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Jim Grant's Tech Tips
‘96 Jeep Grand Cherokee, Low Oil Pressure
Q:
I have a ‘96 Jeep Grand Cherokee with a 5.2 liter engine and the oil pump failed on vacation. The mechanic replaced the main, rod bearings, seal and oil pump. Now when the engine is started cold the oil pressure is fine. After warming up the idling oil pressure is 20 PSI and drops to just above zero when placed into gear. When the Jeep is moving down the road, the oil pressure is 20-30 PSI.
Greg
A:
I’m concerned a bit with your oil pressure readings. The first thing you should do is have a mechanical oil pressure gauge installed and check the readings to confirm that the dash display is acurate. If the mechanical reading is the same as the dash gauge readings then you have problems, and here is where I have a problem. Why were the rod and main bearings replaced? The likely answer is because they were damaged. If they were damaged what did the surface that they rode against look like? I’ll tell you right now, damaged! If an engine loses an oil pump and the main and rod bearings are damaged I can assure you that the surface (crankshaft) that they ride on was damaged as well. So now ask yourself what happens if you place new parts against damaged old parts? I’ll save you the time, you get low oil pressure. When the oil pump failed the rod and main bearings rode against the crankshaft main and rod journals with no lubrication. The bearings are the soft part, but the surface on the crankshaft went down with the bearings. If I’m wrong then you have bad camshaft bearings. In either case, the engine has to come out of the vehicle to repair it correctly. Or should I say pull the check book out to repair it correctly.
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