You’ve got questions. We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re helping looking at oil pressure fluctuation and whether it could mean potential problems.
D.M. Peoria, IL
Q: I have a 1974 Chevy 350 four-bolt engine I have rebuilt for a project car. I had the machine work done by a competent shop and did the assembly myself. I measured the bearing, bore, and side play clearances; most were within Chevy production tolerances, and all were within service tolerances.
The problem is the engine has a very wide range of oil pressure readings (I use 10W30 oil). At cold startup, pressure is at 60 psi. After the engine has warmed up to its normal operating temperature of 200 degrees F, pressure drops to about 22 psi at idle (800 rpm). The pressure will then climb to 40 psi at 1,700 rpm, and continue to climb until it is over 50 psi at highway speed. If I switch to 10W40 oil, I pick up 5 to 8 psi over the engine’s rpm range.
I’ve tried two different mechanical gauges and one electric pressure gauge, and all readings are the same. Although 22 psi is probably OK at 800 rpm, it seems low for a fresh engine. Is it possible that a clearance is opening up when the engine reaches operating temperature?
A: Oil pressure is the resistance to the flow of oil produced by the oil pump. If 10W30 oil is thinned considerably by increases in engine temperature, oil pressure will be lower. If the oil pump has excessive clearance at the gear-to-cover plate area, you may also see erratic oil pressure. However, if your oil pressure never drops below 22 psi when the engine reaches 200 degrees, does not drop as rpm increases, and goes up 5 to 8 psi when using heavier weight 10W40 oil, you’re just fine. You do not have much to worry about.