91' 4.0L 2wd ranger.

I've been reading through some posts about low oil pressure readings. My truck always lists a dangeroulsy low pressure. but from what i've read it isn't actually a true gauge. It is a swtich that reads mid-range when the pressure is good and low when it is bad. Just wanted to know if my understanding is basically correct? secondly, if my reading is always "low" yet still in the normal range, do i most likely have a bad oil pressure sender? also is it worth fixing? i mean, i'll know if i have no pressure right?

 

When you are looking at the truck when it's parked and off...that's the 7 PSI or less oil pressure indication. Anything on the gauge is pretty much 8 PSI or above.

Some indicate further along the scale than others. The best thing to do is to just check the oil pressure with a manual gauge and make sure it's good (the redneck rule is 10 PSI for every 1000 RPM, so 10 PSI at idle, 20 at 2K RPM, 30 at 3K RPM, etc.).

There is also a sender you can buy from an older Bronco that will convert the stock gauge to at least move, though obviously it has no numbers on it. Check the oil pressure manually and make sure it's good!

How low does the gauge show? Is there movement after the engine starts? I.e. when the engine isn't running, note the gauge position, then start the engine and watch it to see if it goes up.

 

I have a smilar problem, and I thought I'd put it here than start another thread. I have a 91 2.3 L Ranger. My pressure gauge stays in the normal area, but when I accelarate it drops to low or it even pegs to very bottom. Has anyone seen this before? I haven't tested to see if it's the sending unit yet. I hear no unusual noises while driving. If seen it do this before and changed the oil and the problem went away. This started Saturday. I had the oil changed, but it still continues. There is no oil leaks. Any help would be great. Thanks

 

Welcome to FTE.

If the gauge hasn't been modified to read actual oil pressure, by using a 80's model resistive sending unit, it may just be a bad switch type sending unit, which causes the gauge to read mid-scale, if the oil pressure is above about 7 psi. We refer to it as a oil pressure idiot gauge, not much better than a idiot light.

If the sending unit is ok, then it could be the gauge itself, or maybe you actually have reduced oil pressure, from a clogged filter, pump screen, or bad pump.

So to be safe, maybe you should rig a mechanical shop gauge to the sending
units port & check the true oil pressure.

If it were mine & the sending unit was bad & needed to be replaced, I'd probably consider doing the modification & installing the resistive sending unit, so the gauge was indicating real oil pressure.
There is a thread in this forum on doing that

Or another option, maybe install a mechnical gauge & line, although I don't like having a live oil pressure line in the cab.

IMO, I'd get it fixed, one way or the other.

 

Mine used to do some similar weird things as duffman's before I pulled the engine. Yes it had several small leaks in a few places. Mine used to read normal (middle of the gauge) only for the first few miles after start up. Then it would fall down to the very bottom of the gauge at idle, and climb almost up to the middle while in motion.

While I had the engine out I replaced all the gaskets, seals, oil pump and the sensor so IDK what 1 thing caused the change, but now it climbs up to the top of the gauge when you start it up and it never moves until the engine is shut off. I have never had a manual gauge to check it but I am fairly sure the pressure is good.

 

thanks for the response. The gauge does move up after starting the truck. i actually havn't been concerned really. however, recently the gauge has been reading lower and lower. but from what i've learned here, that doesn't make sense. if it's reading normal then it's fine. but why does it move at all if every "normal" reading is essentially the same? I guess i'm gonna have to get a manuel reading, just to make sure everything is ok.

 

Get the manual reading, but I'd take a bet it was just your sender or gauge biting the bullet little by little.

 

Can someone identify the NAPA or Motorcraft part number for the replacement sender unit that reads actual pressure that will fit? Thanks.

 

Scroll down to the "Oil Gauge Conversion" link, in the "Misc" section of our "Tech Info" thread, located atop this forums thread listing page.

 

Yep, thanks. Embedded in the thread is a link to a RoguePerformance fix that includes the make and model of all the compatable sending units.

 

One thing to note, though I'm sure this was in the tech article, but either way. The gauge, when converted to full-motion using a pressure sensor rather than a pressure switch, will always indicate slightly low due to the resistor on the back of the instrument panel. This resistor seems to kind of come and go, appearing some years, not appearing other years.

Before installing the new sender, do at least one mechanical check of the oil pressure. I would suggest noting the cold idle, warm idle, and the warm pressure at say 2K RPM with a mechanical gauge. (these values are roughly 55 PSI, 15 PSI, and I want to say 40 PSI on my truck, '94 4.0L). Then hook up your new sender, and with the engine cold, note the position under all these conditions on the stock gauge. That way, you'll be able to look at the stock gauge and say "ok well that's roughly 55 PSI, that's roughly 15 PSI", etc.

 

i doubt anyone has still been looking at this thread, but i just fixed the sender and everything is perfect. i was a little concerned because i didn't know where the oil pressure sender was, but it was totally easy to find and switch out. i guess it's a little trickyer on the 3.0L but on the 4.0L it was a peice of cake. took about 4 minutes. got a little messy though. should have waited about 10 more minutes brefore starting...the oil was still too hot!

 

Good fedback & to hear the fix was simple & easy to do.

Did you just replace the sender, or did you do the oil pressure gauge mod, wih the variable resistance sender, to make it read real oil pressure???

Also it migt be useful to others, to know where on the engine block, you found your year 4.0L's oil pressure sendng unit located.

 

i put a t on mine and put a manual guage in on top of the factory guage. mine goes anywhere from 15 psi to 50.

Matt

 

Sounds about right, I get about 15 at warm idle, 40-45 at warm running, 55 at cold start.