Oil pressure gauge check

[Nut124]

Oil pressure gauge reads alarmingly low sometimes when running hot. Light has never come on. Was planning on a track nite next week but have been worried about oil pressure. Can runs awesome.

Got an oil pressure tester kit from import tool store. Installed it in the oil pressure switch socket.

Found that the Fiat oil pressure gauge reads low and the scale markings seem off. The dash gauge has three marks; The low, the middle (55psi) and the high mark.

Upon starting, at idle, the test gauge reads 44psi. The Fiat gauge reads about halfway between the low and the middle mark.

Warming up, oil pressure drops; The test gauge now reads ~34psi. The Fiat gauge needle left edge is almost touching the low mark. While revving, the test gauge reads 70psi at anything above 4000rpm. The Fiat gauge reads almost, but not quite the 55psi middle mark.

Turning off the engine, the Fiat gauge needle moves way left of the low mark. So, perhaps the low mark on the gauge should not be considered low nor zero but maybe 20-25psi?

I see that my sending unit is russian. Ordered a Veglia, Italian unit.

[18Fiatsandcounting]

The sending units read lower and lower as they age. The process can take decades so sometimes it's not noticeable until it gets extreme. Normal Fiat behavior. The solution is simply a new sending unit as you are doing.

I did take an oil pressure gauge and hooked it up to a power supply and some resistors in place of the sending unit. With a 10 ohm resistor, the gauge needle is at the high mark. With a 250 ohm resistor, the needle is on the zero mark. Less than 10 ohms reads higher than the high mark, and more than 250 reads below the zero mark. So, for the gauge I have, the sending unit should swing between 250 and 10 ohms depending on oil pressure.

My assumption was always that the gauge needle would be in the left "rest position" when the ignition was off. When you turn on the ignition (but don't start the engine), the needle should move to the zero mark. When you start the engine, the gauge should read whatever the sending unit was measuring.

-Bryan

[Nut124]

With my new russian sending unit, the needle does not move when I turn on the ignition. So the rest position, which is 3-4mm left of the zero mark, would be the actual zero pressure, or more.

With this gauge/sender, the zero mark is actually about 20-25psi according to my test gauge. And the 55psi middle mark is 70psi.

[18Fiatsandcounting]

Just for grins, try disconnecting the wire to the sending unit and then put your ohmmeter between the engine block and the connection on the sending unit. If it doesn't read around 250 ohms with the engine off, it's likely that your sending unit isn't behaving as it should. You could also try this with the new Veglia unit when you receive it. Just curiosity on my part.

-Bryan

[seabeelt]

As others have said it’s the sending unit. They go bad sometimes slowly sometimes quickly. I’ve had three different new ones as a test and they all read differently right out of the box. For the most part it’s a feel good gauge. Fiat eliminated them due to all of the complaints. The pressure is also dependent upon the condition of your engine (obviously) the condition of your pump and also rpm higher rpm higher pressure until you hit the relief value

If you are running track and want an oil gauge go with the mechanical direct pressure one with the little hose to the back of the gauge.

All in all your pressure measurements sound normal

[Nut124]

Tested my new Veglia new-old-stock oil pressure sending unit. See image below for results.

This is totally different from the new Russian unit I just installed. This actually would read correct, for the most part.

[18Fiatsandcounting]

Thanks for that data. If my oil pressure gauge was generally within 10%, I would be happy. Right now I kind of treat it like a Yes/Maybe/No gauge in terms of oil pressure accuracy....

-Bryan