I was driving home from school on Monday of last week and noticed that the oil pressure wasn't at it's normal point on the gauge of my GSL-SE. As I drove and the car got hotter, it got lower. I pulled over and watched it as the car sat at idle. It was at this point the pressure on the gauge dropped to below the first (30psi?) mark. I took the car up to 2000 rpm and the oil pressure jumped back to a third of the way between the first mark and the 60psi mark. I babied her home and parked her.

Today I changed the oil and filter in the hopes it was just telling me that it wanted new oil (previous Mobil1 oil had been in there for a little over 3k miles). I started her up and she had a noticeably hard time starting and then, blew a nice cloud out the exhaust. Oil pressure seemed normal. It started and jumped to above the 60psi mark and held there until idle normalized. At first, while it was at idle, it hovered on the first mark on the gauge.

Then I took it for a .5 mile jaunt around my driveway (two and half laps) and let it idle for a second. Much to my dismay the pressure dropped to 3/4 of the way between off and the first bar. I finished my third lap of the driveway and parked the car and the pressure dropped slightly lower this time and at 2500 rpm it only came up a quarter of the way between the first mark and the 60psi mark. Upping the rpm did nothing. Lowering the rpm caused a rapid drop and when it touched the halfway point between the first mark and the no pressure mark I shut it off.

Three things to note: 1) The entire time at idle, the oil pressure needle was bouncing a bit. Not much, but about an 1/8th of an inch, back and forth. 2) over the last few weeks, it has flooded a couple of times, but never to the point of a no start after running the starter for a few seconds with the fuel off (like it did twice today). 3) I am reading the oil pressure gauge as this - off/no pressure mark, first mark/30psi-ish, 60psi mark, 110psi mark.

I'm sincerely hoping someone will tell me that it's just the gauge, but I fear that it is more likely going to be the oil rings and/or bearings because of her age and mileage (211,000 or so).

 

my 85 does the same thing. anything over 2k rpm the gauge is fine, but at idle it falls all the way back to almost zereo...I have had the car for the better part of 2 yrs now so if the oil pressure was actually that low I think the engine wouls have blew by now. On a side note this engine leaks oil out of every dowel pin connection so that may contribute as well lol

 

Yeah, I don't know what the hell just happened, but I started it back up to show my brother what it was doing and the oil pressure was fine. The longer it ran the better it got. When I changed the oil I ran it for fifteen minutes or so and it dropped. This time I ran it for the better part of an hour and it only once dropped below the first mark.... I'm so confused...

 

It could be the gauge in the dash, my girlfriend's 85 gs oil pressure needle is always at 0 and never moves. I've tried replacing the sending with no luck, so it has to be the in dash gauge, it could be yours is going out.

 

72 psi is the normal set point,, when hot with 20W-50 grade oil it will need as much as 3000 rpm to make this

cold, it will do it at idle

normal hot idle is anywhere between 15- 30 psi

 

Okay, I won't worry too much unless I lose a bunch of oil pressure then. Thinking about it, it may have just had gunk on the sender or something. I need to find a good oil gauge to set up on it.

 

Bad oil rings make horrible oil consumption/smoke but oil pressure will be fine until you run out of oil.

The gauge, however, cannot be relied upon.

 

If you got 211 K out of your motor, you are doing good anyway. Change the sending unit and see if that changes anything.

 

She's dead. The oil pressure bottomed out last night about five miles from home. I let her cool down some and started her and she drove for a mile or so with pressure and then it dropped again. Followed the same process three times. While I want to believe that it's just the sending unit, I'm not willing to risk the remaining good parts from the engine to find out. I'm going to check the oil pump and see if that needs fixed, but I'll probably start the search for a good engine for the car. I have an S5 keg, but I'm not sure if it would work properly.

 

See if you can temporarily hook up a mechanical gauge to the engine to verify the oil pressure. You could even use a tee to enable both gauges to operate together, comparing the readings of both simultaneously.
It's crucial to eliminate the possibility of the stock gauge and sender injecting erroneous information.

 

Also, when you start losing oil pressure, it's to the oil control rings inside each side of each rotor - when that happens you'll notice a definite decrease in engine output because the oil is diluting the air/fuel ratio. The car will be a dog compared to a fresh engine. At 211k miles the engine seals are most likely very tired and I've stopped driving mine at 216k due to the above behavior.

Good luck, and I'm betting it's behind the dash (gauge side) vs. sending unit if it's a simple mechanical/electrical issue. The only way to find out is to plumb a mechanical oil pressure gauge in there.