Hi people, got this puzzle that keeps doing my head in . My cars runing fine with no problems except that after a drive i get air reappearing in the top of my radiator .I have a bleed screw on top of the rad so its easy to remove but always comes back .I dont have to keep adding water just venting so it cant be the head gasket right ?

Can't you take a pipe off the bleed screw and connect it to your expansion tank?

Just a thought

Rich

Already done this and it still does it .

Maybe raise the bleed screw end of the radiator a little to get the max amount of air out. Mine does this but my coolant doesn't have any water in it so there's very little air in there either. Maybe consider slowing the water pump to reduce cavitation? 10% reduction in the pulley size can make a difference.

No expert here, but as you have a closed pressurised system, in order to get air in you need to have a source of air at a higher pressure. If you are just clearing out air trapped in the system, then each time you bleed it off the duration of the bleed should be less.

Not sure about the cavitation idea, as cavitation is the creation of vapour bubbles at the impeller of the pump which when they collapse is termed cavitation. This generally leads to destruction of the pump rather than creation of air in the system. You would also be able to hear if your pump was cavitating - doesn't sound pretty.

Your system may be such that you are sucking the coolant so hard at the suction of the pump that you are causing a lower than atmospheric pressure in the expansion vessel and sucking air in through the relief valve, which is then settling out at the highest point (radiator). Fixes for this could be (as suggested) running the pump slower, or fitting larger suction hoses.

Do you have an overflow pipe on the rad neck, and a standard pressure cap? The cap will vent water out when it gets hot and expands, then it allows air back in as it all cools down. If you put the overflow pipe into a small bottle the water will go ito that and then get sucked back into the rad when things cool off. The pipe has to go to the bottom of the bottle of course!
Where abouts in Bedford are you?

I had this problem! I discovered that the level in the header tank was rising and air was accumulating in the radiator. Turned out it was the positioning of the electric water pump relative to the bleed back to the header tank that was wrong and it was sucking when it should be blowing. A few moments reflecting on how the pressure varies as the water is pumped round produced one of those Doh! moments and after a flurry of spanners and various hoze sections all was well.
Bottom dollar is are you sucking air from outside the system or within it.

Thanks for the replies, the header tank is the highest point off the system . The system is not overflowing. Dingz im in Alburgh close where are you ? V8kid i think you may be on the right idea im going to do away with my top vent pipe to the header tank and see what happens ....