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Old 05-19-2017, 10:05 AM   #5
Live, eat, and sleep by TSL
Join Date: Mar 2005
Posts: 1,819
Time to think about that oil gauge!

So I started looking closely at that oil gauge! I pulled the starter off and removed the sender, and then took the gauge out of the car. I wanted to know for certain what pressures things were happening, so I made a little test setup. I purchased a Shop Oil Pressure Gauge from Harbor Freight. It is not the best, but seems to agree with the gauge on my compressor. I wish I had something that was calibrated!! I should mention that I did all this testing with AIR. Air is a fluid too, and I reason that alot of aftermarket oil pressure sensors are actually measuring compressed air in an engine. Think about a mechanical oil pressure gauge. You plumb that in with several feet of nylon tubing. Does oil travel down the tube? Only if you have a leak, or somewhere the oil can displace the air to! So in normal operation, the oil pushes on the air, the air compresses to a certain pressure (the same pressure the oil is pushing with) and the gauge measures air pressure.

I plumbed up a little rig:

There on one side is the shop gauge (reference) and on the other side is a factory pressure switch. The factory switch is M14 X 1.5 thread and I noticed that 1/2 pex tubing was about the same diameter. I was able to forcibly thread the factory fitting into a section of PEX, and then the pex was fitted onto a 1/2 fitting. This was not perfect and had a little leak, but I have an 80 GAL Compressor so a little leakage was acceptable (ie it would not drain the tank). I had an old sender which I had removed from a car (possibly this one but I can't be sure) and it switched on at 4 psi! So 5 psi is "good" to this switch. I tried a brand new sender that I purchased, and it switched on at 6 psi!

WIS says: "Warning lamp comes on when oil pressure is below 0.3 - 0.55 bar" Which is 4.35 to 7.97 psi. So probably BOTH these senders are good within factory spec. My rig was not good enough to tell if the old switch (which read 4 psi) was truly good enough. But the new switch was definitely in the range.

Moral of this story: Do you feel comfortable operating an engine that is running FIVE psi!?!?! Probably should get a real gauge on that high dollar motor of yours.

So now I moved on to the aftermarket iequus gauge. I set this up in the same manner. Remember this sender has dual output, one to run the dummy light and is just a switched light like the factory sender. This is what I checked first. I found that this sender came on at 14 psi. That's almost double the factory spec, but more importantly violates the old adage "10 psi for every thousand RPM". So in my case, it is possible that the light was coming on even when I had "enough" oil for idling.

Now time to check the gauge. I grabbed a 12V wall wart, and hooked up the gauge. I was surprised to see that this gauge was not accurate. Disclaimer, the aftermarket gauge has very few markings, and even some "non-standard" markings. For instance, the gauge has 0, 25, 50, 75, and 100. That means the half way ticks are 12.5, 37.5, 62.5, and 87.5. So it is not really easy to interpolate the number between 12.5 and 25.

At 30 psi it read 25

At 40 psi it read 32

At 50 psi it read 47

But a couple really large errors still to be uncovered! WIS states min oil pressure is 2.5 Bar (36.25 psi) at 2000 RPM and 221 F oil. So I set my pressure to 36 PSI and read 26-28 psi from the gauge!!

So I was missing a benchmark on the car that was critical. Reading 47 psi when the pressure is 50 is not so bad but if you are trying to really see what is going on then you need a gauge with better graduation.

Finally, I set the air to 10 psi, and the iequus gauge didn't read a thing. ZERO. Plus, with a totally stead air supply, the gauge needle did bounce slightly. I had noticed this in the car, but didn't think it was the gauge. A couple of notes on this. This gauge was set up in the BEST possible solution. I checked the voltage on my power supply and it was rock steady 12.36 V. The sender was COOL, and still. Not buried on the back of a HOT engine, and not vibrating like it normally would be. Some might argue that it was because it was cool that it was off, but I argue that it was off by more than 10 PSI! I have been around precision instruments and have had to acclimate some of them for precision work. But that is when you are trying to measure .0001 (make up your own unit here) not 10!

So "In the BIN" with this gauge!
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