I would try the following - top it up to the minimum mark just before a journey you have to make. When you get back, park on the level, leave the warm engine idling, and put something under the car like a big piece of cardboard to show any fluid drips. Mark the coolant level with a felt tip pen, and leave idling for an hour. This test should only cost about £2 of fuel. If the level has dropped and there is no sign of a leak, the head gasket is failing.

If the level hasn't dropped, but only drops when the engine has worked hard over a few hundred miles, the leak could be anywhere - but is probably most likely to be very early head gasket failure, long before the definite diagnostic signs of oil filler cap mayonnaise and over-pressurized coolant reservoir appear.

Can be quite difficult to distiguish airlock and HG failure. I had blowing from the expansion tank for a couple of weeks following a hose failure, drain and flush which I thought (and was told) was an HG (3-cyl engines are prone) but its apparently stopped so was probably just air in the system.

I'm told some engines (VW Golf was mentioned IIRC) are especially prone and need a vacuum fill.

Running the engine with the rad cap off you need somewhere for the coolant to expand to, such as a tight-fitting funnel, and it might boil, which would be alarming, though it would probably purge the air. I chickened out before that point because the funnel was almost full.

I've been told (by Americans, who probably run V8's) that running the engine in normal use to purge air is standard procedure.

Seems risky to me, though effectively that's what I did.

Where there is air cooling must be compromised, and if thats in the head, which is likely to be the highest point on most engines, and where the exhaust valves are, there seems to be a risk of serious damage.

Next time I'll try "burping" it with a big sink unblocker syringe, or maybe use a venturi pump.

Edited by edlithgow on 16/01/2019 at 02:09

If it's working,leave it;there shouldn't be any hot flow thro' it anyway if the system is working correctly.