I've got a car with a radiator punctured near the bottom. There is no coolant in the radiator or in the expansion tank - that much I can see. I assume coolant is gone from the engine block as well (by flowing through lower house into a leaking radiator).

This is pretty much the same condition when we drain coolant from engine block and from radiator doing the flush.

Now, after I replace the radiator and fill it up, some of the coolant will get into the engine block via the lower hose. I expect the air pocket will form in the engine block because thermostat remains closed (it is high on the engine block side and has no hot coolant touching it). As the result, coolant level on the engine block side will be low. The water pump is located quite a bit high as well.

How does the system start to work and circulate the coolant?

1 Answer 1


Most thermostats have a small bleed hole to help filling. This works best when filling slowly as the hole is relatively small. However, most car manuals will also give instructions about the bleeding or burping procedure for that car / engine.

So, consult the manual as it will tell you if a particular hose needs loosening or the front of the car needs raising etc.

  • Also, if available, vacuum filling the system is a good way to ensure the coolant gets to where it needs to be. Also, coolant systems with purge tanks self bleed. There just needs to be enough coolant to reach the water pump. Feb 16, 2021 at 16:26
  • 1
    (author's follow-up on this answer) @solar-mike is exactly right: in the FSM for my vehicle I found that there is an air-relief plug on the engine-block side of the cooling system. FSM instructs to fill the radiator until coolant starts flowing out of the air-relief plug. That definitely eliminates the risk of creating an air pocket on the engine-block side.
    – Dimitry
    Feb 16, 2021 at 21:18

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .