You seem surprised that the GPs do not get hot to the touch when installed in their proper location. Think about this for a minute.
When you tested them outside the engine, the heat had nowhere to go. Of course the bodies got hot. They are close to the source of heat and there is no where for all the heat to go, so it stays on the GP body.
However, when they are screwed into the head, the head makes an excellent heat sink. The threads are a good, clean, metal-to-metal contact path for the heat. So instaed of heating up the GP body, it transfers to the head. The head is a large heat sink and can absorb a lot of heat before you could feel any temperature change.
If the volt meter shows that they are drawing energy, it means they MUST be working--otherwise there would be no current flow and no heat.
I would guess that your GP and circut are fine. Look at the other issues addressed by other answers to your posts.