Under the Hood
---- — I have a 2001 Dodge Ram 1500 with the 5.2-liter V-8 engine. After the truck gets up to operating temperature, the oil pressure drops when I come to a stop. Would this be an issue with the oil pump?
This is a troubling situation, as you mentioned in a follow-up message the oil-pressure gauge drops close to zero at idle. Correct oil pressure is essential to insure a protective film of oil exists between the engine's bearings and crankshaft, as well as the camshaft. Without this, the bearings will overheat, gall, and seize the rotating shaft, destroying the lower end of the engine. Oil flow also lubricates cylinder walls and pistons and valve train components. A common early symptom of low oil pressure is a ticking valve train, followed by knocking sounds, which mean the end is near.
There are several possible reasons for low oil pressure. First ensure the oil quantity and viscosity (5 or 10W-30) are correct and there are no signs of contamination, as fuel can thin oil significantly. Then I'd question the accuracy of the gauge or its pressure sending unit. The best way to do this is to temporarily substitute a mechanical pressure gauge for the sending unit and verify readings under a variety of operating conditions. The minimum acceptable pressure at idle, warmed up, is about 10 PSI.
If the mechanical gauge readings are OK, the truck's instrument panel gauge and wiring can be tested by substituting a variable resistor (potentiometer) for the sending unit. Should the gauge read correctly as the pot is swept to various specified values, a new oil pressure sending unit is needed — a common fix. Try also wiggling the wiring harness leading to the sending unit while a helper eyes the gauge, looking for erratic gauge movement.
If the mechanical gauge also dips very low when the engine is idling and warmed up, I'd try changing the oil — even though it may look OK — and rechecking pressure before tearing down the engine. Low pressure could be caused by a worn oil pump, sludge buildup on the oil pump pickup screen or tube, a loose or cracked pickup tube, excessive bearing clearances, or a combination of these. Oil pumps are bulletproof in design but subject to wear as they are the only part in an engine that runs on essentially unfiltered oil. Oil pumps basically push a large quantity of oil, and the snug fit of the bearings acts as a restriction to flow, resulting in pressure being built. If the bearings are loose, oil leaks more readily back to the oil pan, and pressure won't be as high.
Should it be necessary to access the oil pump or pickup screen and check bearing clearances, your truck is an easy one — compared to many. Once the exhaust system is dropped and a reinforcing strut is moved aside, the oil pan comes down with minimum hassle. A replacement oil pump is surprisingly inexpensive — I'd opt to renew it even if the old one shows minimal wear (in-spec clearances).
I'm hoping you'll find an inaccurate gauge-sending unit. It is normal for oil pressure to rise and fall with engine speed, and it will decrease when the oil becomes thinner when warm. An old but good rule of thumb is you want to see at least ten pounds of oil pressure per thousand RPM of engine speed.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at firstname.lastname@example.org; he cannot make personal replies.