Sporadic oil pressure readings. - Ford Powerstroke Diesel Forum
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Old 11-19-2012, 09:59 AM
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Sporadic oil pressure readings.

I have a 1995 F-250 with the 7.3. My oil pressure gauge on the dash will go from reading a normal reading (splitting the "R" and "M" on the gauge) to just barely being staying on the "N". It is hard for me to blame a bad pump because the readings are good when it's cold but seem to change once the engine heats up. Plus the reading will go from good to low and then back to good again when it's up to normal temp.


-I'm pretty sure all my seals are good.
-The engine does not leak much oil (maybe a 1/4 quart between changes)
-About 189,000 mile on the engine.
-I don't blow white smoke when driving. Only when it starts on a cold morning.
-Coolant is not discolored.
-Truck starts easy

I am new to Deisels and need help to figure out why my gauge is giving me these readings.

Last edited by AWF2; 11-19-2012 at 10:08 AM.
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Old 11-19-2012, 11:53 AM
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On these Ford trucks the oil pressure gauge is really nothing more than a glorified idiot light. This is due to the fact that the pressure sending unit is not a real sending unit (resistance that varies with pressure), but only a switch (ON or OFF, switching at about 6 psi or so with the gauge set to read mid-scale above the 6 psi threshold). The switch is located on top of the High Pressure Oil Pump (HPOP) Reservoir, which is the rectangular box on the top front of the motor (right underneath the front of the decorative engine cover. It has one wire to it. If you ground that wire, the gauge should read mid-scale. If it doesn't, look at the wiring to the gauge or the gauge itself. Cheers!
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Old 11-19-2012, 01:03 PM
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Thank you for your post, Patrick. I do have some question though:

1) If I ground that wire, will it just trick the gauge to read good or will it actually send more oil to the engine? I don't know if there should be a constant psi or if it's normal to vary.

2) Would you recommend putting an aftermarket gauge in? If so, where is the best place to mount it on the engine and what should I see for psi?

I've been told that a bad Cam Shaft bearing can cause bad oil pressure but, that would not give sporadic readings, it would just be low all times. That would also burn oil and blow thick white smoke as well.
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Old 11-19-2012, 06:23 PM
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It will just make the gauge think the switch is ON and make it read mid-scale (supposed to anyway). If there was a real oil pressure gauge the pressure would vary with engine temperature and RPM. I had a real gauge in my 94 for a while and it would read around 70 psi stone cold and maybe down to 20 psi hot (if my memory is correct). There's a couple of spots for a sending unit (one being where the switch is in the reservoir and another down by the oil filter. Personally, I don't find the OP gauge really too useful since low pressure oil problems will usually translate to high pressure oil system and performance problems before engine damage happens (I would rather have pyro, boost and trans temps in the typical 3-guage pillar pod ... although one could add more gauges if desired!). Cheers!
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Old 11-20-2012, 04:15 AM
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Again, thank you! It doesn't sound like that gauge is anything to rely on. If it's reading anything, then there is pressure and that's it. Also, if the pumps fail then I would lose oil pressure and not get high and low readings as I'm driving down the road.
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Old 11-20-2012, 04:23 AM
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As far as losing performance, I'm not sure what to gauge that off of. I just drove it from Maryland to Ohio and was getting about 17mpg on the highway (doing about 60-65mph), I got 19.5 when I drove it to Minnesota from Maryland back in July (but I never went more than 55 on my way to MN).

And one last thing I'd like to add, the acually HPOP is located under the fuel bowl if I'm not mistaken. I do have a slight drip (about a drop every second or two) coming from my fuel restiction sensor. Would that do anything to the HPOP over time?
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Old 11-20-2012, 10:57 AM
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There is really nothing directly under the fuel bowl. The HPOP is on the back of the HPOP reservoir (in front of the fuel bowl) and the fuel pump is in the valley behind the fuel bowl (probably what you are thinking of). The leak in the fuel restriction sensor can cause problems with the wiring down there (especially the Injection Pressure Regulator, or IPR, wiring ... IPR is on the back of the HPOP, 2-wire connector). You can remove the fuel restriction sensor and plug the port with a 1/8 NPT plug, but just don't overtighten it and crack the housing. If your 95 has the fuel restriction sensor on the backside of the fuel bowl down towards the bottom (94 and 95's were this way), you will have to remove the fuel bowl to do it. The FRS just lets you know if there is a plugged filter (doesn't have anything to do with monitored engine parameters).

In a sort-of nutshell, the way the Hydraulically-actuated Electronically-controlled Unit Injectors (HEUI, what we have) works is the low pressure lube oil pump supplies the High Pressure Oil System with oil. The HPOP raises the pressure of that oil to around 3000 psi (it varies on engine needs) and supplies it to the injectors. When the Powertrain Control Module (PCM) electronically commands the injector to fire, the hydraulic oil pressure is used to inject the fuel at 7 times the HP oil pressure. If the lube oil pump is having issues and can't supply enough oil to run the injectors, the truck will run badly or not at all.

Cheers!
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Old 11-20-2012, 12:51 PM
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Putting questions on this forum might be the smartest thing I've done in a while. Your awesome, Patrick.
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Old 11-20-2012, 02:14 PM
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I put this question on another forum. below is a link. Please follow it to give any constructive criticism you may have.

Oil Pressure Sending unit - Diesel Forum - TheDieselStop.com
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