BruceK
Well-Known Member
Recently for a number of reasons I have had to drain my engines of coolant before removing parts for access to the back of the engines. As I have always drained the engines I have never had a problem with airlocks or overheating subsequent to the fact. Recently I had to replace a turbo and because it is high up on the engine I just removed the expansion reservoir and on the coolant side leaving the heat exchanger on and drained the turbo before replacing. I thought this would save me the ~100 odd quid in coolant required each time I drain an engine.
I was unable to sea trial at the time due to replacing the servo pump but on running the engine at idle I got the typical sit forever at 60C before the temps suddenly sky rocketed. After a bit of puckering and consternation and a call to a friend we determined it could not be due to a gasket misalignment on the turbo and highly unlikely to be a recently overhauled coolant pump.
Loads of pipe squeezing later, cracking off the turbo bleed banjo bolt etc later and still no coolant movement through the block.
It was only after removing the thermostat cover and removing the thermostat plates that the cause could be seen. Coolant flows from the pump in three directions with a fourth being the calorifier which we will discount for now. The first direction is around the block, the second around the heat exchanger to the top of the thermostat, but the most telling is the third under the exhaust manifold, round the turbo and back through the manifold to the bottom of the thermostat housing. When the engine is cold the thermostats are closed and so topping up a half-filled system up with coolant creates an airlock in the thermostat housing and pump being immediately below the thermostats which prevents the pump from pumping water around the block, while filling the reservoir does fill the manifold and heat exchanger leading you to believe the system is full and bled.
Removing the thermostat housing and plate you can run the engine for a few second to purge the system of air with minimal coolant loss (surprisingly) before topping up.
Hope that tip might help someone in future. I was almost suicidal in despair at the though of either having to strip the coolant pump or turbo out again, both being pig o' jobs.
I was unable to sea trial at the time due to replacing the servo pump but on running the engine at idle I got the typical sit forever at 60C before the temps suddenly sky rocketed. After a bit of puckering and consternation and a call to a friend we determined it could not be due to a gasket misalignment on the turbo and highly unlikely to be a recently overhauled coolant pump.
Loads of pipe squeezing later, cracking off the turbo bleed banjo bolt etc later and still no coolant movement through the block.
It was only after removing the thermostat cover and removing the thermostat plates that the cause could be seen. Coolant flows from the pump in three directions with a fourth being the calorifier which we will discount for now. The first direction is around the block, the second around the heat exchanger to the top of the thermostat, but the most telling is the third under the exhaust manifold, round the turbo and back through the manifold to the bottom of the thermostat housing. When the engine is cold the thermostats are closed and so topping up a half-filled system up with coolant creates an airlock in the thermostat housing and pump being immediately below the thermostats which prevents the pump from pumping water around the block, while filling the reservoir does fill the manifold and heat exchanger leading you to believe the system is full and bled.
Removing the thermostat housing and plate you can run the engine for a few second to purge the system of air with minimal coolant loss (surprisingly) before topping up.
Hope that tip might help someone in future. I was almost suicidal in despair at the though of either having to strip the coolant pump or turbo out again, both being pig o' jobs.
