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I have a 68 L79 .. fully restored .. after warm up, oil pressure @ idle reads 0-5 psi .. @ 2000 RPM reads 30-35 psi .. I checked the actual oil pressure with a remote oil pressure gauge, and it is fine .. removing/rebuilding the oil pressure gauge is quite a chore .. question: what do others see on their gauge @ idle? (I'm assuming my gauge is in need of rebuild ..) thanks for any comments 1968 Corvette 171
Such readings are 'normal' for the stock oil psi gauge. All you need to know is that a reading of 5 psi on your gauge is FINE! Just refer to that gauge in that way for the future.
P.S. There is NO GM oil pressure gauge which reads correctly throughout its range. They are very REPEATABLE devices; but they are not that accurate. C3 gauges are reference devices; they are only there so that you will notice when something CHANGES SIGNIFICANTLY. Your gauge is perfectly fine for using "as is".
Last edited by 7T1vette; Aug 10, 2020 at 07:45 PM.
So if you're still running the original mechanical gauge:
I had this same issue with 0-10 psi pressure on my original gauge which had worked great for decades.
Checked pressure with secondary gauge and all was well.
Eventually pulled the gauge while I had the dash out and found just a little dust bunny on the needle gear was preventing proper function.
Blew it out and works like new.
The gauge in my '69 is the same - 35psi cold at idle, drops to "0" when hot, just under 35 when driving. Scared the living **** out of me when I was driving it home - thought I lost oil pressure. Still gives me the *******, honestly. I haven't checked if it's the gauge, I assume so.
I'm running 20-50 per the recommendation of several people for big blocks.
You could check to see if the line going from the engine to the gauge is original. I could tell from pulling the carpet up, that it leaked at some point. I installed a new plastic line with new connectors, my SB starts at 70 then drops to 30 ish after the engine warms up.
Don't know about all GM gauges not reading correctly but all my original, untouched read withing 2%-4% compared to my highly accurate Snap-on gauges. I would say get the gauge repaired to read correctly. If you accept that 5 psi on the gauge is normal then you won't worry when it goes to 0. You will eliminate any form of early warning to a problem. By the time you hear the death rattle, it's too late.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
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for reference 10 psi for every 1000 rpm is a good number. If you are running a 10-30 or lower you will see lower pressure at lower RPM because the fluid flows easier when its hot. I have a 68 L79 327 and when I had a flat tappet motor I ran 20w50 mobil1 because it was cheap and had really high levels of zinc, phosphorus and zddp. I never had an issue with any cam with it and the pressures never went below 20psi idling on a hot day. Now I have a roller cam and can run a normal off the shelf oil. I also use a high flow, high pressure cam for the way I drive and the pressure is usually above 30 at idle or cruise
I ran into a similar problem with the oil pressure gauge on my '73 about 20 years ago. I had put it in storage for three years back then while assigned overseas. The gauge was original and had likely never been changed or serviced at that point, certainly not since I'd owned it. A few days after removing it from storage I started the car and noticed I was barely getting a reading on the gauge. I shut it down immediately and checked the oil level which was at its normal level - indicating full. Up to that point the oil gauge had responded as expected on start and after warm-up.
I then realized that if I had no oil pressure my hydraulic valves should be clattering. Since they weren't, I decided to look into the gauge being an issue. The plastic oil line had oil in it and filled fully when the engine was running, but that didn't really tell me anything so I removed the center cluster to get access to the gauge. I turned the pressure down on my air compressor to about 40 psi and after disconnecting the line, forced air into the gauge. The needle barely moved. I then recalled recently reading in Corvette Fever magazine a couple of comments about the Bourdon tube in the oil gauge on older Corvettes getting clogged with oil that had turned to sludge over time. Since the oil in the gauge didn't "flow through" the gauge, once it entered the line between the engine and the gauge, the actual oil in the gauge could easily be as old as the car itself.
I removed the gauge from the cluster and sprayed brake clean into the fitting on the back of the gauge a few times and let it drain out. Initially it came out looking dark and much thicker than oil until after 5 or 6 times it came out clear (the color of the brake clean). I then again added pressurized air to the gauge and it seemed to react normally. I reinstalled it in the car after replacing the oil line with a new copper one and the gauge has read as it should ever since, 40-50 on start and 30-40 after the engine is warmed up.
low oil pressure?
