Help Interpret Oil Guage

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Jun 11, 2002
Fort Smith, AR
I have a '95 Ford Club wagon with a 5.8L V8 engine with 121,000 miles. Runs great.

Before BITOG, I never paid attention to the oil guage on the dash. It always read in the "normal" range. But now that I am a little more oil-anal, I actually look at the oil guage more often.

The guage is not an idoit light. It is a needle guage that ranges from low to high with the word NORMAL printed in between low/high. Usually the needle rests on the letter M with the engine running. Sometimes the needle rests in the middle of the M. Sometimes to the right of the M (ie: higher oil pressure) and sometimes on the letter A (even higher pressure). Once I noticed these fluctuations, I decided to use different brands of oil filters to see if that affected oil flow, and perhaps the guage readings. I do not believe that to be the case now. So far I've used a Pure One, Purolator, Motorcraft FL-1A, and K&N filter. In all cases the needle varied the same. And there seems to be no pattern to rpm's, temperature or road speed. I've seen it settle on the M at cold start up as well as going 70 mph down the freeway. I've also seen it move right to the A at cold start up as well as when driving 70 mph. And of course I've watched it move slowly from the M to the A, and vice versus, as I'm driving.

I guess what I am saying is that I have been unable to observe a pattern to the different readings. My Haynes manual says normal oil pressure for this engine is 40-60 psi @2,000 rpm and normal operating temp. Of course I have no idea what psi any of the letters in the word NORMAL on the guage correspond to.

I'm beginnig to think these "M to A" readings are all normal and if I'd only looked at the guage years ago, I'd have seen the same sort of fluctuation. Does that sound reasonable?
Ford oil pressure "gauges" are fancy idiot lights. they don't move much if at all if the OP is above 10 psi or so. hook up a mechanical OP gauge and you'll see what I mean.

Bret is correct. What you have is an idiot gauge. The sender is the same one that is used for an idiot light. As long as the oil pressure is above the set point, a constant current is sent to the gauge which moves to roughly the middle of the "normal" range. The small variations you see are due to the normal voltage variations in the system.

It is possible to convert the idiot gauge to a real one by pulling the dash, removing/replacing a resistor, and replacing the pressure switch with a gauge sender from a pre-idiot model. A Google search should turn up the exact procedure for your year/model vehicle.

if you have trouble finding articles on finding the resistor in the dash and pressure switch, search over at under the 7.3L IDI forum. Same style gauge that's on the F-series pickups from 80's to early 90's.

One symptom it may be an idiot gauge, or maybe it's just a ford thing, is with the truck running turn on heater full blast, headlight high beams, radio, defroster, etc. The voltage drop will cause the gauge to read about a letter higher. Turn things off, heater especially, and watch the gauge come back down.
You all are absolutely right.

When I was driving the van home yesterday, I started playing with the heater/ac/fan controls. Sure enough, as I increased the fan speed, my oil pressure "guage" readings increase.

PO Ford S.
This is very deceptive of Ford. Shows you can't trust them on anything. My '95 F150 has the idiot gauge and the manual states that the oil "gauge indicates the engine's oil pressure, not the oil level." Well, I guess it all is a matter of what does the word "indicate" mean?

So I bought a nice Autometer mechanical oil pressure gauge and have been very happy with it. Even converting the dash gauge to a real gauge with the resistor jump and different sender won't get you the psi numbers.
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