Having issues with my Ranger, its a 94, 4.0 5spd with 111k. Back in Feb the oil pressure gauge started acting up. As I was driving the needle started moving toward the low side. Took it home and parked it, on the way home gauge went back to normal position and stayed there. Over the next several weeks when i would start it gauge would continue to act up. So i just let it sit until i could find the time to mess with it.
Fast forward to a couple weeks ago, bought a new oil pressure switch and put it in. All seemed good on the first short drive, at first. Drove to a store and when i came out to start it gauge bottomed out, then a second later went to the normal area and basically stayed there but would flicker, got it home and parked again.
In the mean time I went to ford and bought another switch and ordered a oil pressure tester. Tester came in today and going to hook it up tomorrow and see what happens. Question is I've never used on of these before so need to ask a couple questions. Do i let the truck warm up before i take readings, do i take an idle read only or another one at a higher RPM. and finally what oil pressure readings should i be looking for.

 

Any engine noise when the oil pressure is acting out????

Are you using the specified 5W-30 oil & FL-1A oil filter????

Had you just changed the oil & filter when the problem began????

Could be the electrical connector, wire, or corosion problem at the electrical connector for the oil pressure switch.

Good idea to hook up & do a real oil pressure test.
40-60 psi at 2000 rpm, with the specified 5W-30 oil.

Pressure will be higher when the oil is cold & lower when the engine & oil are at operating temp.
Here is what one forum member measures on his 4.0L
Ford Truck Enthusiasts Forums - View Single Post - oil pressure

Let us know what you find.

 

No engine noise, which I take to be a good sign

Proper oil and filter installed, I always use the FL1A or Napa gold if FL1A is not available.

When it originally happened oil was not new, had changed it about 3-4 months earlier so had a little under 2k on it.

I will check the connections, never thought of that.

Going to hook up gauge latter today and will report back the readings i get

 

Yup, No noise = good sign.

Seeing as how you've replaced the op sender, are using the proper weight oil & specified filter & have only 111K on the engine, it sorta sounds like an electrical connection, or wire problem.

So closely check the op senders electrical connector for a tight fit & it should be clean, without corrosion.

If all that checks out, inspect the wire & it's crimp connection to the op electrical connector, for a loose crimp, broken wire strands, or corrosion at the crimp.

Sometimes broken strands will cause the insulation to bulge a little where they're broken underneath. this will usually happen close to where the strands crimp to the connector.

 

okay just took the pressurre readings, let truck run about 20 mins so it was pretty much warmed up i would think

at idle, about 700 rpm once it warmed up and the needle stopped bouncing around getting about 22-24

at 2000 rpm getting about 50-54, really probaly 51-52 but some needle bounce.

For 111k i think those are decent numbers.

So looks like it's ok and either the sending unit i just put in(NAPA) is bad, or i have a bad connection somewhere. Going to install the new unit i got from ford and see if that makes a difference.

 

The bounce on the needle at idle is a bit on an issue. That would indicate split second loss of pressure. The numbers are good but the bounce would indicate some issue.

 

Yup those warmed up op numbers sound good. My old 3.3L I6 would show about 15-20 psi at idle, warmed up & about 50-52 psi at warm rpm.

Could be a faulty new pressure switch, you know, maybe Murphey's Law is messing with you, but I'm still thinking a faulty electrical connection.

When you install the new pressure switch, make sure the block threads are good & clean, so it'll make a good ground connection. I like to use Loctite liquid #565 PST anaerobic thread sealant, as it won't give grounding problems, like PTFE ribbon dope sometimes does.

Keep us posted on your trouble shoot.

 

Read right over the needle bounce at rpm, so wtroger has a good point.

I had initially thought maybe a clogged up op pick up screen, but when you posted no engine noise with the op drop, I kinda forgot about it, but maybe it should be on your suspect list too!!!!

Notied any sludge under the valve covers, or in the bottom of the oil pan when changing oil????

 

Never seen any sludge/gunk in pan when changing oil, when i did valve covers last year that area seemed pretty clean to me also.
The bounce as i called it might be a little incorrect. When i first started the truck with the gauge attached the needle was moving between 22-30 and as it warmed up settled down to the 22-24 area, so maybe it was more of a flicker than a bounce.
One thing i did notice when i installed the new op switch. The electrical connector seemed to slide on very easily. getting it off the original was not the easiest but i can now pull it off this one with little effort, it almost seems like the post for this switch is narrower than the original.
Still haven't driven it yet to see how the gauge will act, that will be a tomorrow task.

 

OK, so no gross internal engine sludge build up on the top end noticed, so maybe the pan is relatively clean,

The next time you do an oil change, maybe try bending a hook on a length of clothes hangar & see if you can rake anything out of the bottom of the pan.
If there are any sizeable deposits in there, you should be able to rake something out.

Could be the oil pump pressure regulator being a little lazy in reacting to control op, I suppose.

The dash op gauge isn't reading true op, as it needs something like 7 psi to have the op switch to close & provide a ground for it to function. Then the dash op gauge will read mid scale, as it's been wired by Ford to be nothing more than a mechanical idiot light!!!!

So with the dash gauge moving around, suggests an electircal problem.

To test the in dash gauge & wiring, you could remove & ground the op senders electrical connector, then the dash op gauge should read steady at mid scale. If not, then look to electrical problems in the wiring, dash circuit board or electrical connector, or the in dash op gauge itself.

The op fluctuation the master mechanical op gauge is seeing, may be caused by something else, like a lazy pressure regulator. So you might have more than one problem.

At least you don't have any untoward engine noise associated with the fluctuations, but keep an eye on it anyway if the new OEM pressure switch doesn't fix things.

Will be intersting to hear how the new op switch behaves, sokeep us posted.