91 F350 7.3idi Non/turbo
250,xxx miles

replaced oil pressure sending unit 9months ago

changed oil 9 mo ago-400 miles, not driven often


had it running for 15 minutes, then the oil pressure gauge dropped like a rock, dummy light came on, I shut it down.

oil looks good/smells good
antifreeze full/ clean

started again
good oil pressure for a minute, then dropping again
shut it down

opened the oil filler cap, and smoke/steam came out slowly for a few minutes.

What is the sensor to the right of the oil filler cap ?

Maybe need new oil, pressure switch-again, gauge on dash, new oil pump--Noooooooooooooooooo !!!

 

The sensor to the right of the filler cap, assuming you mean the big hex head with 2 wires, is the tach sender. The oil pressure sender is on the back of the block.
Get a separate oil pressure gauge, screw it into the same location and test...

 

^^^^^ agreed!

The oil pressure guages on theses trucks are an idiot light but in guage fourm, if you have 7psi or more oil pressure, the gauge show in the middle(ish) area, but below 7psi it drops off like a rock.

Oil pumps don't usualy go just like that, they may lose flow rate, but its usualy main or rod or cam bearings shot that kill oil pressure. Changing the pump is only a bandaid to the real problem.

 

i believe theres a way to connect a real pressure sensor to the factory gage to make it a "real" gage instead of an "idiot gage". i know the pickups 86-earlier used a real gage, and my 87 van does. as mentioned above, either get an aftermarket gage, or rework your gage into a real gage.

with the knowledge that you have a glorified idiot light, it would seem to me that you're getting just barely enough oil pressure when the oil is cold and thick that it keeps the light off, but as soon as the oil warms up a little bit, the pressure drops and the light comes on.

i've heard of some guys running a synthetic oil that was like 0w40 or something - if you're one of those, change it to a more conventional 15w40 or even 20w50 and that might help your problem. if you're not in that camp, then it sure looks like things are wearing out and its time to freshen up the whole engine. the use of heavy additives such as STP or Lucas will thicken up the oil enough that it may delay the need to rebuild it.

on a related note, i have a pickup with the 300-6 which used to lose oil pressure suddenly while driving. the problem was the pressure relief valve in the oil pump, it would get stuck open when it went to vent off a little pressure, and being stuck open would lose all pressure. bringing the engine down to idle speed would reset it and it would have pressure again. the rest of the engine was fine, and changing the pump solved the problem - its been probably 5 years with no more related problems.

 

Ford Oil Pressure Gauge Fix

The above link is what Josh is referring to. I did it on my truck, and it works to an extent. You still don't know what the pressure actually is, but you can see it fluctuate. Actually scared the hell out of me a few months back when the connection on the sender got a little wonky. I just had to pull the wire off and put it back on again and it was fine.

 

That ^^.

You cut a resistor out of the insturment panel and switch to the huge oil pressure sender. Then you'll acctually have one that well shows some sort of pressure and changes when you rev the truck, when its cold it will probably be pegged to high, etc.

Did that on my 351w plow truck when i still ran it and before i started using it for parts. Works good.

The oil pump on the idi is gerotor gear driven off the crank. It puts on tons of volume. The pressure relief valve can get stuck on the oil cooler.

They make replacement valve's for psd but not for idi's.