Ok have to give a little history here I'll try to be brief. About a week ago I noticed my oil guage dancing all over the place when I pull up to a stop sign or light in drive. Pop it in neutral and the gauge goes back to normal reading. Couple days later the oil sending unit failed and poured all my oil out on the highway on my way home. I replaced the sending unit, refilled with oil and it runs fine after the lifters quieted down. Checked oil pressure with a gauge and got 65 lbs. Everything fine right? Wrong. After the engine runs a good while and I mean several miles, sometimes I can run errands to two or three places before anything happens but then the oil gauge starts jumping back and forth again. Pop it in neutral and it settles down. The RPMs only go up about 100 or so when I pop it in neutral but it always settles the needle down to a normal reading.

It had all new oil in it so I went ahead and installed a new Motorcraft FL-1aA oil filter and put in 24 oz of Marvel Mystery Oil to replace the oil lost in the filter change and hopefully clean out the oil pump pick up screen and valve. It was a day or two before the needle started dancing back and forth again and this time not nearly as much. It was bouncing so hard it made noise when the needle hit the stops at each end. Now it's only jumping back and forth within the normal range and still when I stop in drive. It stops if I put it in neutral.

Nothing makes sense to me about this, is my pump failing slowly or is my gauge gone whack? And if it's the gauge why does it stop when I put it in neutral so the RPMs go up a little? Need help from some of you guys like to solve automotive mysteries. :D

 

The sender is a switch, open or closed, and it uses the engine as the ground for that switch.

The gauge has a resistor on the back, when switch is closed the gauge is grounded and needle should go up to a specific spot and stay there, resistor OHMs decides the spot
It can go up or down a bit with RPMs because ALTERNATOR voltage may go up or down with RPMs, but nothing to do with oil pressure

Switch is OPEN when pressure is under 5psi, and closed when pressure is above 6psi

But needle can never go higher than resistor OHM valve, it can go lower if switch opens of course

So you do have a gauge issue

Because you have a 1990 you have the option of changing to a "real" pressure gauge if you want
Use a PS60 sender and bypass the resistor on back of the gauge

How-to here: https://www.therangerstation.com/for...iction.165083/

BUT...............first have a look at the gauge electrics there could be a problem with connector or solder joints on gauge

 

Thanks RonD, That makes sense to me. I was kind of leaning toward it being the gauge and was thinking of replacing it with a real one. I've never taken the dash board and gauge assembly apart on a Ranger and was hoping I wouldn't have to. I'll have to think of what my next step will be. The funny thing to me is the gauge works correctly except when after a period of driving around and the engine is hot for a while and it starts jiggling at stop signs. I haven't read your link yet but can I just install an aftermarket under dash type oil gauge and by pass the old gauge altogether?:

Btw, the new switch I installed is a Standard brand from O'Reilly auto and is a PS240. DO I just ask for ps60 without referencing make and model?

Edit: looking at the PS60 on rock auto the threads look smaller and don't have a chamfer or bevel end like the 240. Not sure that will fit.

 

Just unplug the sender(make sure wire end won't ground), and drive around to see if the gauge stays at 0, it might not if its a short in the dash

Yes, to use PS60 or similar electric pressure sender you usually need a stand off/extension pipe, they are cheap

 

The first sending unit we installed was defective and the needle stayed on zero. When my mechanic grounded it the needle went to the spot it now reads at continually until it starts bouncing around.

He said the needle should have pegged out on max when grounded. Instead it went to between the "m" and "a" in the word "normal". It used to stay on "n".

Now with the second unit installed it stays between m and a in normal..
Do you have a part name or number for that stand off pipe adapter?

 

Mechanic was wrong, temp gauge will do that not the oil gauge, if it setup for a switch

Oil gauge has that resistor that limits its travel to mid gauge, doesn't matter were, i.e. "m" and "a", that is electrical not oil pressure related, so it would be different with 12volt and 13volt and 14volt

Not on a part number its just small NPT pipe

 

Sounds to me like the oil pressure gauge is doing nothing at all if it just reads electrical and not pressure. I may just leave it alone and keep an eye out for the "check oil" idi*t light.

It didn't jump around today at all, just stayed on that "m". Doesn't move when I turn the switch off either but goes straight to zero when the switch is turned on then snaps back to "m" when I start the engine

 

The switch on the engine reads the pressure, under 5psi = open, over 5psi = closed(grounded)

Under 5psi the valve train will start to make the "ticking noise", so you will know if it gets too low

You want Oil pressure between 10 - 50psi, higher than 50psi can cause "washing" on some bearings, the oil sprays out too fast and washes the bearing clean so it over heats

And remember oil pressure is BACK PRESSURE, the oil the engine is NOT using at that time and RPM, which is why it goes UP with RPMs
Oil pump is turning faster so supplying more oil volume but engine can't use the extra volume so it BACKS UP in the main oil passage, where pressure sender is, so higher pressure

Anything above 5psi will get oil up to the valve train, 10-30psi give a little better cooling than 5psi

 

Thanks again for all the info. My primary concern was knowing the oil pump was working properly and I feel pretty comfortable about that now except my mechanic's gauge read 65. But he's a retired (due to injury on the job) Ford mechanic and his gauge is probably 20 or 30 years old and may not be right. I previously thought the gauge read pressure but according to you it's nothing but an idi*t light with a needle. I don't feel comfortable with either of the conversions you linked to and anyway the part that guy got for $13 is now $25 and then I have to get all the fittings etc. So I'm just going to take comfort knowing the pump is working and depend on the oil light to warn me of danger. I check my oil at least twice a week and look everything over in under and around my truck. As long as no other leaks spring up my oil level will be fine this truck doesn't use oil between changes.

Again thanks for all the info and good responses to questions.

 

I just had to add this little post script, before I posted here the mechanic that installed my oil switch (sending unit) told me the oil needle bouncing around like that could be some trash in the oil pump somewhere and suggested I drain out a quart of oil and replace it with a engine cleaning agent. I chose the Marvel mystery oil. The oil was already new but the filter hadn't been changed so I changed it and replaced the lost oil with the MMO. Strangely enough the next day the needle fluctuated a lot less than before. Since the third day after adding the MMO to the oil I've seen no needle fluctuation at all. so I'm still kind of left wondering still was it the gauge or the oil pump? I may never know but I guess it doesn't matter as long as everything works. But observationally it sure seems it was the MMO that cleared up some trash in the system.

 

Yes, that can happen, but it was probably some sludge in the sender passage, if it was actual oil pressure fluctuation you will hear it in the valve train, its not quiet about under 5psi oil pressure, lol.

 

Good point Ron!

 

Had a pouncing oil pressure gauge on my 2002/3.0 auto as well. Took me a bit to realize it was doing little more than acknowledging an open or closed circuit. A firm stop after some hill driving on a hot day would see the needle drop to the left/off (less than 5psi) Truck was relatively new but out of warranty. Some mechanics suggested the very low viscosity oils can slosh and momentarily cause pressure drop. Changed oil to 20/40 and added pipe riser as suggested by RonD and as well installed tee to real pressure gauge. Readings 8 to 35 when warmed up and 20/40 oil.

 

Yes, generally you will have 1psi per 100RPMs with warm oil, so idle of 800rpms would be 8psi, 2,500rpms 25psi, 3,500rpm 35psi but should top out after this

Remember Oil Pressure shows the oil the engine CAN NOT use at that RPM
Oil pressure is tested at the main oil passage before oil goes to the bearings and valve train
So as the RPMs increase the oil pump speed increases, so while the bearings are using a bit more oil the pump is sending way more oil than can be used, so it backs up in the main passage, and the pressure goes up

As long as you have ANY pressure there is too much oil, lol

Under 5psi gravity comes into play, the valve train, at the top of the engine, needs oil and at 3 to 4psi in main passage valve train can be staved for oil, you can hear that

Difference in having 30psi at 5,000rpm and 40psi is 10psi more oil that can't be used, lol, its not better, its not anything, anything over 10psi is just extra unused oil

 

I will say this, when my sender was failing, my indicator needle would move fast enough off and on that it would pulse well above the "regulated" voltage of the gauge. I'm just assuming it was due to inertia and the rapid switching speed of the old sending unit as it was failing.