89 Caravan Low-ish Oil Pressure

3.0 V6 with 106,000 miles, gauges not idiot lights. Fresh oil change with
straight 30 weight (the weather will be hot for the next few months in my
location).

The oil pressure drops down when the motor is hot at idle and sometimes it
touches the bottom line of the wide normal zone. Usually, it is within the
low end of the normal zone. The "Check Gauges" idiot light has never come
on.

However on the oil gauge there is a small box shaped zone "below"
(visually, to the left of) that is *not* colored red or yellow like, for
instance, the low range of the voltage meter.

So I have two questions, if it ever drops in to the area below the large
normal zone, am I in big trouble, or it that box merely the absolute low
end of the normal zone? I read the owner's manual, but it was no help.

Second question, considering the mileage, does anyone have a educated
guess as to which is the most likely cause of the low oil pressure; is it
probably a clogged pick up screen, or is it probably the oil pump?

 

Remember, these gauges are not at all precise. The first thing to do is to
put a mechanical oil pressure gauge on the engine with it hot and idling
to see what your *actual* pressure is. COmpare it to the spec or post it
here. If the pressure is within allowable range, your dash gauge (or more
likely, the sender) needs attention. If it is low, then it's time for some
diagnosis.

You answer your own question here: The area below the normal zone is BELOW
NORMAL. If your dash gauge isn't telling lies, operating the engine with
subnormal oil pressure is a Bad Thing.

Guessing won't get you very far, but a clogged pickup screen usually shows
itself at higher RPMs and/or when the engine oil is cold and thick, not at
lower RPMs and/or when the engine oil is hot and thin.

 

Good point. I had assumed that since the pressure is towards the top of
the normal zone when the motor is cold, and the pressure is much higher
regardless of temperature when the motor is not idling, that the gauge was
working correctly. But still, it could be a bad gauge or sending unit.
I'll look into that.

I've seen old high mileage GM V8 motors whose pressure dropped to near
zero at idle when hot, with no apparent ill effects. I had hoped the same
might be true here.

That figures. Replacing the pump looks a lot harder than cleaning the
screen.

 

Most common cause of low oil pressure at hot idle is increased engine
clearances, due to wear.
Using straight 30 oil will NOT improve this hot idle pressure over a
5W30 or 10W30 oil. If the engine is "experienced" (lots of miles)
using a 10W40 or 20W50 oil will USUALLY raise the hot idle oil
pressure by a few pounds.

If the oil pressure comes up significantly at higher RPMs it is not
likely the oil screen.

Also, is your hot idle set to spec, or has it dropped due to plugged
idle speed control etc????

 

The straight 30 weight was simply what I happed to have four quarts of
lying around, so I used it. My previous vehicle leaked enough that I kept
extra quarts on hand, of whatever happened to be on sale.

Good point, I'll check it today on the way to work. The tachometer is
intermittent, but it usually works. Hopefully it is accurate.

By the seat of the pants, from memory, it feels kind of low. The motor is
so quiet at idle that you cannot hear it inside the vehicle. In Drive,
idling without my foot on the brake, it creeps forward much slower than
most other vehicles I have driven. But it never dies, so I assumed it was
ok.

 

I agree with everything Dan said about checking the actual pressure at the
oil sender using a mechanical gauge, but just wanted to offer that my 1991
Mitsubishi 3000GT SL (same engine: 3.0L V6 dual cam, no turbo) behaves
pretty much exactly as you describe. It currently has 127K miles and has
behaved in this manner for the 2+ years I have owned it. At start up the
oil pressure gauge immediately jumps to the middle range at idle, but as the
engine warms up (3-5 minutes later during summer), the pressure drops slowly
to the low end of the gauge. If I step on the accelerator and increase the
RPM slightly (to above 1K) the pressure rises back to mid range and stays
firmly at upper mid range when driving at highway RPM. If I drive in stop
and go, then the pressure drops at every light/stop.

My gauge appears to be accurately displaying/communicating the pressure and
at higher RPMs seems fine, so I've come to the conclusion that the oil pump
needs to be replaced. I don't plan to do this anytime soon since the entire
engine needs to be overhauled and this will need to be done at some time
down the road when I can take it out of service and afford the work and
coupled with the interval for replacing the water pump and timing belt
again. The reason I want to overhaul the engine is because it burns about a
quart of oil every 1500 miles and the valves/heads are very noisy,
especially at start up and until the engine warms up. I've recently begun
using Mobil 1 10W-30 and this appears to have helped since the valves quiet
down in about 1-3 seconds after start up, as opposed to the 30-60 seconds
when I used regular non-synthetic oil.

Based on what I've read on this and the Mitsubishi newsgroup, my experience
with the 3.0L (and yours too) seems to be pretty much the norm.

Bob

 

Again: Guessing won't get you very far. You are guessing (without any
basis) that changing the oil pump will fix the problem. It may or it may
not. If your insufficient hot-idle oil pressure is due to excessive
internal engine clearances, replacing the pump won't fix it.

 

Hi

This may or may not be a problem Most motors have lifters that require oil
pressure to work. This means if the oil pressure really is low, the motor will
click loud enough that you won't be able to hear yourself think.

Larry

 

Now that you mention it, it never makes any racket even when the pressure
reads low.

 

Assuming the tach is accurate, when hot it idles at 625 to 650 RPM.

There seems to be no way to set the idle; both the Haynes manual and the
sticker under the hood says the idle is preset and can't be adjusted.
Neither bothers to say what the idle is supposed to be.

 

That describes my motor exactly.

[...]

That's the best news I've had all day.

 

Sometimes, a guess is all I've got.

Unless it gets worse, I won't do anything major.