Oil pressure : stock gauge vs. actual reading

[kamaro]

I have been driving with fear from the moment when I swapped in a 3.2 Carrera engine in my '77 911S because of what appeared to be a very low oil pressure, when hot, the gauge would read VERY close to zero at idle and would not go ano further 2.5 even if I rev it past 5000 rpm. I'm using a stock 3.2 sender and swapped in a stock 3.2 gauge. Today I have decided to check the actual pressure using a mechanical gauge with a AN -4 hose connected via adaptor to the oil light sensor outlet behing the intake. I was happy to see I have a very steady oil pressure of 20 psi when idle and hot (after 30 min. drive, engine temp close to 195F) and it would jump to 60 psi when I rev it to 3000 rpm. I have replaced the sender with a porsche genuine one and I have borrowed another gauge and still got the same low reading in the dash!, what do you think is causing that? do I need to run a separate wire from the sender all the way to the gauge?

Here is a youtube clip of the oil pressure :

https://youtu.be/eJtYVKb-DHE

[r lane]

I had a similar problem on a track motor, checked it with a mech gauge whose pressure looked good and ran it. Blew it. Too much to tell right now, but make sure the gauge and sensor are the same bar. Bob

[HarryD]

Had a similar issue but with the gauge reading high. Turned out the original 0-10 bar sender was replaced with a 0-5 one.

[stownsen914]

It's a little odd, but if you're confident that the gauge/sender combo is correct, maybe send the gauge for calibration? Senders and gauges can be off sometimes.

[Walt Fricke]

The '77 came with a 10 bar sender on the 2.7 engine, and a 10 bar gauge. The 3.2 engine came with a 5 bar sender. The two don't match. Cheaper and easier to make the sender match the gauge, so put a 10 bar sender in the motor. That should line up with what your mechanical gauge showed.

[gumba]

I found the gauges to be sensative to voltage. I mounted a mechanical gauge in the engine compartment.

[Walt Fricke]

Harold - these gauges are insensitive to voltage. That is by design. Look up D'Arsonoval (?sp?) to see how this is done. It is a bridge design, where the sensor's resistance is one of the four elements.

[spuggy]

Originally Posted by kamaro

swapped in a 3.2 Carrera engine in my '77 911S because of what appeared to be a very low oil pressure

'77 wants a 10 bar pressure sender to match the 10 bar (0-140 PSI) gauge.

3.2 sender needs to be used with a SC/3.2 gauge, as Walt says.

Originally Posted by kamaro

I'm using a stock 3.2 sender and swapped in a stock 3.2 gauge.
...
I have replaced the sender with a porsche genuine one and I have borrowed another gauge and still got the same low reading in the dash

The senders can go bad - although mine suddenly started giving unreasonably high readings at idle.

Sounds like you could still be mixing 'n matching 5/10 bar gauges/senders. Be careful not to do that.

[kamaro]

Thanks guys for all the great inputs, just for the matter of testing, I'll borrow the sender from my brother's '87 and test it in my car, if it works good then I will buy that dam 100 bucks sensor, I hate to spend that much on a part only to know that something else need to be replaced.

[HarryD]

I have a used 0-5 bar sender somewhere. Email me if you are interested and we can work out a deal.

[r lane]

Take the sender and gauge out of your brothers car. If the pressures on both cars don't come close to matching, run a separate harness. If your car still shows low pressure, then you have a mech problem somewhere. I went through all of this a couple of months ago. Bob

[lvporschepilot]

My dash gauge very often reads just under the first mark, while the digital sensor for my EFI system shows a significant amount more pressure, around 25% more.

[Walt Fricke]

These engines often show what seems like pretty low pressure at idle. I think that is why Porsche shifted to the 5 bar system - so the needle would be farther around the gauge!

At higher RPMS, like 4,000 or so, you ought to get close to or above 10 PSI per 1,000 RPM. This isn't like lunar orbital mechanics, where a tiny bit off sends you off into deep space.

If it is low, is easy to shim up the pressure setting piston. It is the vertical one (cap near the center of the bottom of the engine by the flywheel). Pull the plug and slip a small washer or two under the spring. Surprisingly, you may not get much oil out when you remove the plug. Of course, if the bearings are really worn, or an O ring got messed up on a rebuild, band aids won't do the trick.

As a general rule, these sensors don't wear in such a way that they start reading low. They are a pretty precise, robust design, with a clever slider moving over a resistance coil which is adjusted during manufacture. The resistance isn't going to change over time in any significant way. If the wires wear where the slider passes, that would increase the resistance, which would result in higher readings.

If you pull the wire lug off the sender, the gauge pegs (infinite resistance). Eventually, the resistance wiring can wear through at some point. The symptom there is a rather normal idle, then at some RPM the gauge pegs, and stays there until the RPM drops back.

Solid state sensors these days ought to avoid any of these issues, though if their accuracy at a gauge/display depends in part on the wiring between sensor and gauge, that could be tricky. EGTs all warn against using wiring other than that supplied.

But measurements can be tricky - ever compare the four or five tire pressure gauges you have accumulated to each other?

Walt