Hello, I remember reading here in various threads about a test for knowing if there is a compromise head gasket is to remove the coolant reservoir cap and sound will be heard. I can't seam to figure out if the "sound" is a hissing or a suction. From my understanding a hissing, pressure coming out, is normal, however if it is a suction(pressure going in) then that's the sign of a bad HG. Can someone confirm this? I recently removed my cap and there was a hissing, in which air and tiny splatter of coolant came out of the tank.
Depends on when you open it. If the engine was running recently, the coolant is hot and there will be pressure. If everything is cold, there should not be much pressure difference either way. If there is an exhaust-to-coolant leak at the head gasket, the system may still be pressurized with exhaust. If there is a leak of coolant out, there may be a negative pressure. (Many coolant caps have a relief valve for negative pressure too, though. I've never looked really closely at this one to see.)
Like Chapman said, sometimes based on changes in temperature a small amount of air will blow out, or small amount of air will be sucked in, even in a healthy Prius... A failed headgasket on the other hand will lose significant coolant so the sucking in sound will be much more extreme. But in general, the easiest way to know HG failure is both engine running rough periodically and how much coolant you have to add.
