I am hooking up a aftermarket oil pressure gauge. It was in my truck when i got it just not hooked up. There is oil in the line running from (like it has been hooked up before) i just need to know how to do it thank you.

 

if you have a oil live that goes into the gauge you will need a oil pressure switch that is compatible, the line runs from the switch to the gauge...

 

is this a mechanical or electric gauge?

 

Check and see what type oil pressure sender you have on the engine now. Something is in the hole. If not the oil will shoot out when the engine is started. Unscrew the sender. It might be just capped off or it might be the pressure switch for and electric guage. But you need the threads to match. So take what ever unscrews to the auto parts store and match the threads to the fitting. This ferrel fitting/connector is what will connect the plastic tube to the engine. The adapter kit for direct reading gauges is about $8 at AutoZone.