Oil Pressure. Whats Dictating The Reading Of The Gauge?

[livi]

When I park the car I usually let it idle about 30 seconds before I turn off the engine in order to steady the reading of the temp, oil pressure and level. A very common saying is that if the oil pressure at idle and full operational temperature reads at least 2 bar, its an indication of engine health.

Well, I get different readings all the time. Sometimes it is as low as 1 bar, sometimes more than 3 bar. Note, this is always with a fully warmed up engine at perfect op. temp.

After I installed a SW chip (satisfied) the idle is slightly higher than before, reading around 950 rpm. This is constant however and should not affect the variation in oil pressure I assume.

The car drives like a champ and pulls like a train in all gears. Appears perfectly healthy with very, very low oil consumption.

Question: What dictates reading on the oil pressure gauge and why is mine showing different readings during identical circumstances?

[Flieger]

Do you tap the gauge? With readings at the very low end of gauge scales, accuracy goes down. Airplane gauges are rated for accuracy with and without "tap", where they literally tap the gauge with their finger.

10-15 psi per 1000 rpm is best when at operating temperature. At idle, many people report no discernable oil pressure on the gauge and red lights but the pressure comes up as soon as the revs are raised to 2000 or so. I only check oil pressure at 2000 rpm if I am looking for a warmed up reading. (I do look at it when cold)

I would not worry about it as the car sounds to be running well.

[livi]

Thanks, Flieger!

My question related to the "at least 2 bar at idle is indicating a healthy engine" saying. Very difficult to evaluate when the gauge value fluctuates that much.

I have tried tapping the gauge, does not move the needle at all.

As you say, it is probably no big concern.

Thanks again!

[Flieger]

2 bar = 14.7 psi * 2 = ~30psi

I get about 10 psi or lower on warm idle (180 F) The needle just kind of lays there. I need to raise the revs to get a good reading. I would think 2 bar would be too much at warm idle unless you idle at more than 2000rpm. I idle at about 1200 warm.

When cold, I get 60 psi at about 950 rpm idle. If I rev it to 2000-3000 just easing it out of the garage and warming it up, it gets darn near 100psi

I think 2 bar at warm idle is a little high.

[Flieger]

By the way, what kind of oil cooler setup do you have? Stock Carrera front fender cooler with finned core and electric fan?

I have a front mounted RS style cooler in addition to the engine oil cooler. Oil coolers can cause a lot of restriction of flow with cool oil, raising the pressure.

[Bill Verburg]

Below ~80C the internal t-stat is closed and full oil flow goes from the pump directly to the main oil gallery which parallels the c/s, a % of the oil is bled off to lube the c/s & bottom end. The % varies depending on clearances & revs. The oil temp/pressure senders on a 3.2 are located at the end of that gallery(front of the engine, back of the car). From there the oil is directed to the left or right to feed the cams. Again clearnces and volume of oil wil vary w/ temp & revs

Above ~80C the internal t-stat is open, oil is directed to the engine mounted cooler and then to the main oil gallery as above. There is a temp regime ~80C where some oil is going through both circuits.

lastly the temperature of the oil has a large effect on its viscosity and all oil is different in its performance which also affects oil pressure. At idle there can be very low pressure(enough to trigger the warning light) w/o issue, what you really want is ~ 1bar per 1k rev above idle

all of the above will have an effect on oil pressure

[richardlw]

Gauges are not necessarily very accurate, and as mentioned, sometimes need tapping to move the needle. The pressure is measured as the restriction created by the bearings. So bearing clearance and oil viscosity are the factors that affect it. Higher revolutions raise the volume trying to get through, raising the pressure from maintaining the restriction while increasing volume.

A filter that is not letting the oil get through, either from being clogged and failure of the bypass valve can cause variations.

High foaming in the oil can also cause fluctuations. Make sure your oil is good at reducing foaming, and that you have the right level oil. Too much or too little causes foam.
Idling to reduce turbo temperatures is fine, just don't rev the engine prior to shutting if off or you will leave excess fuel in the cylinders that will drain down the sides and kill your oil.

[RWebb]

one other effector of gauge needle movement is the absence of good ground wiring...

- real world not as simple as the owner's manual...

[dentist90]

As you may already be aware the oil pressure sender works kinda backward: if you disconnect it, the pressure guage will peg at 5bar. My connection at the sender was starting to go and my pressure readings became inconsistent. When I wiggled the wire it broke and my Druck guage shot up. I could look thru the rear window as I touched the wire back to the sender and watched the needle bounce up and down. I'd look into this, because 3bar at idle is too high for a warm engine.
BTW: tapping the guage will not change this problem because it is the electrical signal that is at fault. If you turn your car off, however,and the needle doesn't park down at 0bar your guage is sticking. If your connection is good the problem might lie with the pressure sender itself.

[Ole Petter]

When I bought my 75 2.7S about 3 years ago, the oil pressure at idle was about zero. I never saw the oil-pressure warning light though. The ground-wire at the coil seemed dubious, so I had it seen to... and voila, the pressure read a lot more. The engine even pulled stronger and the slight hesitation at low revs was gone. Very pleased with myself, I was.
The car also ran on Mobil 1 0-30W fully synthetic oil which I was very strongly recommended by the Official Porsche workshop to change for a dinosaur 20-50W oil. That really got the oil-pressure going, and it seems to be constant as well. Earlier it just went up at higher revs.

[livi]

Thanks everyone for the great input! Good education! All said and done, I suppose I can stop taking notice of the exact idle reading every time I park.

[dshepp806]

Livi,

I'd get her fixed so that it works right. Drop that gauge and make sure the connections 'round back are good and tight (after cleaning them)...

I had to replace a section of wire from the pressure sender up to a retaining clamp for the cable..lo and behold, I found a VERY worn place where the metal cable retainer had worn through the jacket (from some idiot not putting it back at a nice 90 degree angle ..instead, it was cocked so that a corner could easily wear through). This may be different on your car...
I also replaced the sender for good measure..all is well now. The entire path was checked...It may be off a slight bit at full scale,..but quite usable still.

As someone mentioned, make sure the ground point used by this gauge is clean and tight..

lastly, the gauge itself could be off (cal) a bit.

Metering problem may not be an issue NOW, but it would be of great service to read pressure correctly when the problem is REALLY there...I just like all my stuff to work..

Best to you,

[Chris'85]

This looks llike the right thread for this question. I recently change the oil in my 85 Carrera. Checked the level when at proper operating temp and the level is right in the middle of the low and high marks. Now for the issue, ever since the oil change the oil level gauge reads high, almost pegged and the pressure gauge, at idle, flucuates wildly above 3 bar and when on the gas the gauge pegs. Since I know the oil level lis fine and the temp is reading normal is this something I need to worry about and is this indicitive of a sensor or wiring issue?

Thanks.

[Flieger]

That sounds like a bad ground somewhere. You should also check the fuses by rotating them in their holders and scrubbing with some contact cleaner.

[Chris'85]

I will check the ground wires and fuses and report back. Thanks for the tip.

[Flieger]

The ground connections I would start with are the wires on the back of the gauges (brown is ground but check them all) and then move on the the oil tank sending unit. On my fiberglass 911, the ground wire was left off because it was connected to a fender mounting bolt. I put a crimp lug connector on the end of the wire and secured it under a nut on a nearby bolt into the chassis.

I have to clean my (uncovered) fuses every now and then.

The ground wire made my oil level and fuel level gauge come back to life after being dead (fuel) and pegged/vibrating (oil level)

Hope this helps

[haycait911]

911's seem notorious for low oil pressure at idle. I talked to a guy who worked in a porsche dealership in the late 70's early 80's. he told of guys coming back with their brand-new cars with the oil light flickering at idle. I go by the 15 psi per 1000 rpm. starting at 2000 rpm.

[Milo2361]

i'm a little curious now... recently my car's pressure gauge seemed a little low.... @ warm idle(950 rpm) it used to read just short of 1 bar, now it sits just above 0 at idle....

2000rpm = 1 1/2 bar
3000rpm = 2 1/3 bar
3500+rpm = 3 bar


doesnt ever go above 3 bar when warm anymore???? is this bad? my 6,000rpm i could have sworn was at like, 4 1/2 bar or so and now it hits 3bar and stays there... no bouncing or vibrations of the needle, no oil light on. Cold idle it sits slightly below 4 bar, and up to 5 bar.

[Bill Verburg]

Originally Posted by Chris'85

This looks llike the right thread for this question. I recently change the oil in my 85 Carrera. Checked the level when at proper operating temp and the level is right in the middle of the low and high marks. Now for the issue, ever since the oil change the oil level gauge reads high, almost pegged and the pressure gauge, at idle, flucuates wildly above 3 bar and when on the gas the gauge pegs. Since I know the oil level lis fine and the temp is reading normal is this something I need to worry about and is this indicitive of a sensor or wiring issue?

Thanks.

different oil can read differently on the gauge, different oil temps will read different on the gauge, different oil states caused by aeration will read differently on the gauge

[Bill Verburg]

Originally Posted by haycait911

911's seem notorious for low oil pressure at idle. I talked to a guy who worked in a porsche dealership in the late 70's early 80's. he told of guys coming back with their brand-new cars with the oil light flickering at idle. I go by the 15 psi per 1000 rpm. starting at 2000 rpm.

that's correct, though they have since changed the internal oil pressure mechanisms and scavange to pressure ratios.