How do I trouble shoot low oil pressure?

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Jun 16, 2003
Edmonton, AB Canada
I just bought a 1989 Ford Club Wagon XL (4.9L six) with only 130,000ish km on it, for $700.00. It seems to run well, with decent power and no smoking, but according to the factory gauge, it has low oil pressure. I don't know what the actual pressure numbers are, but the needle sits on the front of the letter N in the word Normal.

When trouble shooting low oil pressure, what things should I be looking for and what order should I check them? I don't want to go yanking the oil pan off to check the oil pump if there are a bunch of other things I should be checking first.
Bad oil pressure guage/sender. Unless you have a real oil pressure guage, you can see the actual oil pressure and see if it within spec.
Where is the pressure measured from? Is the sending unit before or after the oil filter?

If it is before the filter, then you will get different readings with different filters.
If it is after the filter, then what is your driving style like? In hot weather on long trips it should warm up and thin out the oil, thereby lowering pressure. In cold weather and short trips the oil will not reach operating temp and therefore show more pressure.
How sensitive is the oil pressure signal?

Also, you need to figure out a way to "test" the unit.
If you do searches on oil guages you will find that a bunch of them are garbage. Pretty much idiot lights in the form of a needle and dial.
However, yours being an '89 makes me want to think it is probably just fouled.
Do you know the maintenance history of this vehicle?

Also, note that you probably dont have low pressure. With low pressure you would cook/seize your engine. You probably have a bad dial.
Chances are that with that many miles the sending unit probably has some junk in it and it's not reading correctly, or it's just going bad, OR both.

You can buy an oil pressure guage pretty cheap. Remove the sending unit and screw in the hose for the guage. Start it up and see what it says.

I also agree with the above statement that pretty much all stock oil pres. guages are idiot lights with a needle.
Originally Posted By: tom slick
Does it go up or down with RPM?

I've only driven the van about 10 km since I bought it, but so far it doesn't seem to go up or down with RPM.
"...what is your driving style like? "

No driving style yet, I've only driven it 10 Km since I bought it.

"How sensitive is the oil pressure signal?"

Don't know, I haven't had a chance to really do anything since I bought it this past Saturday.

"Do you know the maintenance history of this vehicle?"

Not very much. I'm the third owner and most of its life was spent in the city as a shuttle for a community centre.
Those engines are so tough, I don't think it matters. You'd hear it if something was really wrong.

PM TallPaul - he's had a few of these engines, and has done some experimenting with oils and oil pressures on them. He'd be able to tell you how accurate the stock guage is, and what to look for if there are problems.
I have an 81 Ford PU that gives similar indications on the gauge. I figured so long as the hydraulic lifters were pressured up and I don't hear any clattering that there must be enough pressure to keep on going. Hasn't let me down so far ( 302 V8 ).
Buy a cheap mechanical guage and hook it up where the oil sending unit would be.Close to the oil filter. Run the tube into the cab of the van. Be sure to bleed the air out of the oil line. This guage will be quite accurite.
That must be a dummy gauge, so all it tells you is you have at least the pressure the switch is calibrated for. Generally, a real gauge run lower pressure at idle and higher at road speed. With dummy gauges the needle just sits somewhere in the safe zone. Wife's '92 Aerostar with this dummy gauge reads in the N and O range on "NORMAL." Variations in needle location probably due to electrical resistance differences between different gauges.

Can jump the resistor on back of gauge panel, add a real sender, and gauge will be real, but not numbers.

As I recall Ford calls it a switch for the dummy gauge and a sender for a real gauge. I've heard that the switch triggers a gauge reading at a fairly low pressure, maybe less than 10 psi. Also I was told the switch is maybe one inch wide, where sender is about twice as wide, but I had a 90 Ford with the fat sender and a "NORMAL" type gauge. Seemed like a dummy to me. I put in a mech gauge and its needle fluctuated.

Anyway, unless the engine is making noises suggesting poor oil pressure, I wouldn't worry about it. A real gauge is nice though. I'd go aftermarket, AutoGauge by Autometer is nice, but a cheap Sun gauge for $17 will do the trick.
Yep. Idiot gauge. TallPaul pretty much nails it. You would hear the lifters if you had true low pressure.
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