oil pressure gauge backwards

[j1mmy]

i bought a 94.5 f350 7.3 powerstroke with 140k miles, a few months ago, and have been working on it since.i finally got to go on the first real 50 mile drive sunday. i noticed i have great oil pressure at idle but as soon as i step on it the gauge drops down almost to the low mark. it just had an oil change, and the truck runs great. it doesn't make any sense to me. usually it works the opisite, low pres at idle and high under throttle. is this just a powerstroke thing from the injectors taking oil or?? is this something i should worry about, what is going on..

[Patrick Feeley]

In the factory installation, the gauge is really a glorified idiot light. A typical oil pressure sending unit will vary the resistance with oil pressure. The one on our trucks is not a typical sending unit, but only a switch (no varying resistance with pressure) that is OPEN or CLOSED at a threshold pressure (I think about 6 psi). If the pressure is above the threshold, the switch is CLOSED and the guage is supposed to read mid-scale. Folks have modified the setup to use a real sender, but the gauge doesn't range all that much. The switch is on top of the High Pressure Oil Pump (HPOP) reservoir (rectangular box on the top front of the motor under the decorative engine cover). If you disconnect the switch wire and ground it, the guage should read mid-scale. Hope that helps, Cheers!

[j1mmy]

why would the gauge be mid way at idle, and drop to almost nothing under power?

[protecdiesel]

i have been working on an 02 7.3 and i have a similar question
mine i can watch the oil level in the hpop go down as i crank it i hope some one answers your question

[backwoodsboy]

Protec, I'm curious. Are you visually watching the HPOP drain while you crank somehow? Or are you talking about HPOP pressure decreasing?

[OnTheBio]

Originally Posted by j1mmy

why would the gauge be mid way at idle, and drop to almost nothing under power?

The gauge you're using is a pressure gauge. It's dropping because it's showing low oil pressure because your hpop is failing/can't keep up/needs replaced.

It seems to be that your oil pressure is dropping because you're "getting on it"....

That's normal. Although it should hold a minimal PSI (i think it's 1900?) at all times driving, a certain psi while cranking (I think is 450?), and I believe a max of (4000?).

Someone can correct me if I'm wrong

Does your gauge show numbers or just say low, norm, and hi?

edit - Does anyone know if a low oil amount would cause low oil pressure or would you run into other problems being low on oil first??

[backwoodsboy]

Nope, not a pressure guage. As mentioned earlier, these guages are a glorified idiot light. For the most part they'll run in the 'normal range' until you have a problem (no oil pressure). If you want an actual pressure reading, aftermarket is you're best bet.
Does the needle drop all the way down to red, or just move in that direction?
Also, have you checked the oil level since you changed it? I doubt this has anything to do with it though...as you'd probably notice hard starting before you noticed the oil guage move.

[Patrick Feeley]

I'm not up on the SD's or 2002 models, so maybe they have an actual oil pressure gauge. The OBS trucks do not. The oil pressure gauge is on the LP oil system. If the reservoir is draining during operation, the LP oil pump might be getting weak or there is a cracked pickup tube. Conversly, if the HPOP can't keep up you should get a CEL. As far as the gauge "moving" it could be anything from a bad switch, wiring, or gauge. Try grounding the wire and see what happens. Cheers!

[j1mmy]

i fixed the gauge.. the sender had a bad connection.. it is still weird how it worked. must of torqued and lost connection under power.. but it is fixed now.. thanks for the help.
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[351fifty]

Originally Posted by Patrick Feeley

In the factory installation, the gauge is really a glorified idiot light. A typical oil pressure sending unit will vary the resistance with oil pressure. The one on our trucks is not a typical sending unit, but only a switch (no varying resistance with pressure) that is OPEN or CLOSED at a threshold pressure (I think about 6 psi). If the pressure is above the threshold, the switch is CLOSED and the guage is supposed to read mid-scale. Folks have modified the setup to use a real sender, but the gauge doesn't range all that much. The switch is on top of the High Pressure Oil Pump (HPOP) reservoir (rectangular box on the top front of the motor under the decorative engine cover). If you disconnect the switch wire and ground it, the guage should read mid-scale. Hope that helps, Cheers!

im interested in how one would make it semi work as a gauge instead of an idiot light