Hi
I have a Mercedes atego 2000 reg, the cooling system pressurises overnight so much that when you take of the cap it comes out like a jet
Any help as to cause appreciated
A seized valve in the header tank pressure cap thus not allowing excess pressure to escape is the simplest answer.
Strange stuff water, it expands as it gets hot and it expands again once below +4C. So given the right circumstances and overfilling the header tank it is possible for what you describe to happen. A lot of money can be spent on investigations if overfilling is not ruled out. I cannot get this issue through to my elderly preserved bus customers who insist on filling the header tank almost to the brim. With a 6ft head of water from the upper deck heating system it is common to find that the relief valve in the pressure cap has lifted overnight. With the correct level of coolant there is no problem.
HOWEVER given a vehicle which only exhibits the symptom described it is again possible for a water cooled compressor with a failing head gasket AND an air drier with a failing non return valve to pressurise the cooling system when all is at rest for a long period.
A simple C02 test should rule out whether there is any engine head gasket problem. If sufficient coolant is lost when refilling an air lock may be introduced, but this is more likely to show up as a problem when running.
Your fitter should be able to carry out simple investigations to diagnose this, however it may require several overnight visits.
Thank you, the system has not been overfilled, definitely no engine cylinder head gasket fault, I am going to replace the head gasket in the compressor failing that I will replace the air drier valve the vehicle is 15 year old so could well be both.