Firstly let me explain why your vape may spit hot liquid into your mouth. It all stems down to the cotton in the coil being over saturated. It has absorbed its maximum capacity of liquid and now there's just liquid swimming all around the cotton and coil. This generally occurs after over priming, (putting too much liquid into the coil when first installed). It can also happen after the device has remained upright and unused for a few days. The cotton will always absorb more and more until it can't hold anymore liquid. This excess liquid will not vaporise correctly due to the sheer amount of it, causing it to pop at you when the coil heats up too quickly! So this explains why hot liquid spits up at you but what about liquid even when you inhale without firing the device? Science! There's too much juice in the coil meaning when you suck, you're bringing up that excess liquid into your mouth even without heating up the coil.
So how do we fix this unacceptable predicament?
It's actually a lot easier than you might expect. First thing to do when any device that spits at you is to use some tissue paper, paper towel or similar and twist the corner to make like a toothpick shape, or wrap around a toothpick or wooden skewer, push down into the airway to remove excess juice, do this a few times until no more juice comes out, small head cotton buds are also useful for this. Another option is to remove the coil from the tank and blow through it to remove any excess juice that might be hiding in there. This generally solves the issue of spitting liquid due to over priming the coil or letting the juice sit too long in the tank.
If the vape keeps spitting at you, make sure that the coil has been screwed in correctly and is finger tight. If it's not seated correctly this can also cause spit back as well as leaking.
If using a tank that takes large coils with a large amount of cotton, check the viscosity of your liquid. Some liquids can have a high PG/VG ratio meaning the liquid is quite thin and is absorbed too quickly into the coil. The above steps can remedy this though but it may be worth trying out a liquid with a higher VG content.
A good thing to do to resolve hot spit back is the simplest of the all. When inhaling, inhale harder than you normally would. It sounds counter intuitive but actually the increased airflow stops the coil heating up too fast causing the hot liquid to spit.