you can have a head gasket go without effecting compression as there is several places it can go-oil to water, oil to bore, bore to water etc, if you think its a head gasket you can get a testing kit which u push into header tank, let the car warm up then thr chemical inside the kit will change colour if gasket has gone , i got told a few times my gasket hadnt gone on my old frst, i knew it had, i got odd miss fire on start up for a few seconds etc. great bit of kit though.