I recently bought a 2003 s40 1.9D, needless to say the previous owner didn't take great care of it before me, but nonetheless i have changed the head gasket, water pump and thermostat. the gasket was clearly blown and it had been using water.
when i put it back together it drove fine for a few weeks but now the temp randomly rises to the red while i'm driving and falls back to normal after 15-20 secs.
i think there is air in the system but i'm not sure that i'm bleeding it correctly, is there a method for this?
when hot there is pressure in the system, often a hissing sound coming from the reservoir cap, could this be a source of air? no leaks that i can find while hot or cold.
also one thing i have noticed is if i check the water level, and add water if required opening the bleed screws etc, it will not over heat for that journey.
if i leave it stopped for a while (like at work for the day), it will then overheat on the return journey, rising to the red for a few secs then returning to normal when i can drive as normal again....
this has me stumped , any help anyone can provide would be greatly appreciated!!
Cheers,

 

check your thermostat. it might be stuck closed and won't open when needed to.

 

Yeah i think i'll put the old one back in for sport... see if that makes a difference,
thanks!
any advice on the correct way to bleed the coolant system?

thanks for your help!