Boiling Coolant

[philwhite]

Bit of an odd one that I wonder if anyone can advise on, basically after driving home last night and parking up I got chatting with a neighbour, and about 5 minutes after shutting the engine off I heard a boiling sound coming from the FD, which lasted about 15 seconds then stopped, then again a minute later it started again lasted about 15 seconds then stopped, this cycle to repeated itself about 4 times then stopped altogether.
   
  I had a good look around the car; there were no visible signs of leaking (including the AST) and no coolant coming from the overflow. Once the engine had cooled enough to check the coolant I had a look and there is absolutely no loss (infact its never lost any in two years - \'touches wood\').
   
  This morning I drove to work rather cautiously and waited after I had shut the engine off and the sound seems to have gone?  
   
  The engine itself is only 15k into a rotechniks rebuild and thus far has had no issues, it starts first time hot or cold, and also I had the whole cooling system checked in July.
   
  Is this anything to be immediately worried about, I have a service and MOT in 3 weeks which I will obviously get this checked, but would it be okay to drive in the meantime?
   
  Thanks,
   
  Phil.

[andrewrx7]

I\'ve had several FD\'s and both would have fits of bubbling after being turned off. Never had any issues. If you consider the heat present in the metal work of the engine/turbos/exhaust, and with no coolant or air flow after the car is turned off, it may not be surprising that the heat soak into the coolant might give the occasional bubbling?

[captainbizzaro]

Could it be water returning from the expansion tank back into the system on the cooldown? Then again I know nothing about FD\'s so highly likely I\'m talking tosh!

[ashc]

+1 for water being sucked back from the expansion tank

[slater1982]

mine does it really badly sometimes but other times not at all, have even seen the coolant boiling right up the neck of the expansion bottle at times. was really worried bout it for a while but its done it since i bought it back in april, guess its just another perk of owning an FD
 
but it doesnt seem to cause a problem, just make sure you check the coolant level regular an it seems to be ok.although dont take that as guaranteed alright, just seems to be ok so far!!

[Prof]

could be lack of pressure in the coolant system (a leak), but if that\'s the case, coolant won\'t get sucked back when it cools down.