Coolant loss and pressure

[Steve Taxi]

My w203 c220cdi 51reg has been loosing a bit of coolant for about 4 months/ 20k miles, no leaks, water pump is sound, no overheating, it seems to have excess pressure in the system and when it gets hot in traffic it blows a bit of coolant from the cap, there are small bubbles in the system forming a foam, the pressure even stays overnight when its cooled down, the top hose is solid to the touch when hot, ok it sounds like the head gasket but if theres a leak from the cylinder to the water jacket why is it still pressurised in the morning, why does it not leak back overnight, anything else I should check before tearing the head off?

 

[BlackC55]

First thing to do is a compression test. Any leak from the combustion chamber into the water jacket will show up here.

 

[Legoman]

check the thermostat isnt restricted or slightly jammed

 

[Steve Taxi]

Took the thermostat out and left it out just to make sure, just made it warm up slower, its not overheating at all.

 

[IAN MAC]

Got to be head gasket, change it before it damages the head.

 

[jberks]

does sound like head gasket. It could be a slight one. remember, the pressures in the cylinder are way higher than in the coolant system so if the leak is slight, it could well be that the coolant pressure isn't high enough to seep back.

 

[television]

With a faulty head gasket, its possible to give the effect of boiling by blowing up the system with a leak from a cylinder, it looks as though it boiling, but its not

 

[MIW615]

Apart from having a compression check carried out I would also have the coolant checked for exhaust gases, there are devises that "sniff" the coolant and detect any leakage. I've used them many a time and it is a good way of confirming a problem does or doesn't exist before you go ripping off the head

Basically the "sniffer" contains a blue fluid and turns green if it detects and combustion leaks.

Something like this ..........

 

[Steve Taxi]

Thanks for your input I am taking the head off tomorrow, any advice would be good from someone who has done it, Steve.

 

[Steve Taxi]

Update, the head is off but its not good, it is cracked across the valves on No4 cylinder, total mystery because I have had the car for 150k miles and its never overheated, its now done 240k, managed to find a used head so should be rolling tomorrow.