1955 PSI Mystery

[RandyK]

Re 1955 Eldorado engine rebuild.  When the engine is started getting read of 35-38 PSI. But quickly goes to 15 PSI. Warns up and drops to 4-5 PSI.  And advice?
Randy
Dallas, Texas

[James Landi]

Hi Randy,

 There's a "rule of thumb" relating oil pressure to rpm's. Some on this thread, I hope, will include it in their response. Here's a thought :I'd try another gauge...also, these older models didn't develop much oil pressure when hot and especially at idle in hot weather.  In fact, I recall reading my '56 owner's manual (decades ago) that advised owners to ignore an intermittent oil warning light when the engine is hot and at idle in traffic.  Additionally, back in the day, folks used 30 single weight oil, and that provided a few extra psi of pressure. A related fact: the manufacturers (GM) of my big block marine engines advises using single grade 30 weight oil and not multigrade oil. Even as recently as the 1980's, Cadillac advised owners of 4100 engines to use single weight 30HD for extended periods of hot weather driving.  (Now modern cars with gauges always seem to have oil pressure gauges that are at a comfortable mid range position).

[Michael Petti]

What oil brand and weight are you using presently?

[Jeff Rosansky CLC #28373]

Are you using an electric gauge?
I chases an oil pressure issue on my 70 for months. Tried everything I could think of.
One really cloudy day I was driving and turned on my lights, and my oil pressure went down about 10psi. Turns out it was an electrical issue.

[J. Gomez]

Randy,

Your Service Manual should have the type of oil listed according to ambient temps range using single or multi grade oil. Also it should have the oil pressure under normal 30MPH at 30-35 lb. and around 15 lb. at idle, the AMA records also shows the same normal 30PMH at 30-35 lb.

If I recall (going from memory here  ) the oil pressure sender will open (light off) at around 6-12 PSI, so at idle and with engine at a normal operating temps per your post and with you reading 4-5 PSI is your oil light off? 

4-5 PSI maybe a bit too low and this could trigger the oil light to come on, but as James and Michael inquired above what grade oil you currently use.