Oil pressure question

[natsgrampy]

89 mercruiser 3.0. Started it up today on the muff in readiness for a fishing trip tomorrow.

It started right up and I noticed the oil pressure gauge went up to 40 from 0 and then back to 0 and stayed at 0, no pressure.

How do I tell if this is an oil pump issue, sender unit, or gauge issue?

 

[Forbes210]

I might sound silly but did you check an oil level?

 

[natsgrampy]

Yeah oil level is fine

 

[Bt Doctur]

With the key "on" ground the sender lead and the gauge should peg full scale =power to gauge and wiring good.
The dirty way= remove sender, start motor, oil should squirt out of hole,
The clean way= install a mechanical gauge , start engine
mech gauge reads oil pressure= replace sender