Expansion tank bubbling?

[RS-Ronan]

Well lads and lassies.
A few weeks back while dropping my GT to my unit she started blowing steam from the rad and expansion bottle even though the gauge was reading around the normal level.
I assumed the worst and thought the headgasket was gone so left it alone for a week or 2 and decided to do a compression test 2 nights ago which came back as follows,

172 172 169 173

That seems to be fine(fair enough theres a slight drop in psi but its nothing to worry about i've been told)
After fiddling with the rad cap I let the car run again and it was fine,no steam or bubbling or a bit.
Did the same tonight but after taking it for a spin and pulling up in the unit,there was no steam or a bit but after the car had turned off you could hear the expansion tank bubbling and it let out some water but once again,gauge was reading as normal.

I'm going to change the rad cap and thermostat asap,also noticed there is very little pressure in the top rad pipe.
Any ideas on whats causing this?

 

[Tim@TBDevelopments]

very little pressure in the top rad pipe is normally a sign the system isn't pressurising properly, so your on the right track with replacing the cap first and retesting

Tim

 

[weeJohn]

More than likely the rad cap, but if the water pump is not circulating the water through the engine properly, it could be boiling in a hot spot in the engine and causing bigger air pockets. When they make their way round to the rad they bubble out into the expansion bottle.

Change the cap first, see how it goes, its hard to test if a pumps not working very well without removing it, then its best to replace it when its off.