swensonm

I have checked the search but unable to find an answer...Gauge reading high at idle 60 psi moves to 75-80 at 60 mph. removed wire from sending unit gauge pegged out. replaced wire turned on ignition and returned to just above 0. Removed sending unit installed manual gauge (harbor freight) warm at idle about 35 psi but the needle is vibrating between 35 and 45.... What might cause this?
thanks Mike

DUB

Mike.
I read you post and you are all over the place so I do not know what you are actually asking "what might cause this?"

What...the fact that the factory gauge does not read the same as the harbor freight one???

OR...

The way the factory gauge responded to removing and re-attaching the wire to the sending unit???

OR...

The fact that the needle is bouncing. Because this oil pressure gauge I am sure is an actual tube that holds oil...Correct?

DUB

gcusmano74

On the 1974 model, the oil pressure gauge used an electric sender right above the oil filter canister. It is screwed into a short extension piece, and looks straight left with one wire attached to it.

swensonm

LOL Sorry
Just a bit frustrated today.
My stock gauge is reading high. Chilton's states 40 at 2000RPM. My car is running 60 psi cold and at 3000 RPM running (60 MPH) about 80 psi. I disconnected the wire at the sending unit turned on the ignition and gauge peg out high. I reconnected the wire ,turned on the ignition and the gauge re set at just above 0. So I think that means the gauge is working. Next I removed the sending unit and installed a manual oil pressure gauge. Reading were better 35-39 psi at idle but the needle never settled down it was "vibrating or jumping around so I could not get a true reading. Is this normal? If not what could be causing the fluctuation?

LOL Sorry
Just a bit frustrated today.
My stock gauge is reading high. Book states 40 at 2000RPM. My car is running 60 psi cold and at 3000 RPM running (60 MPH) about 80 psi. I disconnected the wire at the sending unit turned on the ignition and gauge peg out high. I reconnected the wire ,turned on the ignition and the gauge re set at just above 0. So I think that means the gauge is working. Next I removed the sending unit and installed a manual oil pressure gauge. Reading were better 35-39 psi at idle but the needle never settled down it was "vibrating or jumping around so I could not get a true reading. Is this normal? If not what could be causing the fluctuation?

0Willcox Corvette

Mike... the stock gauge will read high when it's cold... the oil is thick. What does the stock gauge do when it's warm?

You are correct, if you remove the ohms wire from the sender the gauge should peg and when you attach it should read zero with just the key turned on.

But... no book can tell you exactly what oil pressure you should have at a given RPM. There are too many other factors that weigh in on this. ie: condition of the engine, what oil pump is in the engine, the weight of the engine oil and so on.

You can check the gauge based on the scale of 0-90 ohms, this gauge is a linear gauge so you can divide up the increments.

O ohms (key on only) = 0
45 ohms = 40 lbs oil pressure
90 ohms = 80 lbs oil pressure

As far as your bouncing needle with the manual gauge.. I'm not sure how much faith I'd put in that harbor freight gauge for one. You might try a more dependable gauge and see if you get the same results...

Willcox

swensonm

Thank you so how do you test the gauge? and it there a method to test the pressure sensor?

CaseyJones

To test the sending unit, use an ohmmeter as Willcox suggested. The readings he mentioned correspond to the gauge readings. Let the engine get to operating temperature. The use the meter to test the sender. One test lead to the unit and the other to a good ground. If that looks about right, then your gauge may not have a good connection or just be that far off calibration.

swensonm

Thanks I will give it a try...And report back!

swensonm

Wilcox Thanks checked out the gauge and switch based on your recommendations and numbers! A trip to NAPA for a new pressure switch and all is well!
thanks
Mike

jb78L-82

Generally speaking with SBC's the 40 lbs number is correct for a stock engine. I had a stock 73 Nova SS 350 which ran right on those numbers in the late 70's/80's when I owned the car-40 lbs @ 2-2,500 RPM when hot, even with 120,000 miles on the engine before I rebuilt it. My totally stock 78 L-82 with 66,000 miles on the engine also generally runs 40 lbs on the gauge almost always when hot, a little less when idling, a little more at higher revs-50 lbs. I currently use Mobil 1 15W-50 and the numbers are generally the same, a little higher when cold but not much. I have also run 10W-30/5W-30/20W-50 in the past and again the numbers don't move that much, but they do move up or down about 5-7 lbs depending on the weight of the oil.

The point is the gauge should generally stay around 40-45 lbs with a stock engine when cruising on the highway and the gauge does tell you something relative to the engine, if it is stock and never been touched. If the gauge is showing 60-80 lbs on a stock SBC engine something is amiss with the engine, gauge, or sending unit.

ThePabst

40 to 45 psi is plenty. Too much OP can cause overheating, excessive distributor drive ware, bearing float and leaks. Oil is as much a coolant as it is a lubricant, it needs to spend enough time in contact with the metal parts to absorb heat and then dissipate it. The OP is driven buy the same gear that drives the distributor, the increased pressure is developed by the cam/distributor interface. Remember, the oil is pushing back as hard as it is pushing forward... equal and opposite reaction. The oil is also trying to become a body at rest, so the pump needs to impart energy into it to make it move. Takes more power to move more oil, simple. The drive suffers the brunt of the forces. It can also cause undue thrust on the cam as well due to the helical gears.

Leaks... this is the biggie. If you own or ever have owned a vintage SMB of any kind you know all about the Generals' utter contempt for oil control. The increased OP will make the rear main leak big time. It will also blow past the front cover seal, the harmonic balancer will spray this oil ALL over everything. Does your alt belt always slip no matter what you do?? Ummmm. Slow drip from the front of the motor no matter how many times or kinds of gaskets you buy and swap out. The higher pressure also puts more oil up into the valve covers. The drain holes can only drip so much, the excessive oil is gonna go somewhere.

Lastly, bearing float is when oil gets behind the crank bearings on older motors. It can actually lift them and cause them to spin.... Here come the boom!

Bottom line, motor manufacturers spend millions, and I mean MILLIONS on these problems, they know what they are doing... hu, most of the time, almost all motors of this vintage from any manufacturer set the original pressures at 15-20 idle, 40 cruse and 50 at redline. Give or take a little. Need a pump? stick with what she came with. Oh, and C3 gauges are complete COMEDY, as are all GM gauges of this vintage... replace them with modern stuff. I have posted the procedure and wiring dia. elsewhere on this board.

L-46man

Swensonm...I'll try to answer the HFT gauge bouncing question.

For a Hydraulic oil tube gauge to work correctly you need oil all the way through the tube into the Bourdon tube in the gauge. Oil is basically incompressible so it reacts virtually instantly.
If there is AIR in the tube...THAT is compressible and therefore acts like an accumulator....loading and unloading. You won't get a steady reading.

Unkahal