I having a problem with my 98 grand 5.2. When it is idling the oil pressure drops to 0. If I put it in neutral and rev it up a little the pressure goes up to normal. When it is cold it stays normal. Any Ideas?

Mine did that for a bit, then the engine blew.
It's most likely the sending unit for the gauge.

btt

It's the sending unit for the gauge. Go buy a new one from Vatozone or the like and replace it.

If you can rev the engine up and the gauge works..then I doubt the gauge is bad.
The problem is most likely in the engine. When the bearing clearances begin to wear out, there is not enough backpressure at idle from them to keep the oil pressure up. When the oil is cold, its vicosity is higher, and thus is harder to move, and the oil pressure is fine.
An old wise mechanic told me this trick. Rev the engine up at temp. Quickly let off the gas, if the oil pressure spikes a bit before dropping, then the bearings are bad. This usually only works with a quick mechanic gauge though.

To sum up, the engine is old and tired. Not much to do other than a rebuild.

Its hard to believe that my engine already needs rebuilding, it only has 130k miles on it and ive always changed the oil every 3k miles. Is there anything else that might could be wrong?

Take it to a shop you trust and have them look at it.

thanks for the help

thanks for the help

I've had the same problem twice on my yj, replace the sending unit first, or check it with a mechanical guage if you have one.

My YJ had this same issue, although it had the wimpy 2.5l engine. I ran it for 2 years that way. Finally replaced the engine at around 180k miles, but it was still running OK. My solution was to not let it idle too much. When stopped at a light I'd rev it up once in a while to get some oil pumping through. The fact that it was a stick and had no e-brake meant I couldn't walk away from it when it was running. That probably helped too.