My 92 3.0 litre started exhibiting a strange phenomenon lately.
When I start it cold, the oil presure guage doesnt move, it stays at the needle's resting point.
after a short period, typically a few minutes, it swings (abruptly) to the center of the point on the scale which is where it normally runs at.
Every so often the guage returns (abruptly) to the resting point again.
Because the guage shows only normal or zero and never anywhere in between, I tend to think that the sending unit must be faulty, but I just dont know for sure.
Anybody experience this before?
By the way, I will be attaching a test guage to see what the pressure truly is at start up, just to be sure.
Any help appreciated

 

I haven't seen it before, but definitely use a mechanical test gauge to verify. Don't rule out a bad sending unit wire connection either.

If you want to rule out the sending unit, ground the wire and operate. If the needle sweeps like before, then look at the gauge or the wiring. Of course, make sure you have good oil pressure to start.

 

I believe that the oil pressure guage is actually a switch that only has 2 positions, bottom ( none ) and middle (normal). There are instuctions posted previously on how you can convert it to an actual working gauge. I would hook up a mechanical guage to see if you actually have oil pressure low enough ( 4 psi ) to set the gauge to low while the engine is running.

 

Why would it be a switch if the vehicle has a guage on the dash?
I always thought sending unit for guage and switch for idiot light???

 

The oil pressure changes with engine speed and oil temp. The gague I have in my Maze Miata will start at close to 90 when it is cold. It will slowly come down to about 15 after all is warm. If I rev the engine the pressure goes up. The newer Miatas have what looks like a gague but is basically an idiot light. Good pressure and the gague shows in the middle. Low pressure and the gague shows nont.

On my Aerostar it is either none or midrange. Just like the newer Miatas. It's not really a gague at all.

 

Mind you, what car does have anything installed OEM that can be called a gauge?

 

There has been a lot of confusion on this subject. Here is what I know to be "the truth"

1) The water temperature gauge is for real. At exactly the C mark, it corresponds to a resistance of 74 ohms on the sender, which is about 150 F. Near the H mark, the resistance is 9.7 ohms, which is about 295 F, which would really be cooking your engine. The middle of the gage corresponds to about 26 ohms, or 215 F.

2) The oil pressure gauge is bogus. That is just on/off. Below 4.5-5.5 psi, it's off and the gauge reads below the L. Above that, and the switch closes, bringing the reading to slightly above the middle.

That brings us to your situation. I believe you may have a bad engine ground or the connector on the sender might be coming loose. Pull the connector of the sender and crimp it down a little to make sure it's snug on the sender. Mine was doing the same thing (plus a couple of engine misses here and there) until I wised up and started to ground the heck out of the engine. Nothing fancy, just a couple of 12-gage wires with 3/8" eye-connectors soldered onto both ends. One end goes to the bolt on the negative battery terminal, the other end goes to the a convenient bolt on the engine (the bolt securing the oil filler tube is a good candidate). I also did the same to the frame just for the heck of it. Now all that peculiar behaviors and the engine misses just went a way, and the engine is way smoother than before.