  

 

Is this Coolant going in which is at the correct ration.   Coolant is Anti Freeze / Summer Coolant & Rust Inhibitor all in one and has a different freezing and boiling point from water.

 

Be sure what is in the Coolant System is strong enough Anti Freeze if the weather gets cold / freezing in your area. 

As toot says, observe the minimum and max levels of the coolant. It is there for a purpose. If you fill with coolant right to the cap, you have no space for it to expand into as it gets hot. That is why it is called an expansion tank and relies on the compressibility of the air above the liquid. 

Don't top up with water. Use the correct coolant for your car such as G13 and variants.

G12+ is what has been put in by me as I hadn’t a clue when it was done last ??  I did this about 4 months ago and as far as I’m aware I have had no issues since doing this. Only kinda noticed due to not having any warm air from the vents (about a week ago) thus making me check out coolant level and noticing the pressure,,, coolant level when I changed was upto the minimum mark and I have never put any extra in since doing this. I’m kinda thinking that this pressure issue is when I don’t get any hot air from the vents. 
 

I have just topped up tonight like I said because I’ve been trying to release pressure and coolant squirting out in the process 

 

pikpilot I’ve just noticed your G13 spec coolant but I’ve put G12+ could this be the issue ???? Wrong spec maybe ???

Coolant spec is fine, G13 is just what VAG use now.