Oil pressure gauge reads too low

[Histoy]

I recently bought a '68 Coronet R/T and I was concerned because the oil pressure reads too low on a fresh rebuild. It has the Rallye gauge package. I grounded the wire at the sending unit, and the gauge went up to the max reading. The gauge reads 20 lbs at idle and only 30 lbs at 2500 with the engine warmed up. I removed the sending unit and installed a direct reading gauge. It reads 40 lbs at idle and 50 lbs at 2500 rpm. I bought another sending unit and it reads 20 & 30 just like the first one. The sending unit has a small orifice. Has anyone tried enlarging the size of the orifice to increase the reading at the gauge?

 

[eagleone1983]

Putting a bigger hole in the sending unit won't increase or decrease PSI, it's to small of a difference. The same amount of pressure will still be showing up on your gauge. I always heard you want about 10 PSI per 1000 RPM. In that case, wouldn't your readings be ok?

 

[Histoy]

I was afraid that would be the answer, but I had to ask.

The gauge readings would be OK given that rule of thumb you mentioned, however, they are not correct. At 2500 rpm the direct gauge reads 20 lbs higher than the one in the car (50 vs. 30). At 3000 rpm the car gauge still reads 30 lbs but the direct gauge increases to 55 lbs.

 

[HitIt]

I've never fully trusted electric oil pressure guages for this same reason. If you are confident about your direct reading, just take the electric one as a relative number.

Maybe try the guage on another sender or temporarily plug the guage into another car?

 

[Meep-Meep]

Enlarging the orifice will not increase the total pressure reading but I suppose if it was nearly plugged it could give a low reading. Pretty sure the small hole is to delay or damp the signal to the sender. You did all the right stuff by checking with a known good mechanical gauge so I suspect your 50 PSI is real. Does it have a stock pump? If so then 50 PSI does not sound like an issue to me. If you know for sure you have a 70 pound spring then maybe you have excessive clearance somewhere or accumulation of larger than standard clearances. Perhaps try replacing the voltage limiter on the dash circuit board to see if the factory gauge reading changes. Slightly adjusting the voltage to the gauge you should be able to make it read whatever you want.

 

[Histoy]

The engine has a stock oil pump so I'm happy with the readings I'm getting from the mechanical gauge. I'd just like to be able to get the one in the dash to read correctly. I have a spare voltage limiter so I may give it a try.

 

[69 Runner]

Make sure it's the proper sender for the factory gauge.

 

[Histoy]

Both sending units I tried were new and correct for the Rallye gauge. Has anyone tried a different sending unit? I have one for a 100 psi gauge, but I think that the reading on the 80 psi Rallye gauge might be even lower with a 100 psi sending unit.

 

[1badstroker]

I would believe the mech. gage before the electric one.