c70 weird cooling system fault

[evo3500]

Hi all,
Help please, just bought a volvo c70 cab 2.4t gt auto 2000 model has a cooling system fault has done 97,000 miles scratching my head a little over it. Guy told me that car was running spot on then stat stuck shut he fitted new ran it for a day then found coolant blowing out past header tank cap,he fitted a new header tank cap still the same he reported pipes going hard,and after 20 miles or so temp ok but pushing water out of expansion tank cap and coolant level dropped well thats the story ! Also I have noticed cooling fan dosnt seem to be cutting in.

This is where I am put dye test kit over header tank fluid dosnt change colour ( I havnt added a dye to the cooling System) to indicate exhaust gas in cooling system, oil is not contaminated neither is water, only strange thing is if you leave cap off expansion tank and run for a while some white foamy froth like washing up liquid goes like when you add water to it,starts to appear when the circulation begins. No visible external water leaks. car starts perfect from cold or hot no misfire,spark plugs all the same colour no sign of one being washed, exhaust seems steamy on start up although this could be paranoya setting in and this may be normal !
Once warm seems normal, found small leak at header tank pipe have repaired that and am going to fit a new water pump as a matter of course,and try again. Anyone got any ideas or am I just avoiding the inevitable like is it the headgasket ?
Water is going somewhere and the system is pressurising,also had it on pressure test holds pressure !

Is it something i have missed that is specific to volvos I will update after I test with the new water pump.

All ideas theorys welcome,I have been told it is not to common for a headgasket failure.

Thanks

All:(
Simon

[princepugh]

Does the car warm up in a timely manner? Once warm, does the gauge stay in the middle or is it fluctuating? Any idea if he did the temp sensor as well as the thermostat?

[evo3500]

seems to warm up as normal and gauge goes to the middle if that helps

[evo3500]

temp sensor was not replaced,als has the common abs control unit fault on the car abs and srt light coming on and speedo going off,but I am going to repair the board, dont think this has anything to do with the pressurising fault but thought I would mention it just in case

thanks

[evo3500]

quick update fitted new water pump even though couldnt see much wrong with the old one now seems Ok fingers crossed,noticed vvt unit noisey now anyone know where i can get one from.

thanks