Precipitous drop in oil pressure at idle

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At last, an update. My mechanic was swamped and solving problems for other people so it took a week longer to get in than expected.

Brought it in for the really low oil pressure at idle and "while the car is here..." to see if they can find the short causing the blinkers to not work.

The oil: I had changed to Valvoline 20w-50 VR1 and saw a mild improvement of pressure at idle (from 6-8# to 10#). He checked with his experts and that's the very low end of an acceptable pressure at idle, but still lower than he's comfortable with. He will will replace the current pump with a high volume pump. Status of that now is waiting on parts.

The blinkers: They weren't blinking before, but earlier I changed out all the old incandescents for leds in all fixtures but the license plate light and headlights. I had also replaced the turn signal switch (left did not self-cancel), the flasher (leds have less draw and the old flasher may not receive enough voltage) and ignition switch (was "jiggly" and didn't always return fully to the "on" position after starting). None of these made any difference in getting the blinkers to blink. Chassis ground is good.

I then did a continuity test of every single light I could find and didn't find any issues, but then again, I don't really know what I'm doing so surely I missed the short. They found that although one rear brake light is out (when did that happen???), when you apply the brakes all the front and rear side marker lights light up. And when you use a blinker, the front turn signal lights *both* light (but no blinkie). They will continue to investigate.
 

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You've got a grounding problem with your running lights/turn signal lines.
 
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At last, an update. My mechanic was swamped and solving problems for other people so it took a week longer to get in than expected.

Brought it in for the really low oil pressure at idle and "while the car is here..." to see if they can find the short causing the blinkers to not work.

The oil: I had changed to Valvoline 20w-50 VR1 and saw a mild improvement of pressure at idle (from 6-8# to 10#). He checked with his experts and that's the very low end of an acceptable pressure at idle, but still lower than he's comfortable with. He will will replace the current pump with a high volume pump. Status of that now is waiting on parts.

The blinkers: They weren't blinking before, but earlier I changed out all the old incandescents for leds in all fixtures but the license plate light and headlights. I had also replaced the turn signal switch (left did not self-cancel), the flasher (leds have less draw and the old flasher may not receive enough voltage) and ignition switch (was "jiggly" and didn't always return fully to the "on" position after starting). None of these made any difference in getting the blinkers to blink. Chassis ground is good.

I then did a continuity test of every single light I could find and didn't find any issues, but then again, I don't really know what I'm doing so surely I missed the short. They found that although one rear brake light is out (when did that happen???), when you apply the brakes all the front and rear side marker lights light up. And when you use a blinker, the front turn signal lights *both* light (but no blinkie). They will continue to investigate.
If he is going to replace the oil pump, he has to pull the engine. If the engine is out have him check the bearings. No reason to pay to pull and engine and put it back in, just to put a HV oil pump, if the bearings are gone. If the bearings are bad see if he will change them from the bottom, that way you will have a repair that will last you for years, and not maybe for a season or two. After the engine is out and the oil pan and pump are out, it should not be much more to just change your bearings from the bottom. It may just be a couple of rod bearings. If the main bearings are good doing the rod bearings is quite simple and fast once the engine is out.
 
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As said there are many reasons why the oil pressure is low. Adding a high volume pump will increase pressure just from the fact that more fluid will be pushed through the system. However, may not be the best solution since it could be masking other issues.
 
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For now, I think I'm at the end of my low oil pressure saga. Bearings were within tolerance so a high performance oil pump was installed. Limited testing but the only time oil pressure is below the one-third mark on the (factory) gauge is when the car is turned off.

While they had the car in the shop, I asked them to see if they could get the turn signals to blink, which they did, but not at night. Half a success on that, so I'll update my original "Gremlins in the blinkers..." post (link below) instead of this one. And they replaced the front sway bar bushings.

 
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Been there and done that too:)
It is time to pull the engine. I would not run it anymore if I could avoid it, you could have a catastrophic failure with that low of an oil pressure. Minimum oil pressure you need is 7-10 psi per 1000 RPM. At 20 psi you should never get anywhere over 3000 RPM. At 2000 RPM minimum is 14 psi, 10-12 at 2200 is well under that. You will eventually end up having a spun bearing and causing damage to the crank, or worse. If you must run it, see if you can get a 20W-50 oil, or maybe even a straight 50 weight oil with some STP additive or any of those oil additives that are as thick as honey.
I would pull the engine and get the oil pan off to see if the pickup is just full or junk, you could get lucky and the pickup could be clogged and just cleaning it may get you back on the road. If I already pulled the pan I would pull the rod caps and main bearings caps to look at the bearings, if they are fine, I would drop in a new oil pump and put it back together. Being that you have an engine with 140K miles, you are probably in need of a full rebuild.
I am going to put this here, but most will probably be horrified by the thought of doing what I am going to tell you that you can do. But if you are short of funds, it will work. If the bearings are gone and the crank is not in too bad of a condition, you can pull the crank and polish it with emery cloth to remove small imperfections, or you can polish it while it is still on the engine (have to be real careful if on the engine as you do not want the metal particles in the engine), get new bearings of the correct size (your engine should be std/std as from what I can tell it has never been apart but check the back of the bearings) and front and rear seals and put it back together and get it back on the road. Just make sure to keep everything real clean. Best thing obviously would be to tear everything apart and send it to be completely rebuilt at a machine shop, but sometimes funds do not allow for that. I have even done this on just one rod bearing that has gone bad.
In the late 80's I had a modified street race 300ZX turbo, it ran high 12's/low 13's back then. One day I got an obvious bearing noise from the bottom end. I went to the dealer and bought some std rod bearings, took the oil pan off the car (on those cars you could take out the oil pan with the engine in the car), found the bad rod bearing by basically feeling the rods, one had quite a bit of play. Removed the rod cap, removed the bearings and polished the crank with some emery cloth as best we could from underneath the car, then put the new bearings in and torqued the rod bolts back to spec, and we were back racing the next day. Sold the car, probably a year later, never had another issue with that bearing. Yes, it is not the correct way to do it, but when you are young and broke, you make do.
 
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For now, I think I'm at the end of my low oil pressure saga. Bearings were within tolerance so a high performance oil pump was installed. Limited testing but the only time oil pressure is below the one-third mark on the (factory) gauge is when the car is turned off.

While they had the car in the shop, I asked them to see if they could get the turn signals to blink, which they did, but not at night. Half a success on that, so I'll update my original "Gremlins in the blinkers..." post (link below) instead of this one. And they replaced the front sway bar bushings.

Far more often than not I find such "Mystery Behaviors" like this to be related to a poor ground somewhere. It can become a real nightmare tracking them down. Doing a voltage drop test at every ground location works well, but it can be a real PITA. Not that it is hard, it is just tedious. I am so glad I know how to do that as opposed to hiring someone out who knows how to do voltage drop testing on these older cars. It is almost as if it is a dying skill.

Of course, my quick fix to your current situation is to not drive at night when the headlights are needed! In our case we do not drive our vintage pony cars when the streets are not dry, if there is rain, ice, or snow expected, or at night. It is hard enough to drive those old pony cars on public roads in daytime and good weather. Maybe it is just me, but It is as though some folks see those old cars out "in the wild" and become so enamored that they seem to forget that they are driving their own car and need to pay attention to what they are doing. It is as though I can't get far enough away from some of these folks. It is one thing for a car to come up close to ours while stopped at an intersection so someone can take some pictures. But, when on an Interstate doing 65 or so (or more), coming in close for a photo at that speed is not my idea of having a good time. Anyway, I digress... Things are bad enough with night driving as it is, so we opt to only drive them in the best of weather and lighting conditions.
 
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buy a cheap ground strap and connect it from the back of the block to the firewall. Make sure it is making good contact. It’s only a few bucks a very easy to do. Helps to ground the cab of the car. I was having electric gremlins and I added a few ground points, no more weird electrical problems. Any bulb or connection you check add dielectric grease to keep them from corroding. I even added an additional ground from the front of the block to the front clip. All cheap and easy to do. Hope you figure it out!
 
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