Low oil pressure makes me nervous

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nm9stheham said:
Just for info, there is a reason for this.... full synthetics have a much more stable viscosity over a wide temp range. The lower weight viscosity number is tested at 100F, so down at room temp, the standard oil has thickened up while the full synthetic has not and so that is why it seems thin and runny when you pour it. That seemingly 'runny' synthetic will actually be MORE viscous at 200F than a 50W standard oil will be at that high temp.
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I am aware of this thats why I spend close to 70bucks cdn for enough pure synthetic to do an oil change .
While we're at it why the $@#% does Mobil one come in a 4.4 litre jug when it takes 4.7 litres =5 quarts to do an oil change ?
 
Spec for that engine is min 5 psi at idle hot , but I would put a known good Manual gauge on it to be aboustly sure.
 
Biff said:
Spec for that engine is min 5 psi at idle hot , but I would put a known good Manual gauge on it to be aboustly sure.
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Agree w/ this, I`ve even got one under my hood.
 
X2 I had the computer in my wifes old 4.0 jeep shut the motor off one time because the gauge hit zero, the sending unit was crap, changed the sending unit and put a "t" in it with a mechanical gauge just for a sanity check, pressure was fine.
 
sireland67 said:
X2 I had the computer in my wifes old 4.0 jeep shut the motor off one time because the gauge hit zero, the sending unit was crap, changed the sending unit and put a "t" in it with a mechanical gauge just for a sanity check, pressure was fine.
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mine is off each of the rear pressure ports, one 1/4" , and one 1/8" , they both read the same.
 
My 97 Ram 1500 has a 318 in it with 152,000 miles. Had bad mains, so i put main bearings in it and new HV oil pump 4 years ago, as a temp fix. Oil gauge goes midway when cold and drops slightly below mid when griving with 15w40 oil in it. Gauge started bouncing about 2 summers ago, and i thought the motor was done. Sending unit was bad. Changed it, still the same pressures as before.
For kicks one day a few months ago,i dropped the pan again to check the bearings. They still looked brand new and from then on i don't worry about oil pressure. Long story short, if you're concerned about the pressure, mount a mechanical gauge on it to verify if there is any doubt to wether the inaccurate factory gauge is correct or not.
 
I see many comments here regarding the accuracy of the factory oil pressure gauge, don't know why ... the gauge is reading fine and that is the reason for the concern---low oil pressure. Now that I arrived here in Mill Valley along the coast, the mornings are cool in August and the 40 Wt. is reading between the second and third graduation at idle when first starting up. When fully warm at idle, the pressure drops all the way down to halfway between the zero and the first graduation. And that does not bother me, but climbing the grade the other day at "full speed" with the same oil pressure as hot idle pressure is what made me nervous and the reason I started this thread in the first place.
 
Personally, I would want to know what number was associated with "this graduation" and "that graduation".
 
I figure the little white square on the far left corresponds to 0, the first fine line graduation as 20 psi, the second as 40 psi, the third to 60 psi, and all the way over to the right as 80 psi (won't ever see 80 psi).

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famous bob said:
pure guess ! go w/ a mech. gauge
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Bahhh, it moves the gauge needle so it's fine.:D
Unless something is going to be done about it what difference does it make what the pressure is unless it's zero and the motor will tell you that pretty quick without a gauge at all.
There's a lot to be said for blissful ignorance.

(All meant sarcastically of course) but what his he going to do about right now anyway?:D
 
dibbons said:
and the 40 Wt. is reading between the second and third graduation at idle when first starting up. When fully warm at idle, the pressure drops all the way down to halfway between the zero and the first graduation. And that does not bother me, but climbing the grade the other day at "full speed" with the same oil pressure as hot idle pressure is what made me nervous and the reason I started this thread in the first place.
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The straight 40W is acting exactly like it should. If you put in 10W40, then the pressure would not be so high when cold but would be the same when hot. If you want higher pressure at hot idle after climbing the long grade, you'll have to go to an Xw50 grade. Or try an Xw40 pure synthetic to get a bit more pressure stability.

Ultimately, it sounds like: the oil pump is somewhat worn; it ought to keep pressure up much better at road RPM's with straight 40W. No wonder you need the 25W60 or whatever.

Or the pressure gauge is off....which has been said how many times now? LOL Figuring that it reads 'such and so' at certain gradations is meaningless; the gauge moving around means that that the gauge resistance winding is intact and that the sensor is working, but little more.

And BTW, I am not aware of 5 psi being an actual minimum pressure 'spec'.... it is just where the idiot light comes one.
 
Cool, and tnx. I have never seen an actual minimum pressure spec. That is good info!

Of course, what they don't say is temperature conditions for this test, but one would expect it is at average operating temperature, warmed up.

This also confirms, that if your gauge is accurate (which is totally questionable at this point) and your interpretation of the marks is close, then your pump is worn, or perhaps there is a lot of bearing wear.
 
Here is a Mopar oil pressure resistance chart:

Oil Pressure, PSI - 0 10 20 30 40 50 60 70 80

Resistance - 68.0Ω 40.0Ω 31.0Ω 25.0Ω 21.0Ω 18.5Ω 16.0Ω 14.0Ω 12.0Ω

The 25W-60 has been changed into the crankcase and pressure is up a notch. Been driving in 100+ temperatures this past week where the factory coolant temperature needle climbed just to the right side (a hair past the middle point of the gauge) on the freeway at speed. No boilover, though.
 
Of course the resistance all assumes that the oil pressure sensor is still working as it should.....any reason to not temporarily put another gauge on there? That is a pretty standard diagnostic step for this type of issue.
 
I will change the sensor (keep the factory gauge) and use a "T" to add a mechanical gauge (as soon as I decide on or find a USA made oil pressure gauge)
 
Sounds like you would like one of the old Stewart Warner ones! Here is the new ones they sell like that... dunno where they are made now however.

Oil Pressure Gauge

Bet you could find an old one on the sales forums here or on eBay.
 
Do you really live in Baja? I live right across the border if you want to hook up my oil pressure tester.
 
I'm in the San Francisco Bay Area this week, heading back to Baja California Sur in a few more days. I usually overnight in Chula Vista.

During last year's trip up this way, I raced my stepdaughter in her 2004 4Runner at Sonoma Raceway (Sears Point). She beat my little 318 two-barrel twice by two full seconds (16.4 vs 18.4). This year I stayed in the grandstands.

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I purchased two different types/brands of oil pressure sending units on E-bay (not installed yet). One of the sellers claims his sender corrects a OEM defect where the oil pressure reading is different, depending on if the gauge is rising or falling. If true, that might explain why my gauge was reading a little higher when the oil was warm after re-fueling (after restarting-gauge reading more) and a little lower when starting cold and then running the motor continuously until warmed up (driving without stopping-gauge reading less).

I was finding it hard to believe that after a ten minute fuel stop my oil had cooled down enough (and thickened enough) to raise the oil pressure a measurable amount.

from ebay web site:
Patented design corrects a systemic flaw: If your oil pressure goes "up" to say 60 Psi, the original senders will show a different reading than if it comes "down" to 60. This re-engineered sender correct this problem. Accurate at all times, unlike sleazy imports, or even NOS! This is as good as it gets!

Mopar: Oil Pressure Gauge Sending Unit 318-440-426 OEM Charger Dart Cuda GTX R/T | eBay
 
R Ehrenburg DOES have some history with Mopars.... interesting that the old senders had what is called 'hysteresis'. Do you have another gauge yet? I ask because your present sender can be OK but the gauge off, and the new sender would not show you that. You need a 100% independent system for a valid comparison.

And BTW, if your instrument voltage regulator (IVR) in the dash is ****'d, then it will throw off ANY sender/gauge combination.

Since you have found a table of sender resistances versus oil pressure, you can just connect a long wire to the sender, run it into the engine compartment, and use an accurate ohmmeter to record the sender resistances as you drive. Then you can reverse translate to the actual pressures you had. That takes any gauge error and IVR problems out of the readings. I do that in checking the Mopar temp sensor systems, which work in the same way (variable resistance in the temp sensor controlling a thermal gauge movement).
 
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