Sudden low oil pressure

[delco1946]

Why would my oil pressure gauge suddenly be reading low. Yes, I know it always reads low but now it’s reading lower than usual and is no longer responsive when I rev the engine. It also isn’t reading as high upon startup when cold.

Any tips to diagnose this? It seems like either of the gauge is now malfunctioning although none of my other gauges are, or else it’s reading is accurate and which case something is wrong with my engine now.

[viven44]

I have done a couple of quick tests in the past. I am pretty sure I learned it from discussion on the forum

Connect the oil sending unit to the temperature sending unit wire. You can use an alligator clip wire...
-- If the temp gauge behaves like the oil gauge, then your sending unit or something else needs work.
-- If the temp gauge behaves fine like your oil gauge used to, the gauge needs work...
----- If you identify the gauge needing work, confirm by connecting the oil sending unit wire to ground. It should swing all the way past 'High'. If it doesn't REPLACE THE GAUGE as the factory manual might say.

If you have actual oil pressure issues, you will hear it first

I am really thinking you just have high contact resistance, so the gauge is 'lazy'... the wiring probably needs to be cleaned up ... sometimes those contacts can corrode on the post of the sending unit.

I have always just made it a habit to plumb real gauges so I can accurately benchmark oil pressure right from the start...

[BigBrother-84]

I recently noticed similar behavior for Big Brother's oil pressure gauge.  Lower than usual at cold start, and lower-lower when hot.
Never drops out of the "normal" range, but flirting with the lower line.

I'll give a try to Vivek suggestion.

[viven44]

If you are going down this route to test the gauge, you can also use a resistor between the oil pressure sending unit wire and battery ground for a more precise calibration check

Per the factory manual
-- Use of 73 ohm resistor should put the gauge at L graduation mark
-- Use of 22 ohm resistor should put the gauge at mid-scale

I have one of these for random uses like this
https://www.amazon.com/dp/B08FD1XVL6/?coliid=I1Y5TVN6EEZO8M&colid=3IGQF00LDI4AO&psc=1&ref_=cm_sw_r_cp_ud_lstpd_9G5AXQMXMTH3V7CEZBV6

[85lebaront2]

You may have what I experienced on a 390 FE engine in my 1977 F150. The cup plug that retains the relief valve spring, cracked, the tab on the pickup kept the remainder of the plug restrained, but the back side of the plug behind the spring is connected to the inlet side of the pump via a roughly 3/16" hole. Since the pump body is at or slightly above the max oil level, it sucks in air rather then just liquid oil.

[Gary Lewis]

viven44 wrote

If you are going down this route to test the gauge, you can also use a resistor between the oil pressure sending unit wire and battery ground for a more precise calibration check

Per the factory manual
-- Use of 73 ohm resistor should put the gauge at L graduation mark
-- Use of 22 ohm resistor should put the gauge at mid-scale

I have one of these for random uses like this
https://www.amazon.com/dp/B08FD1XVL6/?coliid=I1Y5TVN6EEZO8M&colid=3IGQF00LDI4AO&psc=1&ref_=cm_sw_r_cp_ud_lstpd_9G5AXQMXMTH3V7CEZBV6

Just a word of warning, you'll burn the resistor up very quickly on the mid-scale test, and it'll get quite hot on the L test.

The formula for current is I = E/R and for wattage is I*I*R, meaning current squared times resistance.

Here are some specs:

The chopper/ICVR puts out roughly 6v

And our page on Documentation/Electrical/Gauges says "... each has roughly 12 ohms of resistance."

So if you calculate the current to put the scale at the L mark: 5V/(73+12) = .06 amp.  And power dissipated in the resistor will be .06 x .06 x 73 = .26 W, which is all that resistor is rated for.

But to go mid-scale the current will be: 5V/(22+12) = .15 amp.  So power dissipated in the resistor is .15 x .15 x 22 = .5 watt, which is twice what that resistor is rated for.

And since you are working with heat to move the gauge (they are actually thermometers with a heating element below them) you'll need to run the circuit for several seconds to heat the gauge up.  And the resistors will get very hot - if not burn up.  DAHIK.

[viven44]

Thanks for those calculations Gary. I better have a 0.5W or 1W set handy.

Temp rise also leads to increase in resistance itself so affects validity of results so I agree. The right tool for the job!

I have used it for fuel sending unit approximation before, I assume the fuel gauge has similar challenges in using a resistor?

[Gary Lewis]

Full scale takes 10 - 12 ohms and the current is 5/(12+12) = .21 amps.  So power is .21 x .21 x 12 = .53 watts.  So if you are going to have the ability to measure full scale you may want 1 watt resistors.

[viven44]

I have used it for fuel sending unit approximation before, I assume the fuel gauge has similar challenges in using a resistor?

[Gary Lewis]

All of the gauges, save for the ammeter, are the same.  You may have gotten by with it, but when I tested to create that writeup on the gauges I smoked a few resistors.

I probably ought to rewrite that page to include those calculations so people will understand the amount of power they'll be playing with.  

But right now I'm writing up a 3D page...  

[BigBrother-84]

Gentlemen, I unplugged the connector from the oil pressure sending unit and grounded it.
With key contact on, the gauge needle slowly went to maximum.
I let the contact on, unplugged the connector, and the needle went slowly down to zero.

So I thoroughly cleaned up the connector and the sending unit terminal.

Started up the engine, and the oil pressure gauge needle climbed right in the middle.

Later today was the manure duty, and a hot idling engine gave a lower oil pressure, would say first third.  Higher RPM and the gauge needle goes back to the middle.

So, I think Big Bro's problem was a dirty contact at the sending unit.

Thanks for the advice!

[Gary Lewis]

Glad you got it sorted, Jeff.  

[delco1946]

Well I need to do some more investigating but it does seem to be the oil sender wire. It is toast and I know I hit it while checking spark plugs. Lots of oil around the sender its self. Not sure where the oil was coming ( dripping from valve cover?) from but yuck what a mess.

Not sure how to replace the sender wire as it merges into a larger wire harness ( I hate splicing 😂). The harness looks toasty too but that’s gonna to expensive to try and replace.

It’s so hard to see and get to. No wonder I’ve never paid attention to it before! Thanks for the lead.

[Gary Lewis]

I've seen the sending units leak and squirt oil.  So I'd clean the area up around the sender and try to determine where it is coming from while you are on this mission.

[mat in tn]

absolutely. start by cleaning as well as possible.
it is understandable why the sender is wired the way it is for an assembly line, but if you want the best connection then use a ring terminal. the factory 90 degree, slip on connector is not the best but it has proven to work well enough for many years. you cannot remove or install these senders with a pigtail I'm place so it will need to be done in place and it's a pain to get to, so I understand the slip on. but a better connection can be had. then a bullet connector or any more positively locking connector could be used where it meets the harness.