Question:

I have a 98 Jeep Grand Cherokee w/ approx. 219K mi. When I start it up it takes 15 to 30 seconds before the oil pressure gauge comes up to about 40#. After it runs for a while (5 to 10 min) the oil pressure drops to zero, and in another 5 to 10 min (idling) I start to hear light valve knock. Is it the oil pump or main bearing wear? Is there a way to determine which?

Answer:

Probably a little of both. No way but to remove the oil pan and measure the bearings by removing a bearing cap and using plastigauge to measure bearing clearances. Its almost as much work to get to the oil pump, so its worth investigating the bearings while you are there. I would suggest that if the bearings are within spec, replace the oil pump and consider yourself lucky to get another 50,000 – 75,000 miles from the engine. If the bearings are out of spec, I would look into a rebuilt or used engine from a reputable source like www.gotengines.com.