QuestionI have 1947 chrysler windsor 6 cylinder with fluid drive. Oil pressure reads about 40 lbs when first started. Pressure stays good for about 10 or 15 minutes, then gradually declines to 5 or 10 lbs. Using straight 30 weight non detergent oil. Could it be the gauge?
AnswerYes, it could be the gauge, but we all have a tendency to blame the gauge when it is telling us bad news.
A much more likely explanation is that the bearings are worn and the engine will need a major rebuild soon. However, 10 PSI isn't that bad if that is the reading at idle. In later cars, the "idiot lights" come on at 7 PSI, and the owner's manual says it is permissable for the light to come on at idle when the engine is hot. So if your reading is actually 10 PSI at idle with a hot engine, you aren't ready for the big bill yet!
To see what your oil pressure really is, you can go to an auto parts store and buy a hydraulic (some call it a mechanical) oil pressure gauge - these are operated directly by fluid pressure, one installs them by finding the point at which the factory gauge sender is connected and temporarily plumbing in the replacement gauge, just long enough to get a good reading on what is actually happening with the engine. These gauges just about always have a 1/8" NPT threaded fitting on them and the connection on the side of the engine block will be the same, so you can buy a short piece of high pressure hose (called a "whip" hose) with the right fittings already installed at the auto parts store, with whatever adapters it takes to thread right into the block and onto the gauge. I keep such a setup in my toolbox all the time for checking true oil pressure in engines I am working on - it's nice to know the actual pressure. From that test you can calibrate the dash gauge and rely on it from then on.
However, if your oil pressure is dropping that low at road speed, with SAE30W oil, that is pretty low - start saving your pennies - a major engine rebuild is in the future for you.
You can nurse this engine along for quite a few miles by switching to SAE40W or even SAE50W for summer use - you can find this at most parts stores - they may call it "racing oil". And as a last resort, you can add a can of STP with each oil change - but the mechanic who rebuilds your engine will cuss you for that - that stuff is really tough to get cleaned out of there!
Dick
