Low oil pressure at idle...any ideas?

[BoiseShark]

I was driving my 84 today and noticed that at idle, the oil pressure drops down to about 2 and then when I drive off, it jumps back up to 5. Should I be worried. I have never noticed it doing this before.

If it was my oil pump, wouldn't it be staying low and at a constant low pressure?

I'm afraid to drive it. Sounds normal, no smoke or knock, just a low reading at idle.

any ideas?

[MPDano]

Sounds about right. What weight oil are you using?

[riber.bentsen]

Could be that your oil-pressure gauge simply started functioning properly again

2 at hot idle (I'm presuming it's hot weather) seems reasonable.

[Danglerb]

Some modern oils are designed for modern tight clearance lower flow rate engines and have fairly low hot viscosity. The 928 motor is old school and expects 20W50 to have high enough hot viscosity to make some pressure going through the high flow system.

The 928 has a positive displacement oil pump, each revolution of the crank pumps the same basic volume of oil into the system, so even a lower reading at idle may be "ok" for the engine, not so good for old people nerves.

[Alan in AZ]

I'd be very surprised if it has ever been doing anything but this... it's completely normal.

Maybe you didn't pay attention before to what was going on... it will always peg to 5bar at startup & cold idle - this is a hot idle situation only. As long as you have 5 bar from 3K RPM & up and you don't drop below 1.5 bar at hot idle you are quite OK.

Alan

[hbueno]

I'm currently running 15W-40 and I get about 2.5 at idle and 5 at any revs above idle so I'd say yours is in the ballpark.

Hugo

[dtw]

2 bar at idle sounds completely normal.

[BoiseShark]

I can't tell you how relieved I am to hear all of your responses. Everything fits.

At start up it hits 5 and stays until warm, it's usually somewhere in between 90 and 100 degrees here in Boise, I run non synthetic Castrol 20/50 and I was recently into the pod to figure out why my tack and speedo randomly come to life and die again.

I think it wasn't working before and I lived in the happy world of it fluctuating between 4 and 5.

Thanks again guys. I will sleep tonight

[stepson]

Originally Posted by BoiseShark

I was recently into the pod to figure out why my tack and speedo randomly come to life and die again.

Did you figure out the tach and speedo? Typically it's the rivets in the foil circuit that don't make contact. You can recrimp them or solder them to correct.

[Pete R]

2 bar at idle? I'd pull the motor and rebuilt it so it reads 2.5 at least.

[Alan in AZ]

Originally Posted by Pete R

2 bar at idle? I'd pull the motor and rebuilt it so it reads 2.5 at least.

[BoiseShark]

Just rebuilt it tonight. Sitting right at 2 1/2. Thanks for the advice Pete.

and I did figure out the speedo stepson, it's in the ignition. I can wiggle the key and halfway turn it back to the off position and it starts working. I had the cluster out doing just what you suggested when I apparently fixed the oil pressure gauge.

Not the story for the tach, but going to get a new ignition plug and it might get the tach as well.

[tisilrad]

from working in a garage i can tell you that there are bad oil filters out there that will cause a drop in oil pressure. I have seen it several times. The most recent was a volvo S80 that had had a recent oil change and the oil light started coming on. A simple oil filer change corrected the problem and was confirmed by an external oil pressure test. This is not to say you have this problem,like most guys here say, that is about normal(mine runs 3 bar hot at idle, but my idle speed is a bit high( 900 rpm) due to a vacuum leak.. i gotta fix that...if concerned have the pressure checked to make sure of gauge accuracy.

[Pete R]

Originally Posted by Alan in AZ

That would be a little sarcasm Alan, as there is no real fix for low oil pressure other than a rebuild.

[Mrmerlin]

I hope that when you rebuilt you engine that you got your crank polished,
this will let the bearings spin faster in the bores of the pistons.
The cylinder valves will also run cooler

BTW running BAR SB 3.0 or higher at idle with the engine hot,
so you might have to pull it again to get that last half bar

[Alan in AZ]

Originally Posted by Pete R

2 bar at idle? I'd pull the motor and rebuilt it so it reads 2.5 at least.

Originally Posted by Pete R

That would be a little sarcasm Alan, as there is no real fix for low oil pressure other than a rebuild.

Hardly true... plenty of things you can do... always assuming there is an actual problem.

Check oil level, use heavier oil, check oil temperature, check oil thermostat, check oil cooler, add extra oil cooler, check oil pick-up, check oil pressure bypass springs, change rod bearings, change oil pump.

Alan

[BoiseShark]

Agreed!

[Pete R]

The car is a 79, there is no oil cooler, thermostat ect, He was already using 20/50. It's never the pump, I assume he's not an idiot and it had plenty of oil, and he wasn't overheating. So basically leaves rod bearings which I consider a rebuild.
or we go with the most likely reason ... the oil pressure gauges after 30+ years aren't so accurate and everything is okay

[aluminum]

forget the " bar reading " if your concerned about your Oil Pressure .
But rather hook up a manual type oil pressure Gauge commonly found at most auto stores .
I suggest one with a long line to hook up inside the car > Most have a small plastic line feed .
if your pressure drops below about 10 lbs at hot idle then you most likely have a problem 15 and holding being more common .
Make sure you have proper oil filter a simple mahle oil filter well work fine
Is Common for the pressure Relief valve spring in the Oil pump to weaken also dropping pressure . many instal a Washer behind the spring of the Oil Pump to increase pressure Evan after a rebuild . About 30 to 60 Thousands jumps the pressure Nicely .
Most likely your main Bearings are worn somewhat as well as your Cam Bearings and this well give a 20 % loss most often
50 years experience says read a real Gauge ( non electrical ) and if your wondering if moderately increased pressure well blow some seal in a word no.
More then 50K / 60K Thousand K Miles = decreased Oil Pressure , 70K / 80K more decrease and so on .
One Might Note that a Car with decreased Oil Pressure may run for Years with such .

[Alan in AZ]

Originally Posted by Pete R

The car is a 79, there is no oil cooler, thermostat ect, He was already using 20/50. It's never the pump, I assume he's not an idiot and it had plenty of oil, and he wasn't overheating. So basically leaves rod bearings which I consider a rebuild.

Rod bearings are a rebuild these days ?#!

Believe me oil pumps can have issues, but usually not related to wearout.

Most things I noted can still be done on this car even if not originally stock, there are certainly better 20w50 oils out there (and heavier too). I always give people the benefit of the doubt when it come to being idiots - I've found anything is possible... YMMV

Originally Posted by Pete R

or we go with the most likely reason ... the oil pressure gauges after 30+ years aren't so accurate and everything is okay

No! at 2bar or more at hot idle everything IS OK already - There is simply No Problem to solve here...

Alan