Low oil pressure at idle

I have a 1990 Mitsubishi Montero, with a 3.0L V6 gasoline engine and 160k miles.

My oil pressure light comes on once the engine is warm when at idle (<1000 RPM). From what I’ve read, low oil pressure can be symptom of excessive wear of the internal bearings in the engine (bad) or, perhaps a worn oil pump or clogged pickup tube screen (not so bad).

I had my mechanic take a look; he knows this car well, having worked on it over the last 15 years. He insists that there is no serious internal wear in the engine; that we’d hear it making noise if this were the case. (It sounds the same as it always has when running, no clattering noises or anything). He did the following test with an external oil pressure gauge:

cold: 85 PSI @ 3500 RPM

hot: 50 PSI @ 3500 RPM

hot: 9 PSI @ Idle (~750 RPM)

hot: > 14 PSI @ 1500 RPM or greater

The significance of the 14 PSI being that this is the minimum recommended pressure, i.e. the pressure under which the oil pressure warning light will come on.

He also says that he doesn’t think that there’s any way the engine would be able to produce the 85 and 50 PSI numbers if there were significant wear to the internal bearings.

So my question is, should I take it apart and replace the oil pump and screen? Is there anything else to consider? I’ve started doing all the work on this car myself and before I get into something as complex as replacing the oil pump, I’d like to have confidence that there’s a high probability that it’s going to fix the problem.



I just checked the Haynes manual for my car (2001 Toyota Sequoia, V8) last night. For this car the spec is idle 4-5 psi
3000 rpm 43-85 psi (these are hot motor numbers)

I expect these values are similar for many cars. You can find the spec for your car and engine, but your pressures look OK as reported.

Perhaps the sending unit itself is ready for replacement. Dropping the oil pan and cleaning the screen is not a bad idea. Find out the exact specs for your car, make sure you have a problem before spending any significant money.

What weight oil are you using?? At 160K miles, 15w-40 or even 20W-50 might be of benefit. A rule of thumb, you need 10psi per 1000RPM. Idle oil pressure is not that important especially in a tired high-mileage engine.

Yes, this is a sign that the engine is getting old.

No, changing the oil pump will not fix the “problem”.

Oil pressure is developed by the pump pushing a viscous fluid against resistance. The pump is forcing the oil into the spaces between the bearings and their corresponding surfaces. It’s this fluid barrier that lubricates. As these surfaces wear the spaces become larger, th eoil flows through easier, and the pump is pumping against less resistance. Therefore it has a harder time maintaining pressure. It’s like trying to keep a balloon blown up that has a pinhole.

I think your best solution in to try a heavier weight oil. The more viscous oil will flow less readily through the spaces and the pump can better maintain pressure at idle.

Oil pumps are very simple devices, sort of like meshed impellars, that spend their lives totally bathed in flowing oil. They really don’t wear out that often.

Usually the oil pump is the last thing to go.

What’s been the oil change interval? At least every 5000 miles OR 6 months, I hope.

Before replacing parts I would pull off a valve cover and see how clean the engine is in general. Then I would pull off the oil pan and see what the pickup screen looks like. Then inspect one of the rod bearings.

It is not uncommon to have low oil pressure at idle in hot engine with lots of miles on it, as long as the pressure comes right up when accelerating and running. I would NOT use heavier oil as this may only exacerbate the ability to get oil to the right places on startup. Heavy oil may increase pressure, but actually cause more wear. Stay with the recommended oil…it got you this far. I would leave the oil pump and screen alone, as you probably won’t find a thing.

I would argue that the heavier oil will better allow a pressurized barrier to be maintained between the critical wear surfaces and actually protect the engine better. However I can’t argue with your other points.

And I agree that doing anything to the oil pump or screen would be fruitless

All the other posts are basically right, but I wouldn’t ignore UncleTurbo’s comment about checking the sending unit. I used to have an old '64 Ford pickup. The pressure light started going between flickering and solid on at idle. It turned out that the sensor itself was leaking just a little bit. It wasn’t enough to create a noticeable drip, but it wasn’t reading the pressure correctly. A few bucks and a few minutes & the problem was gone.

Outside of that, the variation in difficulty for pulling an oil pan is pretty large. I don’t know what it takes for this vehicle, but if it is one that can just be dropped without pulling anything else, I’d do it at the next oil change just to check out the screen & such.

Your mechanic is incorrect by stating what he did. It’s entirely and quite likely possible that excessive wear of the bearings can cause low oil pressure and yet the engine will run fine and make no noise.

One item to rule out is a stuck oil pressure relief valve. The mechanic did not note the cold at idle pressure.

At this point the 9 psi is a little low to continue operating. If the mechanic has the capability, have him pressurize the oil system with an oil pot; with the oil pan off; and observe where the major flow of oil is going. If it is back flowing the oil pump there is a pump problem. If it is flowing out of the pressure relief port, a stuck pressure relief valve is likely. If it is flowing from a bearing (or bearings), an overhaul is in order.

Hope that helps. Post back when you find the solution to this problem.