Just bought a used 2002 Grand Cherokee and the oil pressure guage shows zero on starting till I drive a couple of miles. Just like freesherb in March 2011. Can you tell me what else to do besides changing sending unit?

Could be the auto shutdown relay is getting weak. Try swapping it with the horn relay.

How many miles are on the engine? Have you tried to replace the oil pressure switch? is the engine making any type of noise due to low pressure reading?

Two things...when it reads zero, is there any engine noise (tapping or knocking)? After you rev the engine does the pressure read normal at idle and function normally, or does it go back down? Either way, you should not drive the vehicle 'till you have a mechanical gauge (yours is electro-mechanical) put on there and the actual pressure is tested. The condition may be a bad sending unit, a bad bearing or possible obstructed oil pump screen. The repair is obviously different for each one and the price is as well.

Check your oil level first and make sure its at the correct level. A couple of things, if your guage is going up it's a bad guage/sending unit are easiest checks. If it goes way up while driving you probably have an oil passage blocked somewhere causing the pressure to spike maybe a spun bearing (lower motor). Does it make any rattling/knocking noise? If not I would get it checked out ASAP, low/high oul pressure can lead to severe engine damage/failure.

I actually had one do this and it ended up needing a PCM to fix this problem .

ook up a external pressure guage where the sending unit is you will know forsure. with the milage i would guess that the oil pump may be the prblem or to much clearance in yhe bearing.

if you know for certain that the sending unit is correct and the guage is accurate, then yiu may have big problems.....while the engine is running open the oil filler hole.. is everything wet with oil? or does it look dry or just a little lubed? you can allways install an aftermarket oil pressure guage(crappy tire) if you get the same results with that then you may have a week oil pump....worst case scenerio... need an engine rebuild as the moter may have wear issues.does the pressure slowly drop as the motor warms up? at idle ? does it jump up when you step on the gas?

Try replacing the oil pressure sending unit

Any waranties left? Is oil full? Any noises from engine? Did you put a mechanical gauge on it and check "real" oil pressure? (I would not drive it 'till I was sure about actual oil pressure!!)

how many miles are on the vehicle this usually happens when the oi; pump or engine is wore out.