The oil thinning because of temperature doesn't relate to the outside temp, but to the temp of the motor. As the motor warms up, the oil thins. Often the hotter the motor gets, the more the oil pressure drops. Once the motor reaches normal operating temp, the oil should be close to its higher temp range. The lower oil pressure will be with the motor at normal temp (about 5* to 10* above whatever the thermostat the motor has. If the thermostat is a 180*, when the coolant temp reads 180*-190* that should be about the lowest oil pressure your going to see on the oil pressure gauge. I've had a lot of old Mopar motors run many years with 3 psi or 5 psi oil pressure at operating temp, as long as the pressure increases as the rpm raises. Mopar states 10 psi oil pressure at operating temp per 1,000 rpm is OK with their performance motors.
Soooo dodge that bullet!!! Drove to and back from work today. My gauge was reading 55-60psi @50mph (no tach) and 35-40psi at idle. My gauge would rev up as I accelerated.
Congratulations! Now, enjoy the ride. I've seen way too many faulty old electric and mechanical oil pressure and water temp gauges. The first test is always check with a good gauge. What is unfortunate these days is that being new doesn't always mean its good anymore.
I'm glad you solved the mystery and sleep again! Kind of late to the discussion, but if I'm not mistaken (which I never am, but my wife disagrees) most sending units ground through the body and rely on a good physical connection of the threads to the block/heads/manifold. Is it possible your original installation used teflon tape or another sealant that prevented an electrical ground?
I wondered that. I only have one wire going to the sending unit that’s threaded into the block. Maybe I need a separate ground from the gauge to the body/frame?
I had an OT vehile that I changed engines in and had stock gauge problems for a year! One day I was looking in the engine compartment and saw a body to engine ground wire laying along the fire wall that I missed! Hooked it up, all gauge problems when away! Bones
I like the block mounted oil pressure gauge. I would run Mobil 1 synthetic 5W40...With the K&N or Wix filter. Engines need good oil circulation when cold.
That's common even with good bearings, and a fresh engine. Pressure is always higher at startup and drops as the engine gets to operating temperature. But with worn bearings the idle will always be much lower and spread between higher rpm's and idle will be more extreme. It wont rattle until it gets much worse, but eventually is will get bad enough to either rattle or spin a rod bearing. You could try running a heavy straight viscosity oil like Valvoline VR-1 in a 40w or 50w, and you'll see higher idle pressures. But it's a bandaid fix and sooner or later your engine is going to need a rebuild. Not sure how "poking at bearings" would tell you anything? Plastigauging rod and main bearings will tell you something, but often worn cam bearings can cause this issue too, and only way to check them is to mic the cam, and the bearings to see if there's excess clearance.
Be careful putting racing oil in street engines. Many many moons ago my ot car was losing some oil pressure , so I decided to switch from Phillips 10w40 to straight weight Valoline racing oil! In about six months my engine started drinking oil! I tore it down and the oil and scraper ring were gummed up with some grey shit! This was caused by the racing oil not having any detergent in it! Like I mentioned , this was a long time ago, and things may be different, now, but this was my experience! Bones
Good to hear that it's sorted. I had a 1" pressure gauge mounted on a "T" fitting on the block on a Chevy 216, because I didn't trust the original gauge showing 3-4 PSI at idle, but it turned out to be quite normal for this engine. What is that gauge cluster out of, it looks neat?
These were my experiences years ago! I have heard that Valoline has changed the formula in it’s racing oil since then, not sure! Like I mentioned this was my experience some fifty years ago! But I learned a lesson there! Bones
@Boneyard51 is correct about not using true racing motor oil on the street, the additive pack is intended for racing, not street driving, and it can indeed result in the condition he described due to the lack of detergents and dispersants in the oil. But it would appear to me that Valvoline VR-1 carries a bit more detergent additives than typical racing motor oil: https://www.speediagnostix.com/new-oil-analysis/valvoline-vr1 Calcium - 1090 ppm and Magnesium at 528 ppm, less than typical for street motor oil, but enough for 2k - 3k mile oil changes?
Had the same thing with my 1937 Ford coupe with a 430 Lincoln a few months ago. I think using a resistor on a 6 volt quage is a crap shoot
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