About to drop my oil pan

After replacing a rear main seal what would cause a motor to lose oil pressure? The motor always had good pressure until the rear main was replaced and now after the motor warms up the pressure drops to nearly zero. I replaced the sending unit and that wasn't it. The only things I can think of is possibly the pickup was plugged by the shop not cleaning the oil pan well and stirring up gunk or possibly they wrecked the oil pump? Any thoughts on this? The cherokee is a 1990, 4.0, 140,000 miles.

The oil pickup tube is a press fit in the pump body. The FSM has some cautions about being sure there's a good fit when replacing an oil pump and reusing the original pickup. It's possible they whacked the pickup with the pan and loosened the tube in the pump body.

Mind you, that's a wild guess. I don't know how easy or difficult it would be to loosen the seal.

Another possibility is that if they had clearance problems with the pan and removed the oil pump they damaged or omitted the gasket, or didn't bolt it up tight. One way of doing this job is to remove the pump while the pan is lowered but not out, and to replace the pump after the pan is in place but not bolted in. I did it this way, and it can work just fine, but it does require some care and dexterity to get the pump replaced properly.

Would a loose rear main cause this? Maybe on tight enough to seal, but not to hold pressure?


Rev