Ideally when the engine is up to temperature the top hose should be hot and the bottom hose slightly cooler. This is because heat is given off by the radiator into the outside air, so that by the time coolant exits the bottom of the radiator (which by the way is the wrong name for it because it doesn't radiate heat at all, it convects heat) it's lost enough of its heat to cool the engine again. If the bottom hose is stone cold when the engine is hot then you clearly have something wrong. However, it does happen. I would first run the engine and monitor it closely until it reaches the point either where the temperature gauge is getting higher than it usually does, or the pressure in the cooling system (check by squeezing the top hose) is getting uncomfortably high. If by this point the bottom hose is still cold, and the cooling fan has not cut in then go no further. You have a clear problem.

Allow the engine to cool right down before starting any repair work. Check to make sure your new thermostat is fitted the correct way around. Some are in the flow side and others in the return. Whatever yours is the wax bulb must face the engine. If it's the other way around it will never get hot enough to open and the engine will not cool properly. Also make sure the cooling system is properly bled. Do this by removing the coolant temperature sensor or other component in the cylinder head when filling the system. This will allow air to escape. Running the engine will not assist in bleeding if it's not bled properly. The water pump must be fully submerged for it to work. It will not move air.

Some engines are very difficult to bleed of air, though I dunno if the Astra is one of them.

The "professional" solution in these cases is a vacuum filling device, but I don't have one of those.

I've had good results so far with the Daihatsu Skywing (which I couldn''t successfuly bleed otherwise) by allowing the coolant to boil into a funnel jammed into the radiator fill port. Bit scary but it seems to work. I assume the steam displaces the trapped air.

It's impossible for air to get trapped if the cooling system is open at the top, since coolant is heavier than air and will always displace it. Air can only get trapped if it has nowhere to go.

And yet it happens.

It could happen because the cooling system is not open at the top (which I never said mine was) or it could happen because there is a local high point which air rises into, even though there is another point higher up which is open.

If you find that hard to grasp, think of an inverted tumbler in a bucket of water, or a diving bell.

Or continue to believe its impossible. Maybe you will never own a car which will prove you wrong.

I have been working on motor vehicles large and small for a living for over 40 years, and I've never had a serious problem bleeding a cooling system. Now that may just be a complete fluke, or it could be that I have a good understanding of cooling systems, how they work and how to bleed them.

I have an identical Astra. Used the car for the first time today since the original post. Drove about 10 miles and when I stopped I checked top and bottom hoses. Top very hot, bottom quite cool so I don't think you have a problem.

It could be that the radiator is very efficient at dissipating heat, which is a good thing of course. But not only do you not want return coolant being too hot, you don't want it too cool either. The way to ultimately determine whether or not there's a problem is to check the cooling system function with a pressure tester. Any technician diagnosing a cooling system fault should be doing that.

Edited by Railroad. on 11/06/2021 at 14:37

Think that only tells you if you;ve got a leak.

Not at all. Connect the pressure tester to a cold engine. Then start it and allow it to warm up. As is warms the coolant temperature rises as it absorbs heat from the engine. The pressure rises proportionately at the same time. When the thermostat opens coolant will flow through the radiator, giving off some of its heat to the outside air. The coolant then exiting the bottom of the radiator will be cooler than it was at the top. This coolant is now drawn by the water pump to cool the engine again. As it does the pressure will reduce. If the engine gets too cool the thermostat will close again and the temperature and pressure will increase. If/when the cooling fan eventually cuts in because there isn't sufficient air flow to dissipate heat from the radiator the temperature and pressure will reduce again. When the fan stops the temperature and pressure will increase again, and so the cycle continues. The cooling pressure cap is an overall safety valve which will lift if the pressure exceeds its rating. But this shouldn't happen if the cooling system is in good condition because the pressure in the system is regulated by the temperature.

A cooling pressure system tester is an essential tool for accurately diagnosing problems. It seems that there is some sort of enigma attached to cooling systems which makes some people think they are complicated, where in fact they are very simple.

The thermostat is what causes any problems with bleeding. When it's closed the system is effectively divided into two compartments. This is how air can become trapped. That's why a bleed facility needs to be created at the highest part of the cylinder head. If there is a component that can be temporarily removed then air will escape as the system is filled. As I said earlier coolant is heavier than air. Air cannot remain where it is if coolant is pushing on it. That's impossible. Contrary to popular belief, running the engine with a partly bled system will not help it to bleed. Gravity alone will do that. The system must be properly bled before the engine is started.

The MG-F is probably the most tricky to bleed because the engine is at the rear and the radiator is at the front.