Oil Pressure Question

[kings1527]

I had to replace my oil sending unit....again...but this time went with the Duralast PS133 from Autozone. I installed it and when I started up my car, it was sitting right at 80 psi. This didn't seem like any improvement from what I had with the Borg Warner after it failed within a few months.

I found some info on oil pressures and I've been told that upon startup, oil pressure is right around 80 psi until the engine is warmed up and then should fall in a range of around 40-60 psi. One person said that it's always that way with his: he'll start up his car and it'll be at 80 psi and then when it warms up, it'll drop and the gauge will read accurately between 40-80 psi, depending on driving conditions (idle versus throttle). Sure enough, that's exactly what mine did. As soon as I drove for about 5 minutes or so, the gauge was functioning perfectly.

My question is this: is this a gauge 'thing' or an engine 'thing'? Is the engine designed to pump a higher psi until warmed up and then reduce, or is it the sending unit 'warming up'? Anyone know why?

Thanks!

[dmc6960]

Its not an engine thing or a gauge thing. Its an oil thing. The oil starts out thick, but thins out when it gets hot. This is what causes the higher oil pressures when cold.

[kings1527]

Its not an engine thing or a gauge thing. Its an oil thing. The oil starts out thick, but thins out when it gets hot. This is what causes the higher oil pressures when cold.

I never even considered that. Thanks Jim!

[David T]

The general "Rule of Thumb" is 10 psi minimum plus 10 psi for every 1,000 RPM. From that you get almost 20 psi at idle. It will be higher when cold and lower when hot. You should never see 80 psi, that is an instrumentation error. If you suspect a problem you get a good, accurate mechanical gauge and see what the pressure is. Then you compare to the electrical gauge and if they are not in agreement there is a defect with the electric one. usually the sender.
David Teitelbaum

[Bitsyncmaster]

The general "Rule of Thumb" is 10 psi minimum plus 10 psi for every 1,000 RPM. From that you get almost 20 psi at idle. It will be higher when cold and lower when hot. You should never see 80 psi, that is an instrumentation error. If you suspect a problem you get a good, accurate mechanical gauge and see what the pressure is. Then you compare to the electrical gauge and if they are not in agreement there is a defect with the electric one. usually the sender.
David Teitelbaum

Has anyone ever done the mechanical gauge check?

Mine also starts out at 80 PSI and at idle when hot is 40 to 50 PSI. If it's higher than idle speed it hold 70 PSI when hot.

[Jason]

Has anyone ever done the mechanical gauge check?

Mine also starts out at 80 PSI and at idle when hot is 40 to 50 PSI. If it's higher than idle speed it hold 70 PSI when hot.

When I did the cam break in on my newly rebuilt engine I had a mechanical oil pressure gauge hooked up. If I remember correctly it was around 70 PSI at 2500 rpm when cold.

[dmc6960]

I will be doing bench tests of the oil pressure sender in a few weeks to observe its behavior. Tests with a multimeter and tests with a DeLorean oil gauge hooked directly up to it. Will observe its real behavior in the pressure ranges of 0-80+ psi.

[kings1527]

I will be doing bench tests of the oil pressure sender in a few weeks to observe its behavior. Tests with a multimeter and tests with a DeLorean oil gauge hooked directly up to it. Will observe its real behavior in the pressure ranges of 0-80+ psi.

I'm really interested to see the results from this, Jim. Thanks for doing it.

[Dangermouse]

Is there a location on the engine where you could install a mechanical gauge, in addition to the sender for the dash gauge?

[Jeffu]

Just last week I performed a cam break in on a complete rebuild. With a mechanical guage hooked up on initial start up I hit 70 lbs cold, and 25 mins later at idle before I turned the key off it was 42 lbs. This was using Joe Gibbs BR 30 break in oil. The guage (L in picture) was one I bought from Harbor Freight Tools.http://img.tapatalk.com/d/13/07/09/ty2ajeny.jpg

[kings1527]

That's interesting and good to know. Those numbers are real close to what my gauge with a good sender reads.

70 psi on startup sounds pretty spot on. Maybe the car's oil gauge, when functional, aren't that far off. My needle is about a full needle's width below 80 psi on a cold startup which would put it right at about 70 psi. And warmed up, it's about another needle's width or so above the 40 psi mark.

Obviously our car's oil gauges aren't as accurate as your modern test gauge, but for 1980's electronics it's close enough for me!