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just pull each spark plug one at a time, then you thread the tester into yhe plug hole, crank engine over, and see what pressure you have. mudder trucker.
If it's fuel injected, I'd disable the fuel pump(s) so you don't have fuel squirting in there. Crank the engine until the guage won't go any higher (usually 3-4 compression strokes).
Take out all the plugs and wire the carb throttle plate wide open. One type of gauge has a rubber plug that you hold in the plug hole. The other type threads in the plug hole and usually gives more accurate readings.
I use a remote starter switch and jumper to the small terminal on the solenoid to crank the engine. That way the fuel pump is off and you can do everything from under the hood.
As you can see, there are MANY methods of doing this, but the easiest way I have found is to simply disconnect the coil wires. This prevents ANY spark from possibly igniting the fuel. It is still a good idea to go one step farther and use a remote starter like stated above so you don't have to engage the ignition at all, but this seems to work fine for me. If you dont prevent fuel or spark while testing you could wind up with a serious engine fire and possibly worse. I know a guy who forgot to disconnect the coil and ended up acquiring major burns all over his body when the fuel ignited not to mention major underhood damage to the car. Not a smart idea. Play it safe and you will be fine.
Thanks for everyone's input on this, unfortunately i took my truck into the shop this morning only to find out it's fate, time for a rebuild. Does anyone know of a good reasonable place near MN where i can find one?