Hi, I recently bought a 90 Cherokee Laredo with a straight six 4.0 automatic. When I checked it out before buying it the engine was quiet with no knocks. I changed plugs, wires, filters and the oil. Now when the engine warms up a bit it has a knocking sound like if it's a diesel. The noise seems to be coming from the oil pan and if you get the rpms to about 1200 it quiets down almost completly. I added a quart of Lucas heavy duty oil stabilizer and it didn't help at all. It has 140k but I doubt that's accurate because I have a 98 Classic with 120k and the engine on the 90 feels a bit tired compared to the 98. When the engine warms up to normal temp. the oil pressure is about 20psi at idling rpms (about 800). Is the oil pump quiting? Is there something else that I could add that might help? I almost sure that it's not the crankshaft bearings because the knock would increase as the rps go up. Thanks.

The 90 version is a lower horsepower 4.0l where the 98 is the HO version with more horsepower.

Check your motor mounts. Due to tight clearance they will sometimes knock at idle.

My 89 4.0l had about 20 psi at idle and 40+ above idle....