we have a 2000 jeep grand cherokee laredo with the inline 6 cyl, 4.0...last year we had a engine failure with no warning, since have rebuilt the head and bottom got new cam and lifters with new oil pump and gaskets.  Now with have the coolant low light beeping every couple of days and the oil pressure is at 0 psi at idle after warming, it has been suggested by our mechanic to change the oil pump to a high volume pump but none of this makes any sense...suggestions anyone?  

Are you sure your oil gauge is worjing right? I find it hard to belive that your engine would run at 0 psi? And a new oil pump is not gonna fix the low coolant problem.  

Coolant can go one of three places. On the ground, in the oil, or out the tail pipe.

Is there vapor in the exhaust after the engine is warm? Is the oil a milky color?

Why did you loose the cam and lifters? Why didn't the motor get rebuilt if it needed an oil pump and cam and lifters? What was wrong with the head?

High journal to bearing clearance can cause low oil pressure as can a cracked block.

Did the motor overheat? Freeze from cold tempatures?  

take out the bolts to the vaulve cover except two easy to get to and warm it up.. when it first reads 0 psi take off the 2 bolts and off the cover. if there is pressure youll see a happy mess if none youll see next to no oil shooting around. or you could just change the pressure sensor to double check. honestly tho if you brought it to a shop to fix it then they would have to fix it if it was them for free. p.s. i hope you dont run it much  

I'd put a mechanical gauge on it to check the presure.. I had to on mine after it drank a few gallons of water, turned out to be a bad sending unit, it may also be missing a oil plug behind the cam thrust plate unless it sudenly started the problem.

if the low coolantlight is comming on and its not low... same thing, bad sensor, I've had em act up when cold then start working when they get hot