Deciphering Oil Pressure Gauge Increments

[RNajarian]

I searched the brain trust and did not see a thread which addressed my particular question.

Here is the pertinent vehicle details 1983 3.0 SC motor with a 1975S oil pressure sender and gauge. I am using Brad Penn 20/50 oil, and the oil level is always within the specified normal range.

Before I dismantle the Air Conditioning and hook up a mechanical oil pressure gauge I would like to try and decipher exactly what my sender and gauge are telling me.

I am having a hard time determining the indicated oil pressure. With the vehicle off the gauge looks like this;



Obviously the pressure is Zero.

However, with the key at the run position and the engine off this is what the gauge reads;



Oil Pressure at idle with a Cold engine is above the "20" mark. With the engine warm the oil pressure rises "10" per 1000 RPM. At higher RPMs (4000-5000) the pressure maxes out over an indicated pressure of about "30."

My concern is that at idle (850-950 depending on engine temperature) the indicated oil pressure is exactly what you see in the second photograph with key in the on position and the engine NOT running.

Is this consistent with what you guys are getting from your motors. Last thing I want to do is starve my engine of oil

Thanks for bearing with me in this long diatribe and thanks for your help.

[schumicat]

i wouldn't assume that guage is accurate. i suggest replacing the pressure sender and you may find the old one was reading low, or use a pressure test kit to verify the gauge is accurate. with cold oil I think your pressure is low, if the gauge is accurate. ditto your pressure with engine on.

[emac]

I have the same gauge, but I'm just the opposite. When cold I'm between 100--140 as the car warms it drops. When fully warm around 60. Be advised the needle will never hold in one position, due to changes in rpm. Remember never put full trust in the gauge. Check with dip stick, warm to temp, running on level ground. Should be between full and min mark.
Ernie 81sc

[RNajarian]

I may have the answer . . .

I tracked down my owner's manual to a 1980 SC. The section on Oil Temperature/Pressure Gauge seems to answer/allay my fears.



The manual states at operating temperature at 5000 RPM the Oil Pressure should be about 4.5 bar. Regarding my question, at operating temperature at idle it states that it is permissible for the green oil pressure light to light up. However, while driving should the light come on the engine should be immediatly be turned off.

I noticed improved "pressure" when I cleaned the wire at the sender. I will also clean the wire at the gauge, hopefully this will "fix" the problem

[RNajarian]

I would just like to update my progress with this project.

I cleaned the electrical contact at the sender and at the gauge. The corrisponding pressure values have changed.

Engine warm (near operating temperature) at 4000 RPM the pressure is 4.5 bar
Idle (at 850-900 RPM) the pressure is 2 bar

The Green Low Oil Pressure Light lights up at start up (as part of the normal diagnostic) but does not flicker when warm (or hot) at idle, with indicated oil pressure at 1 bar.

Based on this information, I will monitor the system and not make any changes yet, and by keeping oil half way between the MIN and MAX mark on the dipstick I should have enough lubrication of the engine.

The Moral of the Thread

Read the Owner's Manual

[Knight77911]

I know this thread is old, but...is the low tick mark 0 or 10psi. I have the old style numbered oil pressure gauge like the OP.

[RNajarian]

My assumption/best guess is “0” indicates zero pressure and the first hash Mark indicates “10” next is “20” which would naturally indicate 20 psi.