Unsettled Oil Pressure

I have a 51 Chevy pickup with a new GM 350 crate with only 235 miles on it, a 700R4 tranny. When I start it and run it at about 1500 rpm the oil gauge (new Classic) will bounce as much as 45 lbs. I hold it at 2000 rpm and the pressure sits at about 90 lbs. When I drop it to 1500 rpm the pressure drops to 75 lbs. At that point it will vacillate between 65 and 90. Sometimes the movement is erratic and sometimes it moves around slowly.
I have air but this is when nothing is on. I worry there is something wrong with the oil pump.:confused:

you should not have 90psi . is this a electrical gauge ? if so try mechanical. it could be other things but start here first sender could be bad .cold 45to65 hot 25to40 on idle has to do with what oil pump to running and rpm . down the road 45to 65 is fine

If it is a mechanical gauge, try "burping" any air from the line from the block to the gauge. To do it, with the engine running, crack open the connection at the back of the gauge and slowly let the air out, if it's the plastic tube, you can watch the air/oil line rise towards the gauge.

It's not always needed but, getting the air out helps to stabilize the gauge "signal", the air "bounces" the signal as it compresses and expands. This is S.O.P. on our Waukesha diesel generators with "Murphy" safety gauges, ( the gauge has the ability to shutdown the engine). It may help in this case. Worth a try as it won't cost a dime! OH, keep plenty of rags handy if you open the connection to far / to fast it can get messy!

I'm with Pat on the numbers, 90 is awful high. You didn't mention what viscosity grade oil you're using, which may be a factor. Checking with a different (mechanical especially) gauge would be a good proof. If the condition showed there too it could be the relief dumping because pressure is too high.

Like the others say, try another guage. Too many possible variables with electric ones, I prefer the line type.

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Originally Posted by Bob Parmenter

I'm with Pat on the numbers, 90 is awful high. You didn't mention what viscosity grade oil you're using, which may be a factor. Checking with a different (mechanical especially) gauge would be a good proof. If the condition showed there too it could be the relief dumping because pressure is too high.

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yep i was not going to say any thing about a sticky bypass. that why i told him the gauge first.even 50wt still would not be that hi bypass would dump it before then less jammed if moving like he said it the gauge i think