Just recently 1 of our 09 Volvo fM is losing water, it only seems to do it under pressure. The driver said he saw the water coming out of the filler cap the other day. We have changed the cap to start but to no joy. I’ve used the truck myself this morning between barking and Leicester id lost 3 litres! There are dried water marks all around the header tank, oil is ok, and no MAYO.
Any ideas?
theres a blockage somewhere could be the pump, waters just getting hot and expanding try draining the system
Sendall5:
Just recently 1 of our 09 Volvo fM is losing water, it only seems to do it under pressure. The driver said he saw the water coming out of the filler cap the other day. We have changed the cap to start but to no joy. I’ve used the truck myself this morning between barking and Leicester id lost 3 litres! There are dried water marks all around the header tank, oil is ok, and no MAYO.Any ideas?
You are close to change head-Gaskit,i rekon
Its not that bad if Pressure goes from Cilynder into the Water,but could destroy Engine if it goes the other Way.
Just get a Professional to check,but i dont think its Thermostat,and if you fitted a new Top and it didnt solve it,…
Sendall5:
oil is ok, and no MAYO.
Any ideas?
Salad cream would be ok 
Can you smell carbon in the header tank, or see flecks of carbon in the header tank? If so then it’s the compressor head gasket.
Have you checked the compressor pipe to the air dryer for carbon blockage ?
whilst the engine is running and the compressor is building up air switch the engine off then let the pressure go in the header tank, re tighten cap leave for a few mins then let the pressure go again, re tighten again and wait a few more mins, if there is pressure behind the cap now it’s coming from somewhere, most likely the head gasket on the compressor, my fh12 had this problem a few years ago so I fitted a new upgraded head and gasket, had no more bother since!
Thanks Harry Monk, there is no sign of carbon in the tank or smell.
Moose this will be the first thing I try tomorrow.
Thanks for your help!
I’d go with compressor head gasket too
coiler:
I’d go with compressor head gasket too
I’d still check the compressor pipe ,it could be the cause of the head gasket .