Oil pressure

#1

I have a 1995 dodge grand caravan with the 3.3L. The factory installed 3.3 threw a rod and I replaced it with a 3.3 from a local salvage yard. The replacement had 70+k miles. Recently I noticed I had oil pressure(it has a factory gage) when the engine was cold and when I was driving and the engine was hot. When I would come to a stop the oil pressure would bottom out. I replaced the oil pump and it made no difference. I do know that it has the hydrolic lifters but they seem to be staying “pumped up”. I also changed the oil psi sending unit. Just baffled.

#2

Define “bottomed out”. Is your low pressure warning light illuminating?

Oil pressure comes from the pump pushing against resistance, the resistance being forcing the oil through the spaces between the bearings and their respective wear surfaces. If the wear gets such that the spaces become too large the oil gets pushed through too readily and the pump has difficulty maintaining pressure at idle. It’s sort of like trying to keep inflated a baloon with a hole in it.

Unfortunately, there’s no definite way to diagnose this without a teardown. My recommendation is to try an oil with a heavier base weight. That may maintain better viscosity (resistance to flow) and that could help maintain pressure.

#3

Get any warranty on that 70K engine? does it include low oil pressure at idle? did you check out your warranty before you replaced your pump,sounds like you need your warranty.

#4

If the oil pressure is really that low, the lifters would start clattering.

Hook up a mechanical gauge in place of the original sending unit to see what’s really going on.

#5

I agree with using an external guage to check the oil pressure. If the reading is low then you have some crankshaft bearings issues more than likely.

If it’s been a while since this engine was installed then you may also be out of luck.
The salvage yards always sell “guaranteed good” stuff according to them but an annoying percentage of things seem to have problems.

#6

As a general rule, you need 10psi oil pressure for every 1000 RPM. At idle, a tired engine should have at least 5 PSI. What type and weight oil are you using? 15W-40 can be useful in these cases…