they are on the fuel rail, which are above the engine.
they have what looks like a cap that could be used for a tire valve stem,
there is also a schrader valve that is in it.
standing in front of the vehicle, it will be about 1/3 the way from right to left
above the engine.
the fuel rail is what brings fuel to the injectors. it is silver in colour, (stainless or aluminum or chrome looking)