GL: Coolant water

[Jerry Weber]

True, replacing the pressure cap may solve the problem, however the
overheating of the pressure cap should release the overheated water the
overflow jar. That is the purpose of the overflow jar. Just above the lip
where the pressure cap sits is a very small hole about the diameter of a
paper clip. This hole permits the overheated water to enter the hose which
leads to the overflow jar. I have found this hole plugged in the past due to
contaminates in the water and experienced the same effects you are
experiencing. If a new pressure cap does not solve your problem then check
to see if this overflow hole is plugged. Better yet check the overflow hole
first and save the cost of a new pressure cap, however it never hurts to
have a spare on hand.