Alan Cesar
Alan Cesar Associate Editor
3/23/12 10:55 a.m.

I recently started driving a bit of distance to work. 80 mph interstate (~4k rpm) for about half an hour. I also did an engine rebuild ~1 year/10k miles ago. Right now I'm about halfway to due for an oil change. For the record, interstate speed limit is 70 here. And this is a '91 Miata.

My oil pressure is doing some strange things. It was running lower than before on the highway, and I'd see lower than normal pressures at idle after the interstate stretch. That's compared to the readings I'd been seeing in the time since the rebuild. So, 30psi at 4k rpm, just under 15 at idle.

But the needle also floats a bit. It'll fluctuate up and down by a small amount.

Today it got extra weird. I was cruising at slower speeds (say, 65-70 mph indicated). The pressure did some occasional jumping—not just floating—from ~30 to what I'm used to (~45) and back. At idle, it would jump from nearly zero (!!!) to 15-20 psi. That freaked me out a bit, but the sound of the engine hasn't changed and the way the needle was jumping made me think it was electrical. I got to work safely (another 15 minutes of driving).

Oil level is OK. I'm going to clean the terminal soon and see if the problem returns.

Anyone experienced this weirdness? If that pressure continues to drop like this with a clean terminal, where do I look next? Is it time to change the sensor, or look into deeper issues?

Per Schroeder
Per Schroeder Technical Editor/Advertising Director
3/23/12 11:01 a.m.

If it's a quick flicker on the gauge, it's usually a ground issue...

spritedriver28 New Reader
3/23/12 11:08 a.m.

Pull the wire off of the sending unit (under the intake), clean it and smear on some dielectric grease. If that doesn't fix it the sending unit may be wonky.

Keith GRM+ Memberand MegaDork
3/23/12 11:24 a.m.

Yup, that's where I'd start. The terminals corrode, and sometimes the sending units clog up over time.

Xceler8x UltraDork
3/23/12 11:28 a.m.

I'm with the "it's a ground issue" guys. Sounds like you either have a wonky sending unit or a ground issue. Intermittent ground issues can act this way as the sending unit gains and loses contact.

If you clean the ground and make sure it's tight...and the issue persists....What model is your Miata? Early Miata's had an oil sending unit that can pick up spark knock and blip. It's usually a really fast flick as opposed to any reading with longer than 1 - 3 second duration.

Woody GRM+ Memberand UltimaDork
3/23/12 12:22 p.m.
spritedriver28 wrote: Pull the wire off of the sending unit (under the intake), clean it and smear on some dielectric grease. If that doesn't fix it the sending unit may be wonky.

It may be easiest to access if you remove the passenger's side front wheel and go in through the wheel well.

PseudoSport HalfDork
3/23/12 1:59 p.m.

In reply to Alan Cesar:

I had the same problem happen to my 91 Miata. On the highway the pressure was jumping around and when I got off the exit and came to a stop it would jump from 0-15 psi. I pulled over but the engine sounded fine so I wiggled the connector on the oil sending unit and my gauge went back to normal.

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